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Table of Contents
Introduction ........................... ............. .. .. ........................... ........................... .......1
Examining a eld ......................... ........................ .... .... ... ...... ............................ .... 2
Tips for shipping to diagnostic laboratories ... ....................... . . . . . . . . . . . . .......................... 2-3
Stage I: Scouting from emergence to knee-high ... ... ... ... ... ... ... ... ... ... ... .. .. ... ... ... ... ... ... ... ... 4
Stage II: Scouting from knee-high to tasseling ... ... ... ... ... ... ... .. .. .. .. .. ... ... ... ... ... ... ... ... ... ... . 11
Stage III: Scouting from tasseling to maturity ...... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . 16
Disease symptoms ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .... .... ..... ..... ..... ..... ..... ..... . 24
Leaf diseases ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .. ... ..... ..... ..... ..... ..... . 24-26
Smutting diseases ............................................................................................... 26
Virus and virus-like diseases ...................................................... . . . . ......................... 27
Fungal systemic diseases ..... ..... ..... ..... ..... ..... ..... ..... .. .. . ..... ..... ..... ..... ..... ..... ..... .. 27-28
Stalk and root rot diseases ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... . 28-29
Ear and kernel rots ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .... . . ..... ..... ..... ..... ..... ..... .... 29-30
Insect injury symptoms and management recommendations .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . .. .. .. .. . 31-39
Herbicide injury symptoms ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .. .. ..... ..... ..... ..... .. 40-42
Herbicides that may be used or trigger symptomology in corn .... .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. 43-47
Herbicides listed by active ingredient and mode of action .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 48-51
Nutrient deciency symptoms ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .. 52-53
Index .................... ............... ............................ ............................ .................... 54
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IntroductionThis manual is prepared and distributed by Monsanto Company to help farmers, seed dealers, and companypersonnel determine the likely causes of abnormal corn plant appearance. While the primary target is the North
American Region, the principles and many of the situations described in this publication extend to the regions of theworld where corn is grown.
Symptoms may be due to a single cause or the result of two or more interacting factors. Also, because of theirdifferent genetic backgrounds, different hybrids may not have identical symptoms in response to the samecause. Routine eld examinations are crucial in spotting problems or potential problems. Some growers employprofessional crop scouts rather than perform this function themselves. Weekly examinations are generallysufcient.
Once a problem has been identied, its extent and severity must be determined to decide whether correctiveaction is necessary. Many pest management recommendations include threshold levels when control measureswill return a prot. Consult Cooperative Extension Service and chemical company recommendations for control
measures.
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1Diagnosing FieldProblems in Corn
TIPS FOR SHIPPING TODIAGNOSTIC LABORATORIES
Trained, experienced agronomists, crop protection,
research, and sales personnel stand ready to assist you
in diagnosing eld problems. Local seed dealers are your
rst contact when questions arise. Positive diagnosis
often requires identication or conrmation of causes
by a diagnostic laboratory. Most states have some
arrangement, usually through their Cooperative Extension
Service ofce, for accessing expert diagnosis.
Some of these are more formally conducted than others,and cost of the service varies. Also, there are a few private
laboratories that are equipped to provide such services.
Your local Cooperative Extension Service ofce can
suggest companies and provide contact information.
When preparing plant or soil samples for a diagnosis,
follow these instructions offered by the laboratories.
2
EXAMINING A FIELD
Carry the appropriate tools to help effectively scout or monitor elds. Some basic tools to have on hand include:
Tape measure
Knie
Trowel (6 or 8 inch)
Magniying glassClipboard and record keeping materials
Small plastic bags
Water source
Paper towels
CalculatorField marking ags
A spade and a set of nesting pails may be useful if considerable digging is expected. Digital cameras can behelpful in getting a record of insects or disease symptoms.
Be very careful when making area-to-area or eld-to-eld comparisons. Many factors can inuence appearance,including: soil type, slope, and drainage; previous crops; fertility practices; seedbed preparation; date, depth,and rate of planting; pest control; seed lot; and hybrid.
Look for positives, not just problems. Observe and note hybrid differences, as well as the effects of fertilizer andcultural practices.
The following diagnostic key separates plant growth into three primary stages of crop development:
Stage I Emergence to knee-high
Stage II Knee-high to tasseling
Stage III Tasseling to maturity
In the eld, onset of symptoms might occur earlier or later than indicated, and may be observed during more
than one growth stage.
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Provide representative samples of each problem1.observed, as well as a healthy sample from unaffected
plants or plant parts.
If sending leaf tissue, place sections of leaves showing2.disease symptoms between pieces of dry papertoweling or notebook paper.
If sending the whole plant, remove excess soil from3.the roots and wrap the roots in moist paper toweling.Put roots and towel in a plastic bag. Do not place theentire plant in a plastic bag. Wrap stem and leaveswith paper, foil, or cardboard.
Do not add water or crush specimens unnecessarily.4.
Use a sturdy envelope or box for shipping.5.
Avoid shipping at times that are likely to result in the6.parcel lying in a post ofce or freight depot over aweekend or holiday. Overnight delivery, early in theweek, is strongly recommended.
Information will need to be provided with the plant7.specimen. Often, laboratories have specic forms thatare required when submitting a sample. Information
requested on such forms often includes the following:
Variety (hybrid number) of crop.
Location where sample was taken (county,township, and town)..
Date of planting, date problem was rstobserved, and date sample was collected.Indicate whether the problem is better or worsethan when rst observed. Visit a problem
area twice, on dates about a week apart, todetermine whether the problem is intensifying,
spreading, or disappearing.
Crop symptoms as observed in the eld, suchas plants were wilted or leaves appearedspotted. Describe the size of area affected:spots, strips, or the entire eld.
Percentage of plants affected.
Soil type (clay, sand, muck, etc.).
Topography around affected plants, such ashigh ground, low ground, or gently sloping.
Fertility level (include a soil test report, ifavailable) and the amount, kind, and timing offertilizer application.
Pesticides applied (fungicides, herbicides,insecticides, application rate, and date).
Soil moisture situation at and since planting. Ifthe eld was irrigated, indicate the amount anddates of water application.
Unusual recent air temperatures or humidityconditions.
Previous cropping and tillage history.
Types of weeds in the eld.
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2Stage I: Scouting fromEmergence to Knee-High
4
Determine the extent and severity of any problem identied. Is the problem throughout the eld or spotty andlocalized? Has emergence been completed or are there seeds sprouted, ready to emerge? Take accurate standcounts and determine percent of stand achieved.
If replanting is necessary, take steps to prevent recurrence of the cause for poor emergence. Verify that theplanter is operating properly and that fertilizer or pesticide issues have been corrected.
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
No seed1. Planter
Rodents or birds
Improper adjustment; row unit drive notengaged; worn parts; clogged spout; emptybox or tank; wrong plates, disks, or drum;excess or wrong seed treatment.
Digging and partly-eaten kernels.
Normal seed2.
appearance; not
swelled
Unfavorable soil conditions
Poor seed-soil contact
Cold, dry soil.
Inadequate press wheel pressure; improperclosing wheel adjustments; inadequate residuemanagement; dry or cloddy soil.
Normal seed3. appearance; swelled
but not sprouted
Unfavorable soil conditionsFertilizer or pesticide injury
Cold, wet soil.Phytotoxic pesticides or too much fertilizer tooclose to the seed.
Seed dead, rotted4. Seed rots or seedling blights These are accentuated when soil conditionsare unfavorable for germination and seedlinggrowth. Many species of fungi and/or bacteriamay be involved. Fungicide seed treatmentprotects the seed, not the seedling.
% stand achieved = X 100number of plants establishednumber of seeds planted
I the stand is uneven or i there are skips down the row,
dig to fnd the planted seed and its distribution.
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Seed dead, rotted4.
(cont.)
Fertilizer injury
Insecticide injury
Dead seed planted
Unfavorable soil conditions
Fertilizer salts, nitrogen, and potassium drawmoisture and may leave seed in soil too dryto support growth. In-furrow applications aremore likely to cause fertilizer injury than starterfertilizers placed at least 2 inches from the seed.Ammonia toxicity is caused when plantingfollows anhydrous or aqua ammonia applicationtoo closely or where application was tooshallow. This can kill or stunt seedlings. Rootsappear sheared off. Boron and some othermicronutrients impair germination if they are tooclose to the seed.
Some soil-applied organophosphateinsecticides can impair germination if placedin furrow with the seed. Check the label andapply only as directed.
Cold, dry, wet, or crusted soil.
Seed hollowed out5. Insects Seed corn beetle, seed corn maggot, orwireworm (see pp. 36-38).
Sprout twisted or6.
leaves expanded
below ground
Unfavorable soil conditions
Seed planted too deep
Mechanical injury to seed inhandling or planting
Chemical injury
Crusted, cold, or cloddy soil. A cloddy surfacecan allow light to reach the sprout and trigger
leang too soon. In the case of crusting, rotaryhoeing may be benecial.
Fertilizer (see p. 5); insecticides (see p. 5); orsome herbicides such as acetanilides anddinitroanilines (see pp. 41-42).
Slow, uneven7.
emergence
Planter
Unfavorable soil conditions
Seed planted too deep
Seed injury due to improper operation oradjustment, including planting depth.
Cold, dry, wet, or crusted soil. In the case ofcrusting, rotary hoeing may be benecial. Properlybanded fertilizer at planting may help seedlingsovercome unfavorable soil conditions.
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6
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Seedlings pulled or1.
dug up, seed eaten
Bird or rodent damage Chemical repellents may help.
Slow, uneven plant2.
growth
Unfavorable growing conditions
Low fertility
Insects attacking roots
Nematodes attacking roots
Chemical injury
Non-uniform planting depth
Failure of secondary roots todevelop (rootless corn syndrome)
Cold, dry, wet, or compacted soil. Properlybanded fertilizer at planting may help minimize
the effects of some unfavorable growingconditions.
If a nutrient deciency due to inadequateamounts in the soil, is conrmed, considersidedressing or foliar application, depending onthe nutrients involved (see pp. 52-53).
Corn root aphid, corn rootworm, grape colaspis,webworm, white grub, or wireworm (see pp.31-38).
Requires microscopic analysis.
Fertilizer (see p. 5); insecticides (see p. 5);
herbicides such as Balance
; Command
orScepter carryover (see pp. 41-42); or liquidmanure.
Dry, loose soil is not conducive to normal rootdevelopment. This condition is accentuatedby shallow planting and whipping by wind.Cultivation may help by throwing soil around thebase of plants.
Discolored leaves3. Nutrient deciency
Unfavorable soil conditions
Magnesium, nitrogen, phosphorus, or sulfur(see pp. 52-53). Nitrogen deciency results in
a yellow discoloration of leaves. Phosphorusdeciency results in a purpling of leaves due tothe accumulation of anthocyanins. If nutrientdeciency is conrmed, consider sidedressingor foliar application, depending on the nutrientsinvolved.
Waterlogged, cold, or compacted soil. Theseconditions can also affect nutrient uptake andtranslocation.
I plants are abnormal in appearance,
try to identiy one o the ollowing specifc symptoms.
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Discolored leaves3.(cont.)
Insects attacking roots
Nematodes attacking roots
Chemical injury
Wind damage
Frost or freeze
Cold (not freezing) temperaturestress
Anhydrous burn
Mechanical injury
Hybrid differences
Most observed leaf discoloration is due tonutrient deciency induced by damaged orinadequate roots (see nutrient deciency,above).
Requires microscopic analysis.
Fertilizer (see p. 5); insecticides (see p. 5);herbicides such as Balance; Commandor Scepter carryover (see pp. 41-42); liquidmanure.
Abrasion by sand or soil particles. Difcultyestablishing secondary roots.
Check growing point for damage. Seedlingsoften recover. Most pronounced in low-lyingareas.
Cool nights and warm days promote above-ground plant growth at the expense of rootdevelopment. This leads to increased demandby the above ground tissues for more nutrientsthan the roots can deliver. The result can beshort-term deciency symptoms until the rootsystem becomes more developed.
Uneven Corn Plant Growth
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8
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Leaves rolled or4.
puckered, may be
wilted
Drought
Insects attacking roots or stalks
Nematodes attacking roots
Mechanical root pruning
See Section 6, Insect Injury Symptoms (p.31-38), plus chinch bug, cutworm, Japanesebeetle, stink bug, or webworm.
Requires microscopic analysis.
Leaves rolled or5.
twisted together
(onion lea or buggy
whip)
Herbicide injury
Temperature variation
Nutrient imbalance
Hail damage
Acetanilides, dinitroanilines, or phenoxys (seepp. 41-42).
Alternating hot and cold weather, inducingvery rapid growth spurts followed by little or nogrowth.
Boron toxicity or calcium deciency (see p. 52).
Injury to the growing point at this growth stage canresult in short-term wrapping of leaves.
Shredded leaves or6.
eaten plants
Wind damage
Hail damage
Insects
Livestock or wild animal grazing
Armyworm, common stalk borer, corn earworm,cutworm, European corn borer, grasshopper,slug, or webworm (see pp. 32-37).
Look for tracks.
Leaves spotted,7.
striped or dead
Wind damage
Low soil pH
Nutrient deciency
Insects
Disease
Fertilizer or herbicide injury
Sunscald or cold
Hybrid differences
Abrasion by sand or soil particles.
Beaded streaking of leaves, which turn reddish-purple and may die.
Boron, copper, magnesium, potassium, sulfur, orzinc (see pp. 52-53).
Flea beetle, leaf miner, or thrips (see pp. 35-37).
Anthracnose, bacterial wilt, eyespot, Gosss wilt,holcus spot, seedling blights (favored by cool, wetsoil), virus or virus-like diseases (see pp. 24-27).
Anhydrous burn; spray drift; foliar-appliedherbicides such as Buctril, Basagran,Gramoxone, or Blazer; Classic, Scepter, orReex carryover; premix acentanilides post-applied (see pp. 40-42).
Cold nights followed by clear, bright, fast-warmingdays.
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Rows o holes across8.leaves
Insects Billbug, common stalk borer, corn borer,cutworm, or stink bug (see pp. 32-37).
Plants wilt and die9.suddenly
Insects
Wind damage
Disease
Herbicide injury
Frost or freeze
Lightning
Anhydrous burn
Flooded, water-logged soil
Billbug, chinch bug, cutworm, stink bug, whitegrub, or wireworm (see pp. 32-38).
Seedling blights, bacterial wilt, or Gosss wilt(see p. 26).
Triazines or misapplications of glyphosateherbicides (see pp. 41-42).
Check growing point for damage. Seedlings
often recover.Kills everything, usually in circular area.
Buggywhip
Corn plant two days ater rost
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10
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Plants twisted or10.
broken o
Herbicide injury
Insects
Especially 2,4-D followed by wind (see pp. 41-42).
Billbug, cutworm, lesser cornstalk borer, orstinkbug (see pp. 32, 36-37).
Inhibited root11.
development or
malormed roots
Nematode injury
Insects
Fertilizer injury
Herbicide injury
Soil conditions
Requires microscopic analysis.
Corn rootworm, grape colaspis, white grub, orwireworm (see pp. 34-38).
Phenoxys, Banvel, carryover dinitroanilines, andScepter or Classic carryover (see pp. 41-42).
Planting when soils are too wet can causesidewall compaction that can arrest or severelyrestrict corn root development. Look for at-sided or abruptly arrested root systems.
Wind Damage in Corn Field
Hail Damage in Corn Field
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11 Corn Diagnostic Guide
3Stage II: Scouting fromKnee-High to Tasseling
This is the period of most rapid plant growth. Nutrient and moisture demands are high; deciencies will likelyreduce crop yield potential. Problems must be evaluated for economic damage potential before controldecisions can be made. Observe differences due to hybrids and management practices.
I plants are abnormal in appearance,
try to identiy one o the ollowing specifc symptoms.
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Uneven height (tall1.
plants, short plants)
Emerged at different times
Varied nutrient or moistureavailability under droughtconditions
Low soil pH
Herbicide drift
Nutrients concentrated at dry soilsurface
Fallow (idle land) syndrome
Uneven planting depth; uneven soil moisture.
Use of burndown products adjacent to croppedarea.
Nutrients are unavailable to the plant.
Phosphorus is unavailable to the plant (see p.53).
Numerous tillers2. Growing point injury
Favorable early-season growingconditions
Low plant population
Adjacent to open spaces(population gaps) in the row
Hybrid differences
Disease
Mechanical or insect damage.
Optimum moisture, high soil fertility, or both.
Crazy top (see p. 27)
Discolored or dead3.
leaves
Nutrient deciency Nitrogen, phosphorus, or potassium (see p.53).
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12
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Discolored or dead3.
leaves (cont.)
Fertilizer or herbicide injury
High temperatures
Sunscald or cold banding
Insects
Mechanical injury
Frost or freeze
Hybrid differences
Barren stalk
Fertilizer or herbicide on foliage. This tendsto be more pronounced at row ends or whereoverlap application was made.
Noted as scalding or bleaching of top leavesrst.
Cold nights followed by clear, bright, fast-warming days.
Chinch bug, spider mite, or root-attackinginsects, which can reduce nutrient uptake (seepp. 32-37).
Purpling or reddening due to anthocyaninexpression in response to accumulation ofsugars that cannot translocate to the initiatingear.
Whorl leaves dead4.
(deadheart)
Disease
Insects
Chemical injury
Excessive heat
Bacterial stalk rot (see p. 28).
Billbug, corn borer, or fall armyworm (see pp.32-33).
More common following a late over-the-topapplication of certain herbicides.
Temperature exceeding 100F, depending onmoisture availability and genetics of the hybrid.
Leaves spotted or5.
striped
Low soil pH
Nutrient deciency
Chemical injury
Disease
Insects
Beaded streaking of leaves, which turn reddish-purple and may die.
Boron, iron, magnesium, manganese, nitrogen,or potassium (see p. 52-53).
Herbicides, additives reactions, or fertilizers;symptoms may be due to spray drift on foliage.
Bacterial leaf blight, eyespot, Gosss wilt, holcusspot, leaf blights, Physoderma brown spot,rust, sorghum downy mildew, virus or virus-likediseases (see pp. 24-28).
Cereal leaf beetle, corn rootworm beetle,corn blotch leaf miner, ea beetle, spidermite, or thrips; root-damaging insectsinduce nutrient deciency or droughtsymptoms (see pp. 32-37).
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Leaves spotted or5.
striped (cont.)
Wind damage
Hail damage
Genetic stripe
Sunscald or cold banding
Abrasion by sand or soil particles.
Observed only on occasional plants.
Cold nights followed by clear, bright, fast-warming days.
Leaves eaten or6.
shredded
Livestock or wild animals
Insects
Hail damage
Wind damage
Disease
Look for tracks.
Armyworm, billbug, common stalk borer, cornborer, corn earworm, cutworm, grasshopper,Japanese beetle, leaf miner, slug, or wireworm(see pp. 32-38).
Determine if the growing point survived. If it didnot, plant will not produce a tassel. Despite this,ear fertilization may still occur.
Sorghum downy mildew (see p. 28).
Plants stunted, leaves7.
close together with
mosaic mottle or
streaks; leaves yellow
or red
Virus or virus-like diseases Corn lethal necrosis, corn stunt spiroplasma,maize chlorotic dwarf, or maize dwarf mosaic(see p. 27).
Gray or black galls8.
growing on plants
Disease Common corn smut (see p. 26).
Stalks spindly,9.
unthrity, yellow
Nutrient deciency
Excess soil moisture
Excessive plant population
Root damage
Stalk damage
See pp. 52-53.
Insects, nematodes, chemical, or mechanical.
Insects, disease, or mechanical.
Plants wilted or rolled10. Drought
Root damage
Stalk damage
Insects, nematodes, chemical, or mechanical.
Mechanical or insects; chinch bug, common stalkborer, or corn borer (see pp. 32-33).
Top leaves tightly11.
rolled (buggy whip
or onion leafng)
Chemical injury
Cold soil during seedling stage
2,4-D and other phenoxy herbicides; Banvel (seep. 41).
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14
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Top leaves tightly11.
rolled (buggy whip
or onion leafng)
(cont.)
Temperature response or rapidgrowth syndrome
Nutrition
Mechanical injury
Hybrid differences
Alternating hot and cold periods, speeding andslowing plant growth.
Boron toxicity or calcium deciency (see p. 52).
Plants growing up in a12.
curved gooseneck
Recovery after root damage
Recovery after early season environmental conditions
Insects (usually, but not limited to corn rootwormlarvae), nematodes, herbicides (especiallyphenoxys), soil compaction, mechanical injury,or wind.
Curled or stubby13.
brace roots
Wind and wet soils
Dry, hot soil surface
Wind action
Plants twisted or14.
growing o at 90angle
Mechanical injury
Plants leaning or15.
broken o
Wind damage
Livestock or wild animals
Root damage
Insects attacking stalk
Chemical injury, especially iffollowed by wind
Soil compaction
Poor secondary root
development in dry soilDisease
Hail damage
Especially if soil is wet. Wind can cause
greensnap at lower nodes of rapidly growingstalk.
Look for tracks.
Insects, nematodes, chemical, or mechanical.
Common stalk borer or corn borer (see p. 32-33).
Especially Banvel or phenoxy herbicides (seepp. 39-41).
Inadequate root development.
Rootless corn syndrome due to weather and
aggravated by shallow planting.Bacterial or Pythium stalk rots (see pp. 28-29).
Plants wilt and die16.
suddenly
Lightning
Drought
Kills everything, usually in a circular area.
Field areas with low water-holding capacity are rstto show stress.
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Plants wilt and die16. suddenly(cont.) Disease Frost or freeze
Herbicide injury
Bacterial or Pythium stalk rots (see pp. 28-29).
Misapplication of glyphosate, glufosinate,paraquat, or other non-selective herbicides (seepp. 41).
Tassel eeding (while17.
in whorl)
Insects Armyworm, corn leaf aphid, or western beancutworm (see pp. 32-35).
Tassels prolierated18.
into green cluster o
fngerlike branches,
excessive tillering,leaves oten thick and
straplike
Disease Crazy top (see p. 27).
Tassel prolierated,19.
with stringy black
vascular bundles
present
Disease Head smut (see p. 26).
Greensnap at lower nodes o rapidly growing corn stalk
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16
4Stage III: Scouting fromTasseling to Maturity
This period, which commences with the critical pollination stage, includes grain ll, plant maturation, and death.It also includes the critical stage before harvest when standability and ear retention become concerns. Fieldobservations are essential to detect yield-depressing factors during this period.
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Eaten1. Livestock or wild animals
Insects
Look for tracks.
Armyworm, corn borer, corn rootworm beetles,grasshopper, or Japanese beetle (see pp. 32-36).
Shredded2. Hail damage
Wind damage
Disease Sorghum downy mildew (see p. 28).
Dead rosted3.
appearance
Frost or freeze
Drought
Disease
Insects
Anthracnose, leaf blights, or stalk and root rots(see pp. 24-26; 28-29).
Corn borer, corn leaf aphid, or spider mite (seepp. 31-33; 37).
Spotted or dead4. Disease
Chemical injury
Numerous leaf blights such as anthracnose,eyespot, Gosss wilt, gray leaf spot,Helminthosporium leaf spot, northern corn leaf
blight, Phaeosphaeria leaf spot, Physodermabrown spot, rust, or southern leaf blight (see p.26).
Especially spray drift.
Discolored sheath5. Insects
Disease
Corn leaf aphid (see p. 31).
Purple sheath spot caused by saprophyticorganisms existing on pollen trapped betweenthe leaf sheath and stalk.
I LEAVES are aected,
try to identiy one o the ollowing specifc symptoms
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Red or purple color6. Mechanical injury
Barren plant
Disease
Insects
Hybrid differences
Anthocyanins develop in response toaccumulation of sugars that cannot betranslocated to lling grain.
Corn rust, corn stunt spiroplasma, high plainsvirus, or maize chlorotic dwarf (see pp. 24-27).
Corn borer (see p. 33).
White striping with7.
white, downy growth
on upper and lower
lea suraces
Disease Sorghum downy mildew (see p. 28).
I SILKS are aected,
try to identiy one o the ollowing specifc symptoms
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Eaten o1. Insects Armyworm, corn earworm, corn rootwormbeetle, grasshopper, Japanese beetle, orwestern bean cutworm (see pp. 32-35).
None visible, or2.
delayed several days
ater tasseling
Heat
Drought
Nutrient deciency or imbalance
Temperature variation
Insects
Excessive population for
conditions
Especially nitrogen or phosphorus.
Cold nights occurring just prior to silking.
Corn leaf aphid, fall armyworm, or spider mite(see pp. 31-32; 37).
Trapped, balled within3.
the husk
Drought
Nutrient deciency or imbalance
Temperature variation
Hybrid differences
See pp. 52-53.
Cold nights during early silking.
Red or green color4. Hybrid differences Fresh silk color is genetically controlled.
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18
I TASSELS are aected,
try to identiy one o the ollowing specifc symptoms
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Failed to emerge1. Drought or heat stress
Nutrient deciency
Insects
Especially boron (see p. 52-53).
Aphid stress, corn earworm, or fall armyworm(see pp. 31-32).
A mass o leaves2. Disease Crazy top, head smut, or sorghum downymildew (see pp. 27-28).
One or more small3.
ears ormed
Genotype by environmentinteraction
Most common on tillers.
Kernels develop in4.
tassel
Genotype by environmentinteraction
Most common on tillers.
Broken o, stalk5.
tunneled
Insects Corn borer (see p. 33).
I STALKS are aected,
try to identiy one o the ollowing specifc symptoms
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Red or purple color1. Mechanical injury to plant
Barren stalk
Hybrid differences
Insects
Anthocyanins develop in response toaccumulation of sugars that cannot betranslocated to lling grain.
When corn borers tunnels into a stalk, the areaabove the point of entry may turn red or purple.
Barren (no ear2.
enlargement)
Drought
Heat
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Barren (no ear2.
enlargement) (cont.)
Nutrient deciency or imbalance
Insects
Silks eaten off prior to pollination
Disease
Excess population for conditions
Tillers
Timing of silk or pollen sheddisrupted
Mechanical injury
See pp. 52-53.
Corn leaf aphid or spider mite (see pp. 31, 37).
Corn rootworm beetle and Japanese beetle(see pp. 34-36).
Head smut, mildews, virus or virus-like disease(see pp. 26-27).
Seldom produce a fully-developed ear.
Environmental stress.
Broken below ear3. Wind
Disease
Nutrient imbalance
Insects
Weather stress
Mechanical injuryExcessive population forconditions
Delayed harvest
Plants just prior to tasseling are especiallyvulnerable to greensnap by wind.
Stalk and root rots (see pp. 28-29).
Excess nitrogen, insufcient potassium (see p.53).
Southwestern corn borer (see p. 33).
Drought, heat, or other conditions limitingphotosynthesis.
Machinery; livestock or wild animals.
Broken above ear4. Wind
Insects
Mechanical injury
Delayed harvest
European corn borer or fall armyworm (see pp.32-33).
Machinery; livestock or wild animals.
Multiple ears at one5.node
Hybrid differences
Mechanical injury
Cold temperature when earsformed
Disease Mildews, virus or virus-like disease (see pp. 27-28).
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Multiple ears at one5.
node (cont.)
Insects Results from severe silk clipping by insects suchas corn rootworm beetle and Japanese beetle(see pp. 34, 36).
Leaning, but not6.
broken (root lodged)
Wind and wet soil
Poor root development
Insects
Nematode activity
Disease
Nutrient deciency
Herbicide injury
Mechanical injury
Delayed harvest
Hybrid differences
Drought, fertilizer placement, soil compaction,low soil pH, or poorly drained soil.
Corn rootworm and other root feeders (see pp.34-35).
Stalk and root rots (see pp. 28-29).
Especially potassium (see p. 53).
Especially 2,4-D and other phenoxys (see p. 41).
Machinery, livestock or wild animals
Premature death7. Disease
Insects
Frost or freeze
Severe drought
Lightning
Leaf blights or stalk and root rots (see pp. 24-26, 28-29).
Corn borer (see p. 34).
Kills everything, usually in a circular area.
Black mold8. Saprophytic Buildup on dead stalk tissue under warm, humidweather conditions. Timely harvest will deter.
I EARS are aected,
try to identiy one o the ollowing specifc symptoms
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Numerous ear shoots,1.
leay and barren
Disease Crazy top, sorghum downy mildew, virus orvirus-like diseases (see pp. 27-28).
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Sot, glistening smut2.
galls, black and
powdery when mature
Disease Common corn smut (see p. 26).
Poorly flled tips3. Hybrid differences
Nutrient deciency
Interaction between population,hybrid, and environment
Insects
Disease
Drought
Unusually favorable growingconditions after pollination
Suboptimum light, carbondioxide, or temperature duringkernel ll
Especially nitrogen or potassium (see p. 53).
Silks clipped before pollination. Corn borer, cornearworm, corn rootworm beetle, fall armyworm,or Japanese beetle (see pp. 33- 36).
Foliar disease that reduce photosynthesizingarea (see pp. 24-26).
Silks at ear tips were not present when pollenwas available
Cob elongates, but rarely lls up tip kernel.
Reduced photosynthesis; tip kernels abort.
Small malormed, light4.
weight
Nutrient deciency
Drought or heat stress
Plant damage
Excessive population forconditions
Second or third ear on stalk, oron a tiller
Insect-caused plant stress
Disease
Soil compaction
Especially nitrogen and phosphorus (see p. 48).
Mechanical or disease.
Numerous ear-feeding insects (see pp. 32-38).
Leaf blights, stalk and ear rots, virus or virus-likediseases (see pp. 24-30).
Very short husk,5.
remains tight at
maturity; ear tip
exposed
Weather
Hybrid differences
Usually observed after drought or heat hasstopped husk growth, but later favorableconditions permit more normal ear size todevelop.
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GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Barren (little to no6.
grain)
Disease
Pollination failure
Pesticide injury
Head smut, virus or virus-like diseases (see pp.26-27).
Drought, heat, or other stress interfered withsilk/pollen shed timing; insect (clipped silksor caused tassel to abort); chemical injury(especially by growth regulators like 2,4-D andBanvel).
Following drift or misapplications.
Pinched ear syndrome7. Chilling injury during early eardevelopment
Also called beer can ear. Ear is reduced to3-4 inches. Well lled kernels are on bottom
third of ear, cob tissue on middle third, andundeveloped tissue on top third.
Dropped8. Drought
Nutrient deciency
Hybrid differences
Insect damage to shank
Weakened shank.
Weakened shank.
Ears usually drop free of husk.
Husk usually remains on the dropped ear;European corn borer (see p. 33).
Scattered kernel set9. Lack of adequate viable pollenwhen silks are receptive
Silks eaten off before pollinationHeat or drought at pollination
Nutrient deciency
Herbicide injury
Insect or animals.
Phenoxy herbicides or too-late applications ofglyphosate (see p. 41).
10. Kernel eeding Insects
Birds, animals
Corn borer, corn earworm, fall armyworm, picnicbeetle, or western bean cutworm (see pp. 33-35).
11. Rotten (spots or entireear)
Disease
Insects
Birds
Hail or other mechanical injury
Ear and kernel rots or Helminthosporium leafspot (see pp. 29-30; 25).
Corn borer, corn earworm, fall armyworm, picnicbeetle, or western bean cutworm (see pp. 33-35).Followed by weathering or disease.
Followed by weathering or disease.
Followed by weathering or disease.
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23 Corn Diagnostic Guide
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
12. Kernels eaten rom
ear tips
Birds
Wild animals or rodents
Insects Corn earworm, fall armyworm, picnic beetle, orwestern bean cutworm (see pp. 33-35).
13. Tunnels in ear, cob,
shank, or stalk
Insects Corn borer, corn earworm, or fall armyworm (seepp. 32-33).
I KERNELS are aected,
try to identiy one o the ollowing specifc symptoms
GENERAL SYMPTOM POSSIBLE CAUSE REMARKS
Broken seedcoat,1.
popped appearance
Genetic and environmentinteraction
Popped kernels usually occur under hightemperatures.
Horizontally cut or2.
split seedcoat
Silk-cut A genetic and environment interaction. Notvisible until kernels are shelled from the cob.
Pink or red streaking3.
or lengthwise stripes,especially running
over crown
Kernel red streak Caused by toxin secreted during feeding of
wheat curl mite. More pronounced toward eartip. No detrimental effects are known.
Sprouted especially at4.
base o ear
High rainfall and warmtemperature while ear remainederect on stalk
Usually accompanied by mold.
White streaking, also5.
known as starburst
Disease Associated with Fusarium ear rot.
Barren corn Pinched ear syndrome
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5This section will help you identify the common corn diseases based on plant symptoms and conditions. It is notuncommon to have symptoms of several different diseases present at the same time. Symptoms of differentdiseases may appear similar, particularly during early stages of disease development. Laboratory culturing andmicroscopic examination may be required to make a positive identication.
Growth stages during which symptoms generally appear are listed following the disease name and are describedas follows:
Disease Symptoms
LEAF DISEASES
Anthracnose (Stage I, III)Oval to spindle-shaped
water-soaked lesions on
youngest leaves turn tan to
brown with yellow to reddish
brown borders. Heavilyinfected leaves wither and
die. Top-leaf die-back may
occur 4 weeks to 6 weeks
after pollination, leaving
the lower stalk green. The
organism thrives in warm, humid weather. The stalk-rot
phase of the organism is of greater concern than the leaf
blight phase.
Bacterial Wilt and Bacterial Lea Blight
(Stewarts Wilt, Stewarts Disease) (Stage I, III)
Young plants exhibit long,green-gray, water-soaked
lesions with wavy margins,
accompanied by stunting
and wilting which lead to
plant death. Cavities may
form in stalk near the soil line.
Bacterial masses ooze from
cut end of infected stalks or
leaves.
The more common leaf blight phase appears after
tasseling. Leaves are streaked with gray-green to yellow-
green lesions, each distinguished by the presence of a ea
beetle feeding scar toward the base of the streak. Streaks
are long and irregular, turning tan as the tissue dies.
Flea beetles (small, oval, black insects) are the primary
carrier. Incidence of the disease is relative to beetle
population. Sweet corn tends to be more sensitive than
eld corn to this disease.
Common Corn Rust (Stage II, III)
Cinnamon-brown, powdery,
circular-to-elongated pustules
(blister-like growths) can
occur on any aboveground
plant tissue, but especially
on both surfaces of the
leaves. In contrast, pustules
of southern corn rust occur
primarily on the upper leafsurface. Pustules rupture
leaf surface and rusty powder can be rubbed off with
ngers. Pustules become dark brown to black late in the
growing season. The organism thrives in moderate to cool
temperatures and high humidity.
Stage I Emergence to knee-high
Stage II Knee-high to tasseling
Stage III Tasseling to maturity
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Eyespot (Stage II, III)
Small (less than inch),circular, translucent lesions
surrounded by a yellow to
purple margin, give a halo
effect. Lesions occur on
leaves (most commonly as
plants approach maturity),
sheath and husk. The
disease is favored by cool,
moist weather.
Gosss Wilt
(Lea Freckles and Wilt, Nebraska Bacterial
Wilt and Lea Freckles) (Stage I, III)
Young plants wilt and
die. Vascular bundles are
discolored. More common
later-season infections
produce dull gray-green
to orange lesions forming
water-soaked streaks with
irregular margins on leaves.
Within developing lesions,
small, irregular shaped water-
soaked freckles appear. Bacterial droplets may oozefrom the leaf surface early in the morning. Plant injury,
such as from hail or wind damage, enhances infection.
Gray Lea Spot (Stage III)
Gray to tan, rectangular
lesions on leaf, sheath or
husk tissue. Spots are
opaque and long (up to 2
inches). Lower leaves are
affected rst, usually not until
after silking. The organism
thrives in extended periodsof warm, overcast days and
high humidity. It has become
more prevalent with increased use of reduced tillage and
continuous corn.
Helminthosporium Lea Spot
(Northern Lea Spot) (Stage III)
Numerous races of the
organism have been
identied. Symptoms vary
by race. Lesions tend to
be oblong to blocky, and
tan to brown in color. This
leaf spot may also cause a
black, charred-appearing
ear rot. It prefers moderate
temperatures and high
humidity.
Holcus Spot (Stage I)
This organism causes small,
circular to oblong, water-
soaked lesions toward tips
of lower leaves. Later, lesions
become creamy white to
tan, then light brown with
reddish margins. Holcus spot
is very similar in appearance
to paraquat damage.
Rainstorms accompanied by
wind splash overwintering bacteria from the residue ontoyoung plant leaves. This organism does not cause serious
loss.
Northern Corn Lea Blight (Stage II, III)
Long (up to 6 inches),
elliptical, gray-green lesions
that become tan-brown
identify infections caused
by this organism. Infection
spreads up the plant
starting on lower leaves. It
is favored by high humidityand moderate temperatures.
Numerous physiologic races
have been described. Host specic races of the organism
may also attack sorghum.
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26
Phaeosphaeria Lea Spot (Stage III)
Lesions are initially small,round to oval, pale green
or yellow, and scattered
on leaves of mature plants.
Spots become light tan with
reddish-brown margins and
may coalesce into irregularly
shaped lesions. This disease
is most prevalent in areas of
high rainfall and moderate
temperatures.
Physoderma Brown Spot (Stage I, III)
Small yellow spots appear
rst at the base of the
leaf. These spots become
brown and combine to form
chocolate-brown to reddish
irregular blotches, sometimes
as bands of infection across
leaf blades. Sheath, husk,
tassel, stalk, and leaves
may exhibit symptoms late
in the season. Infected stalks may break at a node. This
organism is favored by warm, wet weather.
Southern Corn Lea Blight (Stage II, III)The most common race,
named O, produces small,
elongated (up to 1 inch long),
parallel-sided lesions that are
tan with brownish borders. This
blight primarily attacks leaves. It
is favored by high humidity and
warm temperatures.
Southern Corn Rust (Stage II, III)
Small, circular, orange-to-
light-red pustules (blister-like
swelling) occur on leaves,
especially the upper surface,
and sheaths. Pustules rarely
break the leaf surface. (In
contrast, common corn
rust pustules attack upper
and lower leaf surfaces and readily break through the
epidermis.) This organism is favored by warm, humid
weather.
SMUTTING DISEASESCommon Corn Smut
(Boil Smut, Blister Smut) (Stage I, II, III)
Local infection of any plant
part, even below the soil
surface, occurs through plant
wounds or thin-walled cells
of actively growing tissue.
Resulting galls are rst silver-
white, then become gray
to black powdery masses
of smut spores that are
released when the galls
break open. Galls on leaves
seldom develop beyond
pea-size and tend to harden
and dry without rupturing. High fertility and plant injury
favor the disease. Common smut is not toxic to animals.
Head Smut (Stage III)
Seedling infection results
in systemic development
of the disease as the plant
grows and develops. Tassels
of affected plants may
multiply as a mass of leaves
or be replaced by a black,
smutty mass which ruptures
to release spores, leaving
black, thread-like vascularstrands. Ears may be completely replaced by similar, often
triangular-shaped, smut masses. Occasionally, part of an
ear escapes and produces a few kernels. Hot, dry soil at
seedling stage favors infection. One type of head smut
also attacks sorghum.
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VIRUS AND VIRUS-LIKE DISEASES
Corn Lethal Necrosis (CLN) (Stage II, III)Mosaic patterns appear onleaves and husk. Leaves die
from the margins inward and
plants may die prematurely.
The organism generally starts
at the tassel and works
downward. Barrenness
or sharply reduced grain
production is common. CLN
is caused by synergistic
interaction when plants become infected by maize
chlorotic mottle virus and either maize dwarf mosaic virusor wheat streak mosaic virus. Presently, the disease has
been identied only in parts of Kansas and Nebraska.
Corn Stunt Spiroplasma (CSS) (Stage II, III)Corn stunt is caused by a
spiroplasma, not a virus.
Margins of whorl leaf turn
yellow followed by reddening
of older leaves and yellow
striping which runs the
length of leaves. Plants arestunted, have multiple tillers
and produce numerous small
ear shoots. Root systems
are reduced. Corn stunt
symptom development and epidemiological patterns are
similar to those of virus diseases.
High Plains Virus (HPV) (Stage I, II, III)
Symptoms begin as small
yellowish ecks which often
appear as lines running
parallel to leaf veins. Infectedseedlings turn bright yellow
and quickly die. Plants may
be stunted, older leaves
may become red and then
necrotic, and ear and kernel
size may be reduced.
Considerable symptom
variation exists among cultivars. It is spread by the wheat
curl mite.
Maize Chlorotic Dwar Virus
(MCDV) (Stage II, III)
Younger leaves are yellow
and nely striped. There
is a general yellowing
or reddening of leaves
and plants are stunted.
Most affected plants are
barren. MCDV is spread by
leafhoppers from infected
Johnsongrass and other host
species. MCDV is detected
more frequently and is more damaging than maize dwarf
mosaic.
Maize Dwar Mosaic Virus
(MDMV) (Stage II, III)
Mottling of younger leaves
progresses into narrow, light-
green to yellowish streaks
along leaf veins. Leaves,
sheaths and husks may show
symptoms. Plants are stunted
and ear size and seed set are
reduced. Sometimes, multiple
tillers or ear shoots develop.MDMV is most prevalent in
areas where Johnsongrass grows and serves as a winter
host for the virus. It is spread by aphids.
FUNGAL SYSTEMIC DISEASES
Crazy Top (Stage II, III)
Plants have excessive tillering with rolling or twisting of
newer leaves. The tassel
becomes a mass of leaves;
ears, if present, often do
likewise. Leaves are usuallynarrow, thick and appear
strap-like. Occasionally over-
sized plants will develop.
Infection occurs when young
plants are under ooded soil
conditions; therefore, it is
more frequent in low-lying areas. This is one of numerous
downy mildews that attack corn.
UGA1235014
536657
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28
Sorghum Downy Mildew (Stage II, III)
Leaves are yellow with white
stripes; plants are stunted
and ears and tassels multiply
into leafy masses. Often the
base of leaves are chlorotic
and sharply contrast with
healthy green leaf tips. White,
downy growth appears on
either or both leaf surfaces.
Leaves often split or shred.
This is primarily a disease of sorghum.
STALK AND ROOT ROT DISEASESStalk rots are diseases that are most commonly expressed
as plants reach maturity. Stalk rot of corn tends to be a
complex of several disease-causing fungi and sometimes
bacteria; seldom will only one causal organism be isolated
and identied. Plants with rotted stalks almost always have
rotted roots, too. Usually, but not always, the same causal
organisms are involved. Visual identication is very difcult.Typically, wilting is the rst sign of stalk rot in a eld. In a
few days, leaves turn a frosted gray, ears droop and the
outer rind of the lower stalk turns brown. Fields where
stalk rot is developing should be harvested early to reducegrain losses.
Anthracnose Stalk Rot (Stage II, III)
Early infection may kill plants
before pollination, but onset
usually occurs just before
plants mature. Sometimes
a portion of the plant above
the ear blanches and dies
prematurely (top dieback).
Usually, the entire plant is
killed and several nodes arerotted. Late in the season, a
shiny black discoloration develops in blotches or streaks
on the stalk surface, especially on lower internodes.
Internal stalk tissue may become black and soft, starting
at the nodes. Lodging typically occurs higher on the stalk
than with other stalk rots. The same organism causes leaf
blighting earlier in the season.
Bacterial Stalk Rots (Stage II, III)
At least two organisms have
been identied as causing
bacterial stalk rot. With either,
there is rapid development
of a soft rot of stalks,
accompanied by wilting and
plant death. Stalks twist and
collapse while still green,
disintegrating into a soft
mass often accompanied by
a foul odor.
Charcoal Rot (Stage III)
Charcoal rot can be
expected when the crop
has grown under hot, dry
conditions; it also attacks
sorghum and soybeans.
Lower internodes are
affected, causing premature
ripening, shredding, and
crown disintegration.
Vascular strands remain
intact but are black or charred in appearance. Stalks
become gray-black or charcoal color.
Diplodia (Stenocarpella) Stalk Rot (Stage III)
Lower internodes are
straw-brown, spongy, and
dry. Pith disintegrates,
leaving vascular strands
intact. White fungal growth
may appear on the stalk
surface. Minute, dark bodies
embedded just under the
stalk surface are difcult to
remove.
5361254
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Fusarium Stalk Rot (Stage III)
Rotting of roots, crown, and
lower internodes leads to
premature ripening and stalk
breakage. Split stalks may
show whitish-pink to salmon
color. Disintegration starts at
the nodes.
Gibberella Stalk Rot (Stage III)
Affected plants wilt, leaves
turn dull gray-green, and
the lower stalk softens and
becomes straw colored
as plants die. Pith tissue
disintegrates, leaving only
vascular strands. The inside
of a rotted stalk is pink to
red. Small, dark bodies are
supercial on the lower stalk
surface and can be scraped off easily.
Pythium Stalk Rot (Stage II, III)
This organism attacks individual
plants in localized areas,
sometimes before owering. It
is usually conned to the rst
internode above the soil line,
where rind and pith develop wet
rot. Stalks twist and collapse.
Hot, wet weather favors
development of this stalk rot.
Red Root Rot (Stage III)
Red root rot often occursin plants that are also
stalk rotted. Seminal roots
are most damaged and
adventitious root damage
increases with their age. The
root color of affected plants
ranges from pink to royal red,
with higher soil temperatures
resulting in darker pigmentation. The role of this root rot in
stalk lodging is not fully understood.
EAR AND KERNEL ROTSThese rots can affect ears, kernels, or cobs, reducing test
weight and grain quality. Some rots are responsible for
development of mycotoxins that may contaminate grain.
Positive identication is difcult. Rotting observed in the
eld is often due to a complex of causal organisms, not
just one.
Most ear rots are favored by late-season humidity.
Infections are increased by ear damage by birds or insects
and by stalk lodging that allows ears to come into contact
with the soil.
Aspergillus Ear and Kernel Rot (Stage III)
Greenish or yellowish-tan
discoloration occurs on and
between kernels, especially
near the ear tip. Symptoms
are more prevalent if the husk
does not cover the ear tip.
The rot is favored by hot,
dry weather. It may produce
aatoxins.
Cladosporium Ear Rot (Stage III)
Dark gray to greenish black
fungal growth causes kernels
to appear blotched or streaked
Initial discoloration appears
where kernels are attached to
the cob. Infection eventually
progresses upwards and
infected kernels can be seen
scattered over the ear. Ifcompletely colonized, ears
are dark and lightweight.
This disease is often
associated with damage due
to insects, hail, or frost.
Healthy stalk, left. Diseased
stalk, right.
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30
Diplodia (Stenocarpella) Ear Rot (Stage III)
Symptoms include bleached
husks, white mold over
kernels, and rotted ears with
tightly adhering husks. Early
infection (2 to 4 weeks after
silking) is likely to lead to
complete ear rotting. Later
infections may result in partial
rotting, usually beginning at
the base. Since corn is the
only known host, this disease is most severe when corn is
planted following corn in reduced tillage situations.
Fusarium Kernel or Ear Rot (Stage III)
Scattered individual or groups
of kernels show whitish-pink
to lavender fungal growth.
Infected kernels may also
have a starburst pattern of
white streaks on the cap ofthe kernel or along the base.
Infections are more frequent
on damaged ear tips, and
are favored by dry weather.
Fusarium rot may produce mycotoxins.
Gibberella Ear Rot (Stage III)
Symptoms include reddish
kernel discoloration, usually
beginning at the ear tip.
Husks may rot and becemented to the ear. The
organism is favored by cool,
humid weather, particularly
2 to 3 weeks after silking. It
produces several mycotoxins.
Penicillium Ear Rot (Stage III)
Powdery green to blue-green mold develops on and
between kernels. Infection
usually begins at the ear tips
and primarily occurs on ears
with mechanical or insect
damage. Infected kernels
may become bleached and
streaked. Blue eye occurs
when the embryo becomes
discolored due to the presence of blue-green fungal
spores and can occur if infected grain is stored at high
moisture levels.
Trichoderma Ear Rot (Stage III)
Dark green fungal growth
is found on and between
kernels and husks, often
covering the entire ear. The
disease usually occurs on
ears with mechanical or insect
damage. Infected plants tend
to be widely distributed within
a eld.
Photo Citations:
Clemson University - USDA Cooperative Extension Slide
Series, Bugwood.org (viewed 9/22/10); Corn Stunt
1235014
William M. Brown Jr., Bugwood.org (viewed 9//22/10)
High Plains Virus 5366657
Howard F. Schwartz, Colorado State University, Bugwood.
org (viewed 9/22/10) Bacterial Stalk Rot 5361254
Holcus spot and Pythium stalk rot photos courtesy of DonWhite at University of Illinois
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6Insect Identifcation, InjurySymptoms, & ManagemenRecommendations
APHIDS
(1) Corn Lea Aphid
This small, soft-bodied,
bluish-green to gray insect
colonizes on or in the
whorl, upper leaves, and
tassel, especially under
dry conditions. It sucksplant juices and can cause
wilting, pollen shed failure,
and barrenness. The aphid
secretes a sticky honeydew
that may gather dirt and act as a medium for mold.
Predatory insects, like the lady beetle, may be present
and are important predators of aphids. Scouting should
occur during late whorl to early tassel. Aphid infestations
after 50% pollen shed have little effect on grain yield. A
rescue application of insecticide may be warranted if 50%
or more plants have light to moderate infestations and the
crop is under moisture stress.
(2) Corn Root Aphid
This pinhead-sized, soft-bodied, blue-green to gray-green
insect, may be winged or wingless. Corn root aphid groupsare found in clusters on the crown and along the roots of
young plants. They suck plant juices and cause wilting,
discoloration, and stunting. Corneld ants are generally found
in conjunction with corn root aphid colonies. There are no
rescue treatments for corn root aphid. Cultural practices like
crop rotation and tillage can be effective preventative tactics.
Only the most common or the most economically damaging corn insects in the North American region arediscussed in this chapter. Most, but not all, insect identications can be made in the eld by trained individuals.
As insects progress through their life cycles, they change form and features. The following comments
describe the appearance of damaging stages, which may not correspond with their appearance when damageis being investigated. It is not uncommon to investigate crop insect damage, but nd none of the suspectedinsects. At other times, damage from insect feeding may be detected, but the insect is no longer present.
Use economic thresholds whenever possible and seek further pest management guidance if symptoms areunclear. Generalized thresholds for most pests are provided in this guide. These thresholds may changedepending on the cost of a control tactic or the commodity value. Always conrm that pest numbers havebeen suppressed below thresholds following the application of a rescue tactic. Pesticides may perform belowexpectation due to environmental, chemical, or biological situations, or due to application timing.
While this publication is focused on in-season diagnostics and treatment options of insect pests of corn, anumber of Bt traits are available for several of the insect pests discussed in this handbook. The Corn InsectControl with Monsanto Corn Insect Traits and Seed Treatments table (see pp. 39) provides a list of some of
these products as well as the insects they control. These products provide season-long control of the specicinsects and reduce the need for additional pest management action.
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32
ARMYWORMS
Fall ArmywormThe fall armyworm grows
up to 1.5 inches long and
appears greenish-brown in
color with an inverted white
Y on its head. It chews
ragged holes in the leaves
and usually concentrates in
the whorl of young plants.
The fall armyworm also feeds on the tassel and bores into
developing ears. Early detection is critical since this pest
feeds in protected areas of the plant. It lays round, gray
eggs in clusters of 50 or more, which are usually covered
with scales. Eggs hatch in 3 to 5 days and larvae migrate
to the whorl and adjacent plants. Consider using a rescue
application if eggs are present on at least 5% of plants or
whorl injury is detected on 25% or more plants. Because
larvae feed deep in the whorl of young plants, control with
insecticides can be difcult.
(2) True Armyworm
The true armyworm is
greenish-gray with orange
or pinkish lines along thebody. Armies migrate from
maturing small grains or
grass at night. They devour
foliage, sometimes leaving
only leaf midveins, and chew
silks. Infestations are more frequent in no-till or reduced
tillage systems and the true armyworm is usually only an
occasional pest in rotated corn. Consider an insecticide
application if 35% or more plants in the eld are infested.
BILLBUG
This black or gray hard-shelled snout beetle is active
only at night. Leaves are
punctured while still rolled in
the whorl. When they unfurl,
irregular rows of holes appear
across the leaves. The billbug
also chews into plant stems
at or below ground level.
This feeding can damage
the growing point and cause plants to be bent, twisted
or stunted and sometimes die. Billbug incidence is often
associated with nutsedge. Consider spraying for billbug ifstand loss during seedling stages exceeds 5%.
CEREAL LEAF BEETLEThe adult is a 0.2 inch long, hard-shelled beetle with
metallic blue-black head and wing covers, and reddish-
orange legs and thorax. Feeding on corn is rare and
usually limited to eld borders. Though the beetle eats
completely through the leaf between the veins, plants
normally outgrow the injury and rescue applications
are not necessary. Generally infestations are limited to
Michigan, Indiana, and eastward.
CHINCH BUGThe adult is 0.25 inch long,
and is red and black with
white wing covers. When
crushed, this bug emits a
distinct odor. It migrates
from grasses and maturing
small grains, attacks eld
borders rst and is most
destructive during dry years.
The young nymph (juvenilestage) is bright red, but turns black as it develops. Chinch
bugs cluster in groups behind the sheath of lower leaves,
sucking plant juices and causing wilting and eventual
plant death. Management may be necessary if 10 or
more chinch bugs are found on more than 50% of plants.
Insecticide applications may be limited to only the infested
area.
COMMON STALK BORERThis purplish-brown larva may
reach 2 inches in length. It
has white longitudinal stripes
with a distinct purple saddle
near the middle of the body. It
tends to move out of weedy
or grassy eld borders and
attack corn on eld edges (or
throughout no-till elds). The
larva tunnels into stalks or
whorls. Infestation of plants at the 4-7 leaf stage can leadUGA1234011
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to dead heart. Whorl feeding results in leaves that have
a ragged appearance with injury appearing as a series of
holes across the leaf. Timing insecticide applications priorto borer tunneling is critical. Consider treating if 10% of
V2 plants, 15% of V4 plants, or 30% of V6 plants show
symptoms.
CORN BORER
(1) European Corn Borer
This larvae varies in shades
of tannish-gray and ranges
in size from less than 0.5 to
1.0 inch long. There are two
or more generations per
season. Larvae of the rst
generation feed on leaves
before they burrow into the
stalk. This feeding causes broken midribs and creates a
shot hole appearance as leaves grow out of the whorl.
Consider treating rst generation populations if the moth
ight has already peaked and 5% or more of plants have
shot hole feeding. Second generation larvae feed on and
bore into stalks, tassels, ear shanks, and ear tips causing
top breakage, ear drop (with husk attached), stalk lodging,
and kernel damage. Treatment for second generation
larvae may be necessary if more than 10% of plants have
fresh or hatched egg masses, or young larvae in the
leaf axils. Timing insecticide applications prior to insect
tunneling is critical.
(2) Southwestern Corn Borer
This larvae is white or cream-colored and 0.5 to 1.0
inch long. It has distinct black spots except on the
overwintering form. Two generations feed on corn. The
rst feeds on leaves and creates a shot hole effect
or dead heart in very young plants. Later instars
tunnel in the stalk. The second generation larvae feed
on developing ear tissue, bore into the shank and ear,
and tunnel in the stalk below the ear zone. In the fall,
overwintering larvae migrate to the base of the stalk
and prepare overwintering
tunnels in the stalk below
soil level. These larvae girdlethe plants near the soil level,
resulting in severe stalk
lodging. Treatment may be
necessary for rst generation
populations when greater than
35% of plants hold larvae or
show symptoms. Insecticide
applications for both rst and second generations must
be applied before they begin tunneling. Consider a rescue
treatment for second generation if eggs or young larvae are
found on 20 to 25% of plants.
CORN EARWORMThis larva varies in color from green, yellow, brown to
pink, but all have a yellow-brown head and reach full size
at about 1.5 inches. Since
the insect is cannibalistic,
two or more full-size larvae
are seldom found together.
They may feed on leaves in
the whorl, on
the tassel or
on silks, but
the preferred feeding
site is ear tips and on
developing kernels.
Moreover, ear injury
often leads to ear
molds and rots. The
window for effective
treatment is the few
days after eggs are laid and before larvae tunnel into the
silk channel.
CORN ROOTWORMThis small, white larva feeds on and tunnels into roots
or crowns of young plants. It destroys root systems,
can delay development, and causes root lodging.
Lodging is accentuated by wind storms. Plants may
gooseneck across rows and resume upright growth.
Rescue treatments for control of corn rootworm larvae
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are available, but efcacy
is greatly dependent on the
products ability to move into
the root zone. Continuous
corn or observations of adult
beetles made the previous
year can trigger the use of
control tactics. The adult
beetle may feed on corn
leaves, stripping spots of
the upper surface away. After pollen shed, it feeds on
pollen and fresh silks. Heavy silk feeding can reduce or
prevent pollination and result in partial or complete failure of
kernel set. The beetle can be found in ear tips as maturity
approaches. The general guideline for silk clipping insects
is as follows: control may be necessary if silks are clipped
to less than 0.5 inch and fewer than 50% of plants have
been pollinated. Also, one beetle per plant in late summer
is a useful guideline for justifying whether a control tactic
may be necessary to prevent larval injury in continuous
corn.
(1) Mexican CornRootworm
The adult beetle is light
green and looks similar
to the northern corn
rootworm beetle, but can
be distinguished by a black
stripe on the leg.
(2) Northern Corn Rootworm
The adult beetle is less than0.5 inch long, tan or green
to greenish-yellow without
distinctive spots or stripes.
(3) Southern Corn Rootworm
The adult beetle is 0.25 to 0.5
inch long, yellow or greenish
with six black spots on each
wing cover.
(4) Western Corn
Rootworm
The adult beetle is about0.25 inch long, black and
yellow striped or black
with a yellow tip on the
wing cover. It cannot
be visually identied as
to species.
CUTWORMThere are many species of cutworms that attack corn
and other crops such as cotton, tobacco, and many
vegetable crops. Cutworms are widely distributed; some
species migrate to Corn Belt states from the South and
many overwinter there. Young larvae typically feed on leaf
margins and larger larvae feed below or at the soil surface.
They chew into or completely cut young stalks, causing
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Corn rootworm feeding
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36
JAPANESE BEETLE
This shiny, green-bodiedadult beetle has copper- to
bronze- colored wing covers.
As an adult, it grows to about
0.5 inch in length and feeds
on corn leaves and silks. Leaf
feeding by this pest appears
skeletonized or lacy. Larvae
are white with a brown head
and feed unobtrusively on
the roots of plants. They can be differentiated from other
white grubs by the V-shaped pattern of bristles on the
raster. The general guideline for silk clipping insects, suchas an adult Japanese beetle, is as follows: control may
be necessary if silks are clipped to less than 0.5 inch and
fewer than 50% of plants have been pollinated.
LEAF MINERThis small, white larva or
maggot tunnels between leaf
surfaces, leaving long blotchy
tunnels within the leaf. This
pest seldom reaches economic
proportion in corn.
LESSER CORNSTALK BORERThis black larva with white
bands burrows into the stalk
base of young plants. This
borer causes wilting and plant
deformities like twisted, bent
or often barren plants.
PICNIC BEETLE(Sap Beetle,
Scavenger Beetle)(to right)This small, dark beetle
usually has four orange or
cream colored spots on wing covers. It frequently inhabits
ear tips as corn approaches maturity and often appears
where primary insect pests or birds have damaged ear tips.
SEED CORN BEETLEThis small, 0.25 to 0.33 inch long brown ground beetle
damages the germ and hollows seed before it germinates.
The beetle also attacks emerging seedlings, causing spotty
stands. Damage is greatest if germination has been delayed
by the environment. Current seed treatments generally
provide good control.
SEED CORN MAGGOTThis yellowish-white spindle-
shaped larva, is about 0.25
inch long and may eat
the entire kernel before it
germinates. Often times it
leaves only the seed coat
behind. Wet, cold, and heavy
soils are associated with this
pest that can create spotty,
uneven stands. Modern seed
treatment provides good control. Otherwise, there is no rescue
treatment for seed corn maggot and replanting may be the
only option.
SLUGThis soft-bodied, slimy and
legless grayish creature
hides under residue. It is
active at night and leaves a
telltale silver-colored slime
trail on the soil surface. It
feeds using a rasping action
on the lower stalk and leaves
of young plants. This feeding
often removes only onesurface of a leaf and the symptom is more common in no-
till systems. Insecticidal baits are available for slug control,
however, injury seldom warrants this expense.
SOUTHERN CORN LEAF BEETLEAdult beetles are dark brown and can be difcult to nd in
the eld because they are often covered with soil. Beetles
feed on stems and on the edges of leaves of seedlings.
Injured plants appear ragged. When beetles feed in large
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numbers, plants may die. This beetle is most problematic
in elds that have not been cultivated.
SPIDER MITETwo species can cause
severe damage to corn:
banks grass mite and two-
spotted spider mite. Spider
mite problems are more
prevalent when temperatures
are high and humidity and
rainfall are low. The tiny (about
the size of a pencil dot), eight-
legged creature feeds bypiercing individual leaf cells and sucking out the contents.
Damage is usually rst noted as plants approach tasseling
and continues through grain dent stage. Lower leaves
appear blotched and chlorotic and continued feeding can
cause leaves to die. Damage symptoms progress up the
plant with time. The mite spins a white web on the leaf
surface where it feeds.
STINK BUGSeveral species of green or
brown stink bugs occasionally
attack corn. The adult (shieldbug) has a hard, angular back
and wing covers, and gives
off a foul odor if crushed. It
inserts its piercing-sucking
mouthparts into the base
of young plants and can kill
the growing point or distort
further growth.
SUGARCANE BORERThis tan larvae has
indistinct brown spots on
each segment. It feeds in
whorl-and reproductive-
stage plants similar to
the southwestern corn
borer, except that it does
not girdle the stalk. Yield
losses occur due to
reduced ear weight. The sugarcane borer is found in
Florida, Louisiana, Mississippi, and Texas.
THRIPSSeveral species of this tiny, slender insect occasionally
feed on leaves of young corn plants. As an adult, it
develops wings. With mouthparts tted for rasping and
sucking, thrips remove the green surface layer in tiny
streaks. Individual leaves have a speckled appearance and
elds, or affected areas, may look silvery.
WEBWORMThe garden webworm and sod webworm attack young
corn. It appears as a gray to yellow-green, spotted, bristly
larva about 1 inch long. This pest gets its name from the
ne web it spins from its silk-lined underground nest. The
larva uses this web to travel to plants at night and return
to its nest during the day. Similar to the cutworm and
wireworm, the webworms primary feeding site is at or
just below the surface. Injury becomes more obvious as
damaged leaves emerge from the whorl. Larvae also feed
on the underside of lower leaves.
WHEAT CURL MITE
(Kernel Red Streak)
Kernel red streak is causedby a toxin secreted during
feeding of the wheat curl
mite. It is most common on
yellow corn, but may occur
on sweet corn, popcorn,
and white corn. Streaks are
more pronounced toward ear
tips, especially if kernels are
exposed. Streaks vary from
dark red on yellow kernels to pink on white kernels. No
detrimental effects from the discoloration are known.
WHITE GRUBThe larva has a thick,
soft body with three
pairs of legs just behind
a brown head. It ranges
from 0.125 to 1.5 inches
long depending on
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38
age, and it characteristically curls into a C-shape when
disturbed. There are several species with 1 to 4 year life
cycles. The white grub feeds on developing roots of youngplants, and causes stunting, nutrient deciency symptoms
and death. Heaviest infestations occur where corn is
planted into killed sod. There are no rescue treatments
for grubs and the percent stand loss is the best gauge for
determining if replanting is worthwhile.
WIREWORMThe larva is shiny and slender
with a yellow- to brown-
colored hard body. It ranges
in size from 0.5 to 1.5 inches
long, depending on age and
species. Damage to seed or
young plants reduces stand
and vigor. It damages the
germ, stunts plants by root
pruning or kills the growing
point by boring into the base of the stem near ground
level. Modern seed treatments prevent damage to seeds.
Like grubs, there is no way to combat this pest after
stands have been thinned. Use percent stand loss as a
gauge for determining if replanting is worthwhile.
Photo Citations:
Frank Peairs, Colorado State University, Bugwood.org (viewed9/23/10) Armyworm 5364223
Clemson University - USDA Cooperative Extension Slide Series,Bugwood.org (viewed 9/23/10) Billbug 1234011
University of Georgia Archive, University of Georgia, Bugwood.org (viewed 3/11/2011) Grasshopper 4709020
David Riley, University of Georgia, Bugwood.org (viewed
9/23/10) Lesser Cornstalk Borer 2511032
Alton N. Sparks, Jr., University of Georgia, Bugwood.org (viewed
3/11/2011) White grub (scarab) 1327103
Corn earworm damage
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Below
Ground
Western corn rootworm larvae Trait Trait Trait NC
Northern corn rootworm larvae Trait Trait Trait NC
Mexican corn rootworm larvae Trait Trait Trait NC
Wireworm (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
White grubs (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Grape colaspis (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Seedcorn maggot (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Color Key to Activity Insect activity conferred by:
= Control Trait, Insect icide 250 rate, or multip le
= Suppression Trait, Insect icide 250 rate, or multip le
= No Control NC = No Control
Protection for Above and Below Ground PestsProtection forAbove Ground
Pests
Trait NameGenuity
SmartStaxGenuity VT
Triple PROYieldGard VT
TripleGenuity VT
Double PRO
Trait Logo
Seed Treatment Poncho 250
Insects
AboveGround
European corn borer Trait Trait Trait Trait
Southwestern corn borer Trait Trait Trait Trait
Corn earworm (ear feeding) Trait Trait Trait Trait
Western bean cutworm Trait NC NC NC
Black cutworm Trait/250 Rate 250 Rate 250 Rate 250 Rate
Fall armyworm Trait Trait Trait Trait
Sugarcane borer Trait Trait Trait Trait
Common stalk borer Trait Trait Trait Trait
Lesser cornstalk borer Trait Trait Trait Trait
Corn flea beetle (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Corn leaf aphid (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Chinch bugs (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate
Southern corn leaf beetle(seedling stage)
250 Rate 250 Rate 250 Rate 250 Rate
Corn Insect Control with Monsanto Corn Insect Traits
and Seed Treatments
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40
ACCase InhibitorsAryloxyphenoxy Propionic Acids and
Cyclohexanediones
(Including AssureII, Fusion, Poastand Select Max)
Symptoms of damage from
acetyl coA carboxylase
inhibitors include chlorosis
(yellowing) of newly formed
leaves with possible
reddening or purpling ofolder leaves. Tissues at the
growing point turn brown
and eventually decompose,
a symptom called deadheart.
Sublethal doses result in pale white to yellow streaks
between leaf veins.
ALS InhibitorsImidazolinones, Sulonylureas, andTriazolopyrimidines(Including Accent Gold, Beacon, Classic, FirstRate,
Lightning
, Pursuit
, and Scepter
)
These herbicides alter the
function of the acetolactate
synthase (ALS) enzyme and
affect root and shoot growth
and development. Corn
injury typically shows up as
reduced root systems, often
described as bottle-brush
roots. Roots often grow at
or parallel to the soil surface
and may turn brown. Stems andmidribs can purple, and the stem will be
short and thick below the whorl. Mid
to late-season symptoms include short
internodes, malformed leaves, poor root
systems, and pinched ears.
Herbicides with sulfonylurea chemistry
are labeled for use on both corn and
soybeans. Products in this family may
persist in the soil, particularly if soil
pH is above 6.8 and post-application
rainfall has been limited. Thoughthey have the same general chemistry,
different herbicides are used on corn
and soybeans, and injury of corn
may develop the season following
sulfonylurea herbicide application to
soybean elds.
Most observed herbicide damage is due to misapplication, but hybrid and environment interactions willsometimes cause injury symptoms to appear. Herbicides used to control weeds in the previous crop must betaken into consideration, as some herbicide carryover may affect the following corn crop.
Various tankmixes of herbicides are widely used. Be alert to plant injury that can be caused by individualtankmix components or by an interaction involving two or more components in the tankmix itself.
Corn plants frequently outgrow the effects of herbicide injury and their nal yields may not be noticeably
lowered.
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41 Corn Diagnostic Guide
Synthetic AuxinsBenzoic Acids, Phenoxys, and PyridineCarboxylic Acids
(Including Banvel, Butyrac, Clarity, and Crossbow)
These herbicides, also
known as plant growth
regulators, are translocated
to meristematic tissue and
interfere with cell formation
resulting in abnormal root and
shoot growth. Symptoms
include twisted whorls or
buggy-whipping where
the leaves do not unfurl,
abnormal brace roots,
root proliferation, brittle
stalks, and poor pollination.
Growing conditions
are a factor in damage
expression.
Auxin Transport Inhibitors
Semicarbazones(Including Distinctand Status)
These herbicides are primarily active against broadleaf
plants, but injury symptoms may occur in corn under
certain conditions. Injury is similar to growth regulator
herbicides.
Carotenoid Synthesis InhibitorsIsoxazolidinones
(Including Command)
Shallow planting or stress
conditions that slow seedling
metabolism can increase the
potential for injury. Damage
to corn seedlings the season
following an application may
occur, especially if soil pH is
below 6.0. Affected plants are
distinctly white or bleached.
Most corn plants recover and
new leaves have normal color.
Cell Division InhibitorsAmides
(Including Degree, Dual II Magnum, and Harness)Amides (also known as
acetanilides, acetamides,
or chloroacetamides) are
meristematic growth inhibitors
that are translocated to the
shoot and leaves. These
products can cause seedlings
to leaf underground or trap
seedling leaves so they
cannot unfurl, giving a ladder
effect. Damage is more likely to take place with cool, wetweather occurring immediately before emergence.
EPSP Synthase InhibitorsGlycines(Including Roundup agricultural herbicides and
generic ormulations o glyphosate)
These non-selective foliar-
applied herbicides are
translocated and interfere
with amino acid synthesis.
If spray drift reaches corn,
leaves wilt, turn brown,
and die. Sublethal rates
can cause phenoxy-like
symptoms. Glyphosate applications that occur too late to
corn with Roundup Ready 2 Technology can result in poor
kernel set.
Glutamine SynthaseInhibitorsPhosphinic Acids(Including Igniteand Liberty)
Inhibitors of glutamine synthase
are nonspecic and will kill
plants that have not been
modied to express resistance
to them. Symptomology will
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42
range from light speckling on leaves and yellowing to
plant death,depending on the degree and intensity of d