BRE 211: Principles of Agriculture and Forestry

BRE 211: Principles of Agriculture and Forestry

Lecture 5

Plant Diseases Plant said to be diseased when

Its chemistry or structure has been subjected to an abnormal, sustained alteration.

An injury is caused when a leaf pulled off a tree A fungus, bacterium or virus enters a plant and

deprives the plant of nourishment or alters normal functions of the plant.

There a dysfunction of a plant due to lack of a nutrient resulting in undesirable symptoms.

For a disease to occur it is necessary to have a susceptible plant, an agent causing the disease, and a suitable environment are all necessary for disease to occur.

Causes of Plant Diseases Plant diseases are caused by:

Non-living agents including low temperatures, high temperatures, atmospheric impurities, mineral deficiencies and mineral excesses

Living agents such as fungi, bacteria, a few higher plants, nematodes, algae, viruses, mycoplasmas, and viroids.

Fungi and bacteria cause the majority of plant diseases.

Viruses are minor causal agents but their damage is severe and they are more difficult to control.

Control of Plant Diseases The objective is to reduce the disease effect below

the economic threshold. Most important point in controlling a plant disease is

choosing the best control method or series of control methods for

Use of more than one control measure (systems control) is often needed.

Principle of control is to find the part of the life cycle of a living pathogen that will succumb to available control methods and utilize the control methods early in relation to pathogen propagation.

Most plant disease control methods are preventative. Some eradicative techniques are available but should

not be relied upon for consistent control.

Conventional Methods of Plant Disease Control Include

Physical: Growing of crops in green houses where diseases are

kept out of touch with the growing crops especially for high value crops.

Legislative: Use of laws and regulation to prevent the importation of

diseased planting materials into a country and to restrict the spread of diseases in other areas

Objective of legislation is to prevent dangerous diseases from colonizing new areas.

Legislative control involves Quarantine Eradication regulations Certification

Conventional Methods of Plant Disease Control Cultural control

Manipulation of agronomic practices employed in crop production to influence the incidence of certain diseases.

Basic principle is the disruption of the development and life cycles disease causing organisms either by denying them their food or by exposing stages in then life cycle to adverse conditions so that they are killed.

Relatively cheap and effective. Poses minimal danger to the environment. Methods involve:

Cultivation of the soil Crop rotation Trap cropping Resistant crop varieties Mixed cropping Good husbandry practices

Conventional Methods of Plant Disease ControlBiological Control

Deliberate use of organisms (parasites, predators and pathogens) to reduce populations of disease causing organisms.

Such natural enemies may be arthropods (insects and mites), bacterial protozoan, fungi, viruses, nematodes or even vertebrates (birds, toads, fish).

Conventional Methods of Plant Disease Control Chemical control

Most common and easily applicable method. Plays a significant role in solving the food and

wealth problems of tropical countries. Advantages include:

Relatively easy and cheap Produces quick results Can be repeated as often as desirable Broad-spectrum action of many chemicals makes the

method control many diseases singly or combination

Conventional Methods of Plant Disease Control Disadvantages of Chemical control

Chemical control is repetitive thus wasteful Chemical applied rarely controls the disease

completely. Chemicals are toxic substances and the

residues that remain in the plants can cause harm wildlife, fish and humans

Cause environmental pollution and ecological disturbance.

Repeated use can cause disease resistance. Chemicals provide only a temporary solution to

disease problems Chemicals expensive to manufacture.

Weed PlantsA weed could be defined as:

A plant growing where it is not wanted A plant out of place or A plant whose usefulness has not been

discovered.Plants are also considered weeds when

they interfere with utilization of land and water resources or otherwise intrude upon people’s welfare.

Weed PlantsSome other plants are weeds because

they are poisonous to humans and livestock or are generally obnoxious.

Useful plants growing where they are not wanted such as volunteer maize in cowpea fields and rice seedlings in soybean fields are considered as weeds.

Characteristics of weedsThey have a tendency to grow in an

undesirable locationThey have competitive and aggressive

habits They are wild and have rapid growth

habitsThey are persistent and resistant to

control and eradicationThey have a high reproductive capacity

Characteristics of weedsThey grow densely or in large

populations around economic plantsThey are useless and undesirableThey have spontaneous growth They are harmful to humans, animals

and cropsThey have unattractive sight

Classification of WeedsDone according to

Life form Life span Growth habit Habitat

Classification of WeedsLife form

Narrow-leaf weeds Leaves have parallel veins and the growing

point of the plant has protective layers of leaf sheath at early vegetative stages e.g. grasses and sedges

Broad leaf weeds Leaves of these weeds have net venation

Classification of WeedsGrowth habit

Independent weeds Grow independently

Parasitic weeds. Establish from seed in association with host

plants which supply foodLife Span

Annuals Perennial

Classification of Weeds Annuals

Complete their life cycle in one growing season. They produce large quantities of seeds

Perennial Require more than two growing seasons to

complete their life cycle. They are adapted to long-season crops such as

cassava and plantation crops. Have perennating structures such as tubers,

rhizomes or stolons. Simple perennials live for many years but

reproduce only by seeds.

Classification of WeedsPerennial

If the aerial plant is cut, the basal portion may produce new shoots vegetatively.

True perennials live for many years and will produce new plants either by seed or by vegetative propagules.

Many true perennial weeds have either lost their ability to produce seeds or produce very few viable seeds and thus rely mainly on vegetative propagules

Classification of Weeds Habitat Aquatic weeds:

Grow in soils that either have standing water or are permanently wet

True aquatic weeds may be: Floating hydrophytes if they are in contact with water and

air only Emergent hydrophytes if they are in contact with

substrate water and air Submerged hydrophytes if they root in the substrate but

do not emerge above the water Non-aquatic weeds:

Cannot complete their life cycle in any of these high moisture regimes.

Economic Importance of weeds Are important components of our ecosystem. Although they cause greater loss in agriculture they

also provide food for people and animals and protect the soil from erosion.

Losses Caused by Weeds Competition:

Compete with crop plants for light, water and nutrients reducing yield and quality of produce.

Act as an alternative host for other crop pests and diseases Weeds may harbour many fungal, viral and bacterial

diseases as well as insect pests. Weeds also provide food for birds, rodents and their predators.

Reduce human efficiency Some weeds cause allergies and poisoning.

Losses Caused by Weeds Increase water management cost :

Aquatic weeds interfere with use of water for irrigation, recreation and fishing

Increase production costs: Weedy crops are prone to insect and

disease infestation and thus more resources are spent on controlling these pests and diseases.

Weeds increase labour costs Increase cost of inputs such as herbicides

and machinery.

Weed Control Methods Cultural Methods

Includes all aspects of good crop husbandry used to minimize weed competition with crops.

The methods include: - Burning Hand-weeding Mechanical weeding – use of machines Mulching Tillage – cultivation Mowing Flooding Good Cropping system – each crop has its

characteristic weeds Preventive weed control – use of clean seed

Cultural weed control methods are often laborious, unattractive and ineffective in some plants.

Weed Control MethodsBiological Control

This is the control or suppression of weeds by the action of one or more organisms accomplished either naturally or by the manipulation of weed, control organisms or environment.

Specific control methods Fallowing Live mulch Use of low growing crops to shade the soil

surface suppresses weeds.

Weed Control MethodsSpecific biological control methods

Appropriate modification in plant population

Higher plant populations reduce weed competition.

Spatial arrangement Growing crops with different canopy

structures will give good shading that suppresses weeds

Weed Control Methods Chemical Control Herbicides kill, suppress or modify weed growth to

prevent interference with crop establishment, growth and production of economic yield.

Advantages Reduces early weed competition in crops thus increasing

yield. Reducing labour requirements for weeding. Pre-emergence and post-emergence herbicides enhance

timely weeding Makes it possible for a farmer to cultivate larger hectarage

with efficient weed control. Disadvantage:

Causes environmental pollution.

Weed Control Methods Integrated weed management

Suppresses weeds by combining one or more weed control methods. Some farmers may still prefer to plant seedlings directly in the soil. This often means, however, that considerable damage is done to the seedling on extraction thus delaying its development.

Environmental and socio-economic constraints make integrated weed management one of the best options for weed control in the tropics.

Light soils, high or low rainfall; a wide range of crop types and cropping systems, abundance of persistent weeds all combine with low persistence of many herbicides to make absolute reliance on chemical weed control difficult.

Crop Harvesting Done when the useful plant parts reach

maturity. Maturity usually related to the age of the crop

when harvestable parts have accumulated maximum dry matter.

At maturity Harvestable part ceases to increase in size Colour changes due to ripening especially

in fruits Senescence and loss of vegetative parts

occur. In grains, there is reduction in moisture

content.

HarvestingHarvesting at maturity ensures

maximum yields but delayed harvesting will lead to losses.

Harvesting is sometimes advisable, particularly in grain crops where it may be necessary to reduce the moisture content in the field to facilitate handling and storage.

In fruit and leafy crops high moisture content is preferred.

HarvestingPotatoes should be harvested when soil

is not excessively wet.Residue should be properly disposed by

burning or incorporation in the soil after harvesting Helps in preventing a carry over or

dissemination of any pests and diseases present on the harvested crop.

It also helps in enriching the soil.

Harvesting Time of harvesting depends on:

Characteristics and quality requirements of a crop. Harvest produce when foliage is dry.

Grain crops can be left in the field to dry down to 30 per cent moisture content or less in dry weather.

However this may result in: Losses from shattering and shedding of the grains

or lodging of the plants. the grains may absorb water and sprout on the

plant Grains may be bleached thus reducing quality. Insect and fungal attack may become serious

Post – Harvest HandlingRough handling of produce should be

avoided.Harvest and discard rotted fruit last.Do not allow harvested fruit to remain in

direct sun.Soil adhering to harvested fruit, tubers,

or bulbs should be allowed to dry and then lightly brushed off.

Soaking or washing produce to remove trash or soil can result in more storage rots than if not done.

Post – Harvest Handling If washing of produce is done

It must not be done in water that is cooler than the produce Blemished produce should be discarded prior to washing. Adding bleach to clean wash water for tomatoes reduces

bacterial soft rot. Fruits should not be allowed to be deeper than 12 inches or

for more than 3 to 4 minutes. Storing produce dry and within cooled or air

conditioned areas will reduce post-harvest rots. Discard all produce with rot prior to handling or

storage. Periodically, inspect stored produce for rot and

discard rotted produce.

Post – Harvest Handling Agricultural produce may be directly

consumed or sold after harvesting or it may be treated to facilitate transportation and storage.

The treatments of harvested crops include: Drying: Natural drying or Artificial drying Aeration

Crops are dried in order to Prevent germination of seeds Retain maximum quality in the grain or forage by

preventive deterioration Reduce moisture content in order prevent insect

attack and microbial infestation.

Natural Drying Harvesting is done in the dry season exposing it to

the sun can dry the produce conveniently and adequately.

In humid areas the most practical method of drying would involve Partial drying in the field followed by further drying either in: Shallow layers on damp-proof platforms protected from

moisture in the form of dew or rain or Containers that permit dry air movement through the

materials. Excessive or prolonged drying leads to:

Bleaching Wrinkling Scorching Case hardening,

Artificial drying Use of heat to dry produce in conditions where

natural drying is not possible or convenient. In small-scale peasant agriculture, artificial drying is

limited to such quantities of produce as can be accommodated in the fireplace.

This usually includes seeds required for planting next season. Maize, beans etc are hung on horizontal grids over the

fireplace to dry. The produce dried over the fireplace is

characteristically tainted by smoke oduor In some instances, raised grain stores are built so

that the produce can be fired from below. Drying with hot air can be done.

Artificial dryingLow and high temperature drying can

also be done In low-temperature drying, the air

temperature is raised to 5-10 ºC above the surrounding temperature and drying is completed in 3-4 days to avoid deterioration of produce.

In high temperature drying, the air temperature is raised 15-60ºC above the surrounding temperature and drying is completed within few minutes

Aeration Aeration is necessary to

lower grain temperature equalize temperature through the bulk of produce remove unpleasant oduors and fumigants reduce moisture content slightly

Air can be used to dry stored produce when it is dry and warm.

Humid air is unsuitable unless it is heated to slightly above the surrounding temperature before it is passed through the produce for storage.

Storage of Harvested Produce Storage life of seeds varies with

Species Environmental conditions in which the seeds are stored.

Seeds must be stored under moisture conditions that retard respiration and enzyme activity within seeds. Moisture content is crucial for extended storage life of most

seeds. Seeds usually contain 20% moisture or more at

harvest and must be dried to retain maximum viability. Some seeds e.g. citrus however lose viability when dried. Many seeds dry naturally in the field except when conditions

are extremely humid. Others require drying after harvest.

Storage of Harvested Produce Most seeds retain viability best at low relative

humidity. A relative humidity of 65% or more is unsuitable for seed

storage. The optimum temperature for long-term storage is in

the range 18°C to 0 °C. Since moisture and temperature are interrelated, it is

important to ensure that when one is high the other must be low. For most seeds, a temperature between 0 °C and 10°C and

relative humidity of 50-60% is adequate to maintain full viability for at least one year.

Controlling the storage atmosphere, through reducing the oxygen content and increasing the carbon dioxide content, both of which reduce respiration, can extend the storage of seeds.

Pest Damage to Stored Produce High temperature and high moisture content

predispose grains to damage by insects and disease organism.

At 13% moisture and 21°C insects actively damage grains.

They respire and raise the moisture content of the grain leading to spoilage by heating, moulding, decay and further damage by the insects.

At 9% moisture and 5°C, insects are very inactive and spoilage can be avoided.

Besides insects and rodents, fungi also damage stored grains.

The predisposing factors for fungal damage are moisture temperature, physical damage, infection and duration of storage.