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Feb 03, 2016
Diseases and Pests in Wheat and Rice
Diseases and Pests in Wheat and RiceWHEATPrincipal diseasesBarley yellow dwarfCommon buntEyespotWheat leaf rust
Barley Yellow Dwarf
Barley yellow dwarf is a plant disease caused by the barley yellow dwarf virus, and is the most widely distributed viral disease of cereals. It affects the economically important crop species barley, oats, wheat, maize, triticale and rice.Biology
Barley yellow dwarf virus (BYDV) is a positive sense single-stranded RNA virus; the viron is not enveloped in a lipid coating. The virus is transmitted by aphids, and the taxonomy of the virus is based on genome organisation, serotype differences and on the primary aphid vector of each isolate.Pathology
When aphids feed on the phloem of the leaf, the virus is transmitted to the phloem cells. Once inside the plant, the virus begins to replicate and assemble new virons. This process requires significant metabolic input from the plant, and causes the symptoms of barley yellow dwarf disease.Sources and spread
There are two main sources by which a cereal crop might be infectedBy non-migrant wingless aphids already present in the field and which colonise newly-emerging crops. This is known as "green-bridge transfer".By winged aphids migrating into crops from elsewhere. These then reproduce and the offspring spread to neighbouring plants.
"Green bridge" sources must be ploughed in as early as possible. Alternatively, a desiccant herbicide should be applied 10 days prior to cultivation. Insecticide sprays may be used at crop emergence.Drilling dates prior to mid-October favours attacks from winged migrant aphids. However, yield penalties may be experienced from late drilling. Insecticide sprays in this instance are therefore aimed at killing the aphids before significant spread can occur.
Common BuntThe bunts and smuts are caused by fungi that affect the seed of wheat and other small grains and grasses. Only loose smut and common bunt occur in Oklahoma. However, Karnal bunt, which does not occur in Oklahoma, is a concern because grain that tests positive for Karnal bunt can not move freely into international markets.Symptoms
Plants with common bunt may be moderately stunted but infected plants cannot be easily recognized until near maturity and even then it is seldom conspicuous. After initial infection, the entire kernel is converted into a sorus consisting of a dark brown to black mass of teliospores covered by a modified periderm, which is thin and papery. The sorus is light to dark brown and is called a bunt ball. The bunt balls resemble wheat kernels but tend to be more spherical. The bunted heads are slender, bluish-green and may stay greener longer than healthy heads. The bunt balls change to a dull gray-brown at maturity, at which they become conspicuous. The fragile covering of the bunt balls are ruptured at harvest, producing clouds of spores. The spores have a fishy odour. Intact sori can also be found among harvested grain.
Millions of spores are released at harvest and contaminate healthy kernels or land on other plant parts or the soil. The spores persist on the contaminated kernels or in the soil. The disease is initiated when soil-borne, or in particular seed-borne, teliospores germinate in response to moisture and produce hyphae that infect germinating seeds by penetrating the coleoptile before plants emerge. Cool soil temperatures (5 to 10C) favour infection.The intercellular hyphae become established in the apical meristem and are maintained systemically within the plant. After initial infection, hyphae are sparse in plants. The fungus proliferates in the spikes when ovaries begin to form. Sporulation occurs in endosperm tissue until the entire kernel is converted into a sorus consisting of a dark brown to black mass of teliospores covered by a modified periderm, which is thin and papery.Management
Control of common bunt includes using clean seed, seed treatments chemicals and resistant cultivars. Historically, seed treatment with organomercury fungicides reduced common bunt to manageable levels. Systemic seed treatment fungicides include carboxin, difenoconazole, triadimenol and others and are highly effective. However, in Australia and Greece, strains of T. laevis have developed resistance to polychlorobenzene fungicides.Eyespot
Eyespot is an important fungal disease of wheat caused by the necrotrophic fungus Tapesia yallundae and Tapesia acuformis. It is also called Strawbreaker. Eyespot is more severe where wheat is grown continuously and when the weather is cool and moist. Treating crops against eyespot with fungicide costs millions to farmers and is complicated by the pathogen becoming resistant to the more commonly used fungicides. Symptoms
The eye-shaped elliptical lesions which give eyespot its name form on lower stem bases near to the soil surface. The lesions are straw yellow, often with black pupil-like dots in the centre, and are bordered by greenish-brown to dark-brown rings. In cases of severe infection stems are weakened at the point of infection which makes the host susceptible to lodgingDevelopment of infection
It is more severe if wheat is grown continuously in same field over the same period. The fungus grows as mycelium which penetrates successive leaf sheaths throughout the growing season. High humidity, cool, and damp weather at the soil surface favours disease development. Whilst dry and hot weather causes the leaf sheaths of the plant to dry and fall off, taking the inoculum with it, thus lessening disease.Invasion of fungi
Invasion of Pseudocercosporella herpotrichoides'" in wheat initiates with release of enzyme for breaking the plant cell wall. A specific sequence of enzymes is employed; without these enzymes the fungus would not be able to invade the plant cell. The fungus is present in the plant stem, it causes disease by affecting nutrient and water supply to upper parts of the plant, weakening the plant stem.Plant defences
Wheat cells release hydroxyproline glycoprotein (HRGP) in their cell walls. Secretion of HRGP is dependent on the signal induced by the fungal elicitors stimulating the transcription of genes encoding HRGP accumulation in the cell wall. In the case of wheat HRGP is less accumulated allowing for more easy invasion by the fungus.Method of control
The best method of control for eyespot disease is breeding for resistance. Currently the gene conferring resistance to eyespot is the Pch1 gene. To generate resistant culitvars plants containing this gene are bred with others to pass the gene to their offspringCrop rotation is also important in reducing the extent of disease because eyespot fungi live on debris of the previous crop. Cropping the wheat with alternate non-host crops and with set-aside periods of at least one year helps to lessen disease.Use of fungicide can be effective in the short term but is not a long term solution as the pathogen can develop resistance to fungicides. Application of chemicals is also costly.Wheat Leaf RustWheat leaf rust, is fungal disease that affects wheat, barley and rye stems, leaves and grains. In temperate zones it is destructive on winter wheat because the pathogen overwinters. Infections can lead up to 20% yield loss - exacerbated by dying leaves which fertilize the fungus. The pathogen is Puccinia rust fungus. Puccinia triticina causes 'black rust', P.recondita causes 'brown rust' and P.sriiformis causes 'Yellow rust'. It is the most prevalent of all the wheat rust diseases, occurring in most wheat growing regions.SymptomsSmall brown pustules develop on the leaf blades in a random scatter distribution. They may group into patches in serious cases. Infectious spores are transmitted via the soil. Onset of the disease is slow but accelerated in temperatures above 15C, making it a disease of the mature cereal plant in summer, usually too late to cause significant damage in temperate areas. Losses of between 5 and 20% are normal but may reach 50% in severe cases.Control
Varietal resistance is important. Chemical control with triazole fungicides may be useful for control of infections up to ear emergence but is difficult to justify economically in attacks after this stage.RICECommon Types of DiseasesRice BlastSheath BlightStem RotTungro
A Rice Leaf with Rice Blast
SymptomsInitial symptoms white to gray-green lesions or spots with darker borders produced on all parts of shootOlder lesions elliptical or spindle-shaped and whitish to gray with necrotic bordersLesions wide in the centre and pointed toward either endLesions may enlarge and coalesce to kill the entire leaves
Causal agent or factorA fungus causes rice blast. Its conidiophores are produced in clusters from each stoma. They are rarely solitary with 2-4 septa. The basal area of the conidiophores is swollen and tapers toward the lighter apex.The conidia of the fungus measure 20-22 x 10-12 m. The conidia are 2-septate, translucent, and slightly darkened. They are obclavate and tapering at the apex. They are truncate or extended into a short tooth at the base.
Mechanism of damageConidia are produced on lesions on the rice plant about 6 days after inoculation. The production of spores increases with increase in the relative humidity. Infection tubes are formed from the appressoria and later the penetration through the cuticle and epidermis. After entering the cell, the infection tube forms a vesicle to give rise to hyphae. In the cell, the hyphae grew freely.ManagementManipulation of planting time and fertilizer and water management is advised.Early sowing of seeds after the onset of the rainy season is more advisable than late-sown crops.Excessive use of fertilizer should be avoided as it increases the incidence of blast. Water management pra