Field Crops> Wheat (Triticum aestivum L.)> Diseases

Wheat crop suffers from several diseases which reduce its yield and quality. The major diseases of wheat are rusts, alternaria leaf blight, loose smut, Karnal bunt and powdery mildew.

Rusts

Rusts are caused by three different species of the fungus Puccinia. Brown and yellow rusts are particularly impprtant in the majro wheat growing areas of the north-western part of the country. Black rust appears quite late in these areas and normally cannot cause substantial damage except in wheat fields sown very late. However, black rust appears in severe forms in the southern, central and eastern India and causes significant damage.

Brown-Rust: This rust is caused by a fungus known as Puccinia recondita tritici. The small, round-oval uredial pustules of brown rust develop mainly on the leaves but in severe attacks, these appear on sheath and stem as well. The uredial pustules are bright orange in colour. These are never in rows but are either in small clusters or irregularly scattered.

Bruwn rust spreads fast on susceptible wheat varieties during warm and humid environmental conditions. In case of severe attack of the disease, plants mature early, produce light and shrivelled grains and poor development of root system. This rust is most widespread in the country and generally the most damaging in several areas.

Yellow Rust: This rust is caused by the fungus, Puccinia strciformis. In the mild attack of this rust, uredia are formed chiefly on hte leaves, but in case of severe attack, these appear on leaf sheaths, stalks and glumes as well. The uredial pustules are oval in shape and lemon-yellow in colour and are formed in long streaks or rows. The pustules of yellow rust are smaller than those of brown rust. Appearance of lemon-yellow pustules in rows is characteristic feature of yellow rust. In severe attacks, however, these rows are not disinct, as large patches of the plant become covered with crowded pustules.

Unlike other rusts, yellow rust is restricted to those areas, where temperature is considerably low during the wheat growing season. Thus yellow rust is a problem of the hills, foot-hills and nort-western part of the country. Since yellow rust develops early in the season, damage ot young plants is severe.

Black Rust: This rust of wheat is caused by the fungus, Puccinia graminis itici. The uredial pustules occur on stem, leaf sheath, leaves and earhea but the stem is often most severely affected. The pustules of black sust are reddish-brown in colour and elongated in shape. This rust spreads fast under relatively warmer and humid environmental conditions. By the time the crop matures, dark, black, enlongated telial pustules are formed. The name black rust was given to this rust due to the prominence of its dark black telia.

Black rust, though prevalent all over the country, appears in epidemic forms in the southern, central and eastern India, which are characterised by high temperatures in the wheat growing season. Black rust is a killer of wheat plants and in severe infection there is heavy crop failure.

Control Measures: Adop the following mmeasures to control the wheat rusts:

1. Grow rust-resistant varieties of wheat recommended for a particular area. It is safer to divide the total farm area for wheat cultivatiohn under 3-4 different varieties of wheat. In other words, sowing the total aea with a single variety is unsafe.

2. Avoid late sowing of late maturing varieties. Late sown crop is more exposed to rust damage.

3. Donot use excess nitrogenous fertilisers, as very high nitrogen dose favours rust development. Use of high dose of potash is known to favours rust development. use of high dose of potash is known to reduce rust infection. apply balanced doses of nitrogen and potash as recommended.

4. Damage by wheat rusts may be minimised by spraying Zineb or Dithane M-45. Spraying is recommended only when a good crop of wheat is threatened by rusts. Spraying is not economical on a poor crop of wheat. Spray 0.2 per cent Zineb or Dithane M-45 (active material) and 0.1 per cent Sandovit (spreader-sticker) as outlined below:

(a) Give first spray when rust pustules are seen. This normally occurs during the last week of January or first week of February. About 375 litres solution per hectare is enough for this application.

(b) Give second spray 10 days after the first. Use 750 litres solution per hectare.

(c) Give third and fourth sprays at an interval of 14 days. Use 1000 and 1250 litres solution per hectare, respectively.

Loose Smut

Loose smut has now become a major problem of wheat crop. This destructive disease of wheat is caused by a smut fungus, Ustiloto nuda tritici. Generally, diseased plants are indistinguishable from the healthy plants before heading, but diseased plans produce heads earlier than healthy plants. However, diseased plants of wheat variety Sonalika (RR-21) can be dedetected before ear emergence also by a characteristic streaking and yellowish of the flag leaf which starts drying fromthe tip and later becomes brownish in colour. Flag leaves of diseased plants of Sonalika can easily be seen in the field from a distance. Terminal sympotom in all the eties is the production of black powder in place of wheat grains in the most every ear of the diseased plant is converted into black powder.

The black powder is a mass of olive-green microscopic sporres of the smut tungus. When spores are blown to flowers of healthy plants, they germinate on the female organ, stigma and lead to infection of the developing seed. The smut fungus thus reaches in the mature grain and remains dormant there until germination of the grain. From superficial examination at harvest time, infected grains cannot be distinguished from the healthy ones. When these grains are sown in the following season, the fungus inside the seed becomes active as the germination starts. The fungus grows inside the plant first behind the growing point, keeping pace with apex of the growing shoot. As the ear foramtion starts, there is an accumulation of hte fungus in the floral parts, which ahe completely destroyed due to formation of the black powder.

Control Measures:

(1) Growing of loose smut resistant varieties is the best method of control.

(2) Since loose smut fungus totally depends on wheat seed for its survival and carry-over from one season to another, growing of disease-free seed is the only alternative method of control available at present for large scale adoption. Treat the seed withVitavax at the rate of 2.5 g per kg of seed before planting.

(3) At the time of ear emergence, visit the seed crop regularly and locate plants with diseased ears. Uproot the entire plant while covering the diseased ears with a paper envelope so that black powder does not spread. Destroy it by burying under the ground or by burning.

(4) Solar heat treatment of infected seed is highly valuable in the hot summer areas of the plains to make the seed disease-free. On a bright sunny day in the last week of May or first week of June, soak teh seed is water for about 4 hours in the morning and then dry it under hot sun in the afternoon. Store the well-dried seed for use in next wheat season. The seed soaking activaties the dormant fungus in the seed and heat of hte sun kills it.

Karnal Bunt

Karnal punt or partial bunt, first observed in experimental seed plots at Karnal, (Haryana) in 1930, was regarded as a very minor disease of wheat till recently. But now this disease has become a problem in manyplaces in northern India, mainly in the Himalayan foot-hills and Tarai regions. Practically all the commerical varieties of wheat grown at present are susceptible to it. The disease reduces both quantity and quality of wheat. Flour prepared from wheat with more than 3 per cent infected grains smells fishy and is unfit for human consumption.

This disease is caused by the fungus called Neovossia indica. The disease can be recognised when the grains have developed. A portion of the grain along its groove is converted into a black powdery mass. The embryo tissue, except isn very severy cases, is generally not destroyed. The black powder gives a foul smell due to presence of trimithylamine. In a plant hill, all the ears are not affected and in an ear only few grains ae infected.

Control Measures: Since spores of the Karnal bunt fungus are soil-borne as well as seedborne and infection occurs at the flowering time by air-borne sporidia, the disease is difficult to control in hte absence of resistant varieties of wheat. However, the following methods can minimise occurrence of Karnal bunt.

(1) Avoid planting highly susceptivle wheat varieties like HD-2009(Arjun), UP-319, WL-711 and HD-1982 (Janak) in the Tarai areas.

(2) Use disease free seeds. Seed-borne inoculum can be killed by treating seed with mercury fungicide (e.g., Ceresan or Agrosan G.N.) at the rate of 2.5 g per kg seed. Preliminary tests indicate that seed treatment with Vitavax is also effective for killing spores.

(3) Avoid continuous cropping of wheat in the same field. Practice field sanitation and deep ploughing in summer.

(4) Avoid excessive irrigation particularly during flowering of wheat crop.

Alternaria Leaf Blight

This disease is caused by the fungus, Alternaria triticina. Lowermost leaves are always the first to show the sign of infection , which gradually spreads to the upper leaves. The disease first makes its appearance as small, oval, discoloured lesions, irregularly scattered on the leaves. The sports become irregular in shpae as these enlarge and take up dark brown to grey colour. A bright yellow marginal zone is sometimes seen around the spots. As the disease progresses, several sports come closer and cover large leaf areas, eventually resulting in death of hte entire leaf. In case of severe attack, leaf sheaths, awns and glumes are also infected. Black powderly spores of the fungus cover the lesions at this stage under moist conditions. These spores are disseminated by wind and cause disease on healthy leaves and plants. The disease spreads very fast under warm and humid conditions. Heavily infected fields present a burnt appearnace.

Control Measures: Seed-borne infection can be controlled by treating seed with Vita-vax at the rate of 2.5 g per kg seed.

Apply adequate fertilisers and irrigation.

Alternaria leaf blight can also be controlled by Zineb or Dithane M-45 sprays. The fungicide may be sprayed as described for rusts. It is beneficial to mix urea at the rate of two to three per cent with Zineb spray at first and second sprays.

Powdery Mildew

Powdery mildew disease of wheat is caused by a fungus, Erysiphe graminis tritici. This disease is characterised by formation of white powdery growth of the fungus on the upper surface of leaves. The white growth contains mycelium and spores of the fungus. Later on, the colour of the powdery growth becomes grey or brown. Under severe disease conditions, leaf sheaths, stems and earheads may also be covered by the fungus. Infected plants become stunted due to reduction in size and number of leaves and produce poor yield. This diease is common in hte sub-mountaneous areas of northern India, certain parts of Rajasthan and the hills.

Control Measures: Grow varieties resistant to the disease.

Burn crop refuge in the field after harvesting is over.

If losses are expected to be hhigh, spraying with a mixture of Dithane M-45 and Karathane has been found beneficial. Prepare mixture by mixing 16 parts of Dithane M-45 and 4 parts of Karathane 25 wettable powder. Spray mixture at the rate of two kilogrammes in 1000 litres water per hectare. About three sprays should be given at an interval of 10-15 days. Amount of water for different sprays may be decided on the basis of growing stage of the crop.