गेहूँ के बीज जनित रोग
Wheat is the second most important cereal crop in India after Rice. Being one of the most important staple crop, it fulfils major nutrition requirement of the human population since times immemorial.
Seed borne diseases of wheat viz., Karnal bunt (Tilletia inidica), loose smut (Ustilago nuda tritici), head scab (Fusarium spp) and tundu or ear cockle (Clavibacter tritici and Anguina tritici) are major constraints affecting both quality and quantity of the wheat grain production leading to low market prices.
Besides affecting quality, several diseases may have toxic effect on human beings and animals due to the toxins produced by pathogens. Infected wheat seeds can be carried to long distances through various means leading to spread of diseases across different areas. The early detection of seed borne diseases is essential in managing these diseases.
These pathogens are being identified through various conventional, culture based and molecular methods. The conventional methods includes direct inspection of dry seeds, incubation tests, washing test, soaking test, blotter tests, embryo count test and filter and centrifuge extraction technique.
The Sodium hydroxide seed soak method was used for detection of Karnal bunt and many PCR-based methods are being used nowadays for detection of Tilletia spp and Fusarium spp. Pathogen free seed production is an important step in ensuring increased productivity and quality of wheat.
Some of the important seed borne diseases of the wheat are given below.
1. Karnal Bunt
Karnal bunt was discovered in 1931 at Karnal (Haryana) by Mitra M. The disease occurs in India, Pakistan, Mexico, Iraq and Afghanistan causing severe losses. The disease is listed as quarantine disease by more than 20 countries in the world.
The disease occurs in northwestern parts of the India and it is reported to cause yield loss of up to 20% in susceptible varieties. The quality loss caused by the pathogen is a major problem in addition to yield losses.
It affects export as well as market value of the grains. The quality of the wheat flour prepared from disease affected grains smells fishy (due to chemical triethylamine) affecting the consumer end products like chapatti.
It is reported that wheat samples containing more than 3% karnal bunt affected grains is unfit for human consumption.
Symptoms:
This disease is caused by fungus Tilletia indica Mitra or Neovossia indica (Mitra) Mundkur. This disease affects only grains and symptoms become evident only when the grains develop fully.
The whole grains are either partially or fully converted into black powder of fungal spores. Only few grains in an ear head will be affected. These bunted grains emit fishy like odour due to production of triethyamine.
These bunt spores may blow off due to air or may fall on ground and spread and infect other plants at the time of flowering leading to further spread of infection.
The primary source of inoculum is either soil or seed contaminated with teliospores. It also spreads through air as well as contaminated farm equipment. The spores can remain viable for 2-10 years in soil, straw and manure.
The application of excessive nitrogen fertilizers and irrigation at flowering time pre dispose the plants to this disease.
Management:
Management of Karnal bunt is difficult due to its mode of survival and spread. Growing resistant varieties is the most economical method to manage this disease.
Farmers should avoid sowing susceptible varieties, should use disease free certified healthy seeds for sowing. Durum wheat are tolerant to this disease therefore wherever the disease incidence is low, durum varieties may be planted.
Irrigation/Sprinkler irrigation at the time of flowering should be avoided. One spray of Propiconazole 25EC (Tilt 25 EC) @ 1ml/l water should be sprayed at 50% flowering and can be repeated after 15 days.
In disease prone area, the one spray of Propiconazole or two sprays of Trichoderma viride at tillering and ear head emergence stage could be given.
2. Head Scab
It is also called as head blight, caused by several Fusarium spp. F. graminearum (Gibberella zeae), F. avenaceum and F. poae are the most common Fusarium spp. responsible for the disease. This disease also affect both yield and quality of the wheat grains. Several mycotoxins (like deoxynivalenol) are produced by Fusarium spp. affects quality of the produce.
Symptoms:
Flowering time is the most susceptible period for fungal infection. Initial infection is characterized by whitening and drying of individual spikelets or entire parts of the ear.
Florets become infected just after blooming, grains are often not formed, while other infected spikelets have grains with various deformities, shrunken and with pink-white coloration.
3. Loose Smut
It is one of the common disease of wheat across wheat growing areas of the world. The disease occurrence depends on environment conditions especially at ear head emergence as well as varieties grown.
Light showers combined with cool, humid climate especially at flowering time favour the loose smut infection. The entire earhead will be converted into smutted heads affecting yields drastically.
Symptoms:
This disease is caused by fungus Ustilago tritici (Ustilago segetum var. tritici). The disease can be seen when ear head emerges out of the flag leaf, which occurs few days earlier than the healthy ones.
The entire earhead will be converted into dry mass of balck spores. Initially earheads appear as black and can be differentiated from other healthy green spikes. Generally the affected plants will be stunted in growth with less tillers.
All the grains in an earhead will be replaced with black powdery mass of spores. These spores further carried by wind and infect other earheads. The fungal mycelium infect seed embryo and survives as mycelium inside the seed.
The mycelium remains dormant in the seed and serve as primary inoculum for next crop.
Disease management:
Use healthy and certified seeds for sowing. Early roughing of affected plants in the field. Seeds soaked in normal water were dried in during sunny days from morning till evening was also used to manage this disease.
Seed treatment with chemicals carboxin (Vitavax 75WP @ 2.5 g/ kg seed), carbendazim (Bavistin 50WP @ 2.5g/kg seed), tebuconazole (Raxil 2DS @ 1.25g /kg seed),
Seed treatment with combination of Trichoderma viride @ 4g/kg seed and carboxin @ 1.5 g/ kg seed, is effective. Early roughing of infected plants in the field must be done.
4. Alternaria Blight
This disease causes significant yield losses in wheat growing countries of the world. Its epidemic was observed in India during 1964 – 66 in eastern Uttar Pradesh. It is caused by fungus Alternaria triticina and Wheat is the only host. This disease is seed and soil borne and inoculum survives as conidia on the surface of seeds and mycelium inside the seeds.
Symptoms:
Wheat plants of 40-45 days old are generally affected with the disease. The discoloured, oval lesions appear on the lower leaf surface which progresses upwards.
These lesions enlarge and coalesce into irregular, dark blotches, often with chlorotic margins. However, youngest leaves are not usually affected. Severely affected fields appear to be burnt, leaves will become dry and infected seeds are discoloured and shriveled.
The pustules are covered with black spores under humid conditions. A temperature around 250 C, coupled with high humidity, favours the disease.
Management:
- Use only healthy, certified seed for sowing
- Hot water treatment: Seeds soaked in cold water for 4 hours followed by 10 minutes in hot water at 520 C.
- Seed treatment with Vitavax @ 2.5 g/kg seed
- Spray Propiconazole (Tilt 25EC @0.1% 1ml per litre) can control the disease.
5. Yellow Ear Rot or Tundu disease
It was first reported from Punjab province of India and now it occurs in India, Egypt, China and Australia. This disease is caused by the by a bacterium Clavibacter tritici and a nematode Anguina tritici and these bacteria and nematode are never observed individually.
There will not be any grain in the ears of affected wheat.
Symptoms:
Twisting and crinkling of lower and middle leaves of wheat plant due to which growth will be retarded and stem becomes deformed. Yellow slime is produced on the stem and inflorescence, which dries up to form sticky yellow layers and cause curling and twisting of the spikes.
Under humid conditions, oozing of the slime is visible from affected plant tissue. Under dry conditions, the slime becomes hard and turns to dark yellow or brown in colour.
Most of the grains in the ear head are replaced by galls due to nematodes and nematodes can survive upto 10 years in dormant condition in these galls.
The bacteria survive in the nematode which serve as vector. Low temperature of 10-150 C with high humidity of 70-100% the bacterium multiplies and expresses itself in the form of yellow slimes on the leaves of young plants. The disease cycle is repeated when seeds with galls are used for sowing.
Management:
Use of healthy certified seed, roughing and destruction of affected plants and ears, summer ploughing, separation of galls in the wheat seeds by soaking seeds in 2-5% brine solution in which seed galls will float and can be separated easily, Crop rotation in affected fields is also recommended.
6. Ear Cockle Disease
It is caused by a nematode Anguina tritici found throughout the wheat growing countries. It is mostly found in some parts of northern India especially the states of Bihar, Jharkhand, eastern U.P. and Chhatisgarh.
Symptoms:
Basal swelling of the stem at the ground level with a whitish tinge is the first visual symptom further leading to basal enlargement of the stem, rolling, twisting and crinkling of leaves. Infected seedlings are severely stunted and show profuse tillering with a few productive tillers.
The juveniles of the nematode reach growing point and start feeding ecto parasitically. On initiation of the leaf primordia, these nematodes act as endoparasite and initiate production of galls. The galls and the grains develop side by seed in the same floret. In case of severe infections the plant may die prematurely.
Nematode survive in galls and spread year after year through the seed contaminated with cockles or nematode galls. The seed contaminated with cockles or galls when sown, the galls absorb water from the soil, become soft and liberate the second stage juveniles which infect the seedling.
Management:
Use of healthy certified seed, roughing and destruction of affected plants and ears, summer ploughing, separation of galls in the wheat seeds by soaking seeds in 2-5% brine solution in which seed galls will float and can be separated easily, Crop rotation in affected fields is also recommended.
Authors:
Prakasha T.L.
ICAR-Indian Agricultural Research Institute, Regional Station, Indore-452001
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