Diseases of plants caused by fungi
Early Blight of Potato
The disease is quite common in India, and occurs on about three week old plants. Since this blight occurs earlier than the ‘late blight’ of potato (caused by Phytophthora infestans), it is called ‘early blight.’
- Field sanitation and rotation of crops can minimize the disease.
- Regular spray of fungicides like Dithane Z-78 controls the disease.
Blast Disease of Rice
Pathogen Pyricularia Oryzae is the enemy number one of rice crop and occurs in India frequently.
Spindle-shaped spots, ash-coloured in centre with brown margins, appear on the leaves, culms, glumes, and the stalk (neck) of the panicle. The ears are also infected and bend downward due to rotting of the stalk.
Grey Blight of Tea (Thea Sinensis)
This is the most common blight disease of tea leaves and does considerable damage to the crop. First, small brown spots appear which later spread and cover the entire leaf blade. The old lesions turn grey and masses of conidia appear as black dots on the grey background. The infected portions turn brittle and fall off, leaving irregular cuts on the leaves.
- The blighted leaves should be collected and burnt to prevent infection next year.
- Spraying of the crop should be done frequently by Bordeaux mixture.
- Water-logging favours the disease and, therefore, should be avoided.
Loose Smut of Wheat
Plants, produced by internally-infected seeds, contain the hyphae in every nook and corner of their body. But marked symptoms appear only when ‘ears’ come out. Sometimes, the infected plants are stunted in growth. The ears instead of containing grains contain black mass of spores. The sori in the early stages are covered by a thin membrane derived from host tissue but later the membrane ruptures and the spores become a loose mass; hence the name ‘loose’ smut.
- Seeds for sowing should be obtained from reliable places where the disease does not occur.
- If there is any doubt, the seeds may be treated by ‘hot water method’. The method was discovered by Jensen in 1889. Seeds are soaked in shallow warm water contained in pots and then spread out in the scorching sun during the day. The hyphae die due to the heat of the sun, much quicker than the embryo. The safety line is very thin and there is every chance of killing the seeds. But the risk is worth taking.
- Use of resistant varieties is the best method of avoiding the disease.
Black Stem Rust of Wheat
It is a macrocyclic, heteroecious rust as it produces all the five known types of spores (basidiospores, spermatia, aeciospores, uredospores and teliospores) and needs two hosts – wheat and barberry, to complete the life cycle. Uredo-and teliospores are produced on wheat while spermatia and aeciospores are produced on barberry. Basidiospores are produced on promycelium formed by teliospores on germination.
- Rust-resistant varieties of wheat are available and their use is the safest and cheapest method of control.
- For a heteroecious rust, like the black rust of wheat, it might be expected that eradication of one host – the economically unimportant host, barberry, may control the disease by cutting down the life cycle of the fungus.
Smut of Maize
Common smut of corn, caused by Ustilago maydis, is easily identified by tumor-like galls that form on actively growing host tissues and contain masses of dark, sooty teliospores. Throughout most of the world, common smut is considered to be a troublesome disease of corn.
Ustilago maydis infects two hosts: maize (Zea mays) and teosinte (Zea mexicana). Like other smut diseases, common smut of corn derives its name from the sooty masses of teliospores found on infected host plants.
The most obvious symptoms are tumor-like galls that vary in size from less than 1 cm to more than 30 cm (0.4-12 inch) in diameter. All meristematic tissues are susceptible to infection. Galls are found most frequently on ears, tassels, stalks, nodal shoots, and mid-ribs of leaves.
- Use of resistant varieties is the safest and cheapest method of control.
- Soil sanitation- The soil should be sprayed with fungicides to kill the fungus present in the soil.
Late Blight of Potato
The disease is worldwide in occurrence and is also reported from all parts of India. Epiphytotics never occur in the plains, but are frequent in the hills. The non-availability of high relative humidity during the crop season (October-December) prevents the disease occurrence. The fungus, present in potato tubers, fails to survive the high summer temperature in the plains. The disease recurrence occurs through ‘seeds’ (tubers) stored in cold-storages.
The name late blight, is given to this disease because, compared to another disease-early blight (caused by Alternaria solani), it occurs late in the season. The brown spots appear on leaves only at the time of flowering; grow very fast and soon cover the entire foliage. Ultimately, the infection reaches the underground portions of the plant and infects the tubers. Independent infection of tubers also takes place by zoospores present in the soil.
- The seed tubers should be free from the disease. This requires strict seed tuber inspection at the cutting time. This measure will eliminate direct infection.
- Storage of potato tubers in cold storage rooms reduces or even checks the progress of the rot.
- Considerable success has been achieved in the perfection of resistant varieties of potato at the potato breeding station, Simla. Growing these will provide an increasing opportunity to combat the disease.
- Resistance alone has not effectively checked the disease. Therefore the complete control of blight is accomplished by the application of protectant fungicides.
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