Leaf Blight of wheat is caused by infection of which of the following fungus ?

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Correct — option (2), Alternaria triticina. This is the fungus for which the disease is named: it was described as a new species in India in 1962 by Prasada and Prabhu, in a paper titled for the leaf blight of wheat that it causes, and the species authority Prasada and Prabhu is still carried with the binomial. Alternaria blight is now among the most damaging foliar diseases of wheat in the warmer wheat-growing tracts of eastern and central India. The symptoms begin as small oval discoloured specks on the lower leaves, usually appearing after the plant has begun to form its ear; the specks enlarge into irregular brown lesions with a chlorotic yellow halo, coalesce until whole leaves are scorched, and can spread upward to the flag leaf, which is the leaf that feeds the developing grain. Killing the flag leaf early cuts the period over which the grain can fill, so the loss shows up as shrivelled grain and reduced weight rather than as a reduced number of ears. The fungus survives between seasons on infected crop debris and on seed, and it spreads within a season by air-borne conidia, so warm, humid weather late in the season favours an epidemic. Management therefore combines clean, treated seed and the destruction of stubble with resistant varieties and, where warranted, a foliar fungicide timed to protect the upper leaves. Note that all four options are printed in italics in the paper, as the binomial names of fungi conventionally are.

Why the other options are wrong — (a) Uromyces is a genus of rust fungi, and its familiar hosts are legumes — beans, peas and related crops — rather than cereals. The rusts of wheat belong to a different genus altogether, Puccinia: stem or black rust, leaf or brown rust and stripe or yellow rust are the three that matter in India, and they are among the most historically destructive wheat diseases anywhere. Their symptom is quite unlike a blight in any case: rust appears as raised powdery pustules of coloured spores that burst through the leaf surface, which is why the diseases are named for the rusty deposit they leave on hands and clothing.  (c) Ustilago nuda is a smut fungus — the loose smut of barley, closely allied to the loose smut fungus of wheat — and smut damage looks nothing like leaf blight. Instead of lesions on the foliage, the emerging ear is destroyed and replaced by a mass of black powdery spores that blow away, leaving a bare rachis. The infection is also carried differently: loose smut is seed-borne, the fungus lying dormant within the embryo of an apparently healthy grain, which is why it is controlled by seed treatment or hot-water treatment of seed rather than by any spray applied to the leaves.  (d) Helminthosporium sativum, now more usually placed as Bipolaris sorokiniana with the sexual stage Cochliobolus sativus, is the nearest of the three wrong choices and the one a candidate is most likely to be caught by, because it too attacks wheat foliage in warm, humid conditions and is part of the same broad foliar disease problem in the eastern plains. Its disease, however, is conventionally named spot blotch, and the same fungus is also responsible for seedling blight and common root and foot rot. The species erected specifically for the leaf blight of wheat, and named in the paper that described it, is Alternaria triticina.

Concept: Wheat diseases are grouped by the tissue they attack and by the symptom they produce, and the grouping is the fastest way to keep the fungi apart. The rusts, caused by Puccinia species, erupt as powdery pustules on leaves and stems and are dispersed as air-borne spores that can travel long distances, which is why rust epidemics move across regions in a season. The smuts and bunts, caused by Ustilago and Tilletia species, destroy the ear and replace the grain with spores; they are typically seed-borne or soil-borne and are managed at sowing rather than in the standing crop. The blights and blotches, caused by Alternaria and Bipolaris among others, produce necrotic lesions on the leaf blade that spread and coalesce, killing photosynthetic tissue during grain filling. That last group does its damage indirectly: the ear is already formed, so the loss is not in the number of grains set but in how well they fill, and the flag leaf matters disproportionately because it supplies much of the carbohydrate that goes into the grain. Management follows the mode of survival and spread — treated seed against seed-borne pathogens, stubble destruction and rotation against debris-borne ones, resistant varieties against all, and timed foliar fungicide where the economics justify it.

State PCS 2024 Q57 — Science & Technology PYQ with Answer

State PCS 2024 Q57 — Science & Technology PYQ with Answer