MPSC 2024 Prelims Paper-I · Question 57 of 100

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.

• (1)Uromyces tritici — 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.
• (3)Ustilago nuda — 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.
• (4)Helminthosporium sativum — 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.

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.

MPSC's agriculture and general science questions test crop-to-pathogen matching, and the Commission draws on the standard list of Indian crop diseases taught in agricultural science: leaf blight and the three rusts of wheat, blast and bacterial leaf blight of rice, late blight of potato, red rot of sugarcane, tikka disease of groundnut, wilt of pigeonpea. The safest preparation is a table of crop, disease, pathogen and, where it is distinctive, symptom, because the Commission sometimes asks for the pathogen from the disease and sometimes the reverse. Where two fungi attack the same crop and the same tissue, as here, the distinguishing detail is the name of the disease each is conventionally credited with. It is also worth recognising the genera themselves: Puccinia means rust, Ustilago means smut, and Alternaria and Bipolaris mean necrotic leaf lesions, so even an unfamiliar species name can be placed by its genus.

• Leaf blight of wheat is caused by Alternaria triticina, described as a new species from India in 1962 by Prasada and Prabhu, whose names are carried as the species authority.
• The disease appears as small oval discoloured specks on the lower leaves that enlarge into irregular brown lesions with a yellow halo, coalescing until whole leaves including the flag leaf are scorched.
• Damage falls on grain filling rather than on grain number, because the lesions destroy photosynthetic tissue after the ear has formed, producing shrivelled and light grain.
• The pathogen survives on infected crop debris and on seed and spreads within the season by air-borne conidia, so warm humid weather late in the crop favours epidemics in eastern and central India.
• The other options belong to different disease groups — Uromyces is a rust genus of legumes while wheat rusts are Puccinia species, Ustilago nuda is a loose smut fungus that destroys the ear, and Helminthosporium sativum causes spot blotch and root and foot rot.

• Choosing another wheat pathogen that also attacks the leaf, when the disease named in the stem belongs to a specific species
• Confusing the disease groups — rusts produce powdery pustules, smuts destroy the ear, blights produce necrotic lesions on the leaf blade
• Assuming a species name containing the word for wheat must be the wheat pathogen in question, since several genera carry such epithets
• Overlooking the genus as a clue, when Puccinia, Ustilago and Alternaria each signal a distinct kind of disease before the species is even read

Crop disease questions in MPSC papers are one-line matching items — name the pathogen of a named disease, or the disease caused by a named pathogen — and the distractors are always genuine pathogens, usually of the same crop or of a closely related one. Because the examinable list is short and stable, this is among the most efficiently prepared corners of the agriculture syllabus. Expect binomials to be printed in italics as they are here, expect the occasional outdated genus name to be used, and expect at least one distractor drawn from a different disease group of the same crop.

No directly related past PYQ was found.

• practice — not a real PYQ

  Loose smut of wheat differs from most foliar diseases of the crop in that it is controlled chiefly by which of the following measures ?

  • (a)Foliar fungicide spray at the flag leaf stage
  • (b)Seed treatment, because the fungus is carried within the embryo of the grain
  • (c)Flooding the field before sowing
  • (d)Removal of alternate hosts from field bunds

  Answer(b) Seed treatment, because the fungus is carried within the embryo of the grain — loose smut is seed-borne and lies dormant inside apparently healthy seed, so a spray applied to the standing crop cannot reach it. Chemical or hot-water treatment of seed before sowing is the standard control. Removal of alternate hosts belongs to the management of certain rusts.

• practice — not a real PYQ

  Stem rust, leaf rust and stripe rust of wheat are caused by species belonging to which of the following fungal genera ?

  • (a)Alternaria
  • (b)Ustilago
  • (c)Puccinia
  • (d)Fusarium

  Answer(c) Puccinia — the three wheat rusts are Puccinia graminis for stem or black rust, Puccinia triticina for leaf or brown rust and Puccinia striiformis for stripe or yellow rust, all producing powdery pustules of coloured spores that burst through the plant surface. Alternaria causes leaf blight, Ustilago the loose smut, and Fusarium head blight and wilts.