UPPSC 2022 Prelims GS Paper-I · Question 34 of 150

Correct — D, A-3, B-4, C-2, D-1. Take the three unambiguous rows first and the fourth follows. A, 'citrus cracker' — the paper's rendering of citrus canker — is bacterial: Xanthomonas citri, a rod-shaped Gram-negative bacterium with polar flagella, which leaves raised corky lesions on fruit, leaves and twigs; so A goes with 3. B, red rot of sugarcane, is the classic fungal disease of the crop, caused by Colletotrichum falcatum (teleomorph Glomerella tucumanensis); it is recorded in some 69 countries and has destroyed one commercial cane variety after another, so B goes with 4. C, the 'Krishnakant' condition of potato, is black heart — not an infection at all but a physiological disorder in which the inside of the tuber darkens and dies because oxygen cannot reach it, in waterlogged soil or in badly ventilated storage; so C goes with 2, deficiency of oxygen. That leaves only 1, insect, for D, the 'Sahu' or 'Sehu' disease of wheat — the ear-cockle and tundu complex. One caution on that last pairing is given below and in the concept note: the actual agent of ear cockle is a nematode, not an insect. The mapping A-3, B-4, C-2, D-1 is option (d).

• (a)A-1, B-3, C-4, D-2 — It misplaces all four. Citrus canker is bacterial rather than an insect problem; red rot is fungal, not bacterial; the potato disorder is a matter of oxygen supply, not a fungus; and wheat's ear-cockle disease is not caused by a lack of oxygen. The single easiest check is red rot — a textbook fungal disease of sugarcane — and this option gets it wrong.
• (b)A-1, B-2, C-3, D-4 — This is the 'straight run' option, matching A to 1, B to 2, C to 3 and D to 4 in order — a pattern worth distrusting on sight. Every pair fails: it makes red rot an oxygen-deficiency disorder and the potato condition bacterial, when it is exactly the reverse.
• (c)A-4, B-1, C-3, D-2 — It swaps the two microbial agents onto the wrong hosts, calling citrus canker fungal and the potato disorder bacterial, and it makes red rot of sugarcane an insect problem. Red rot is caused by Colletotrichum falcatum, a fungus; the borers and the pyrilla are the insect pests of cane, and they are a separate story.

Plant diseases are classified first by the kind of agent behind them, and the four possibilities in List-II cover three of the main classes. Bacteria (citrus canker) and fungi (red rot) are parasitic pathogens, spread from plant to plant. Animal pests — insects, mites and nematodes — are a third class, which damage tissue directly or carry pathogens with them. The fourth entry, oxygen deficiency, belongs to an entirely different family: physiological or non-parasitic disorders, produced by the environment rather than by any organism, and therefore not contagious. Potato black heart is the classic example — waterlogged soil or an unventilated store starves the tuber's interior of oxygen, its cells suffocate, and the flesh blackens from the middle outward.

The route through this item is elimination, and it is worth noting where the examiner has been loose so that a student is not misled. 'Citrus cracker' is not a standard English disease name; it is citrus canker. 'Krishnakant' is the vernacular name for black heart of potato. And the fourth row is genuinely imperfect: the 'Sehu' or 'Sahu' disease of wheat is ear cockle, caused by Anguina tritici, a plant-parasitic nematode three to five millimetres long that turns the grain into brown-to-black galls and can take 30 to 70 per cent of the yield; in the related 'tundu' or yellow-ear-rot form it works together with the bacterium Rathayibacter tritici. A nematode is a roundworm, not an insect. Since List-II offers no 'nematode' entry and the other three factors are each firmly taken, the examiner has filed the nematode under the general animal-pest head, and the elimination still delivers option (d). Learn the row as 'wheat ear cockle — nematode, coded here as insect', not as an insect disease.

• Citrus canker is caused by the bacterium Xanthomonas citri — Gram-negative, rod-shaped, with polar flagella — and shows as raised, corky, water-soaked lesions on fruit, leaves and twigs.
• Red rot of sugarcane is caused by the fungus Colletotrichum falcatum (teleomorph Glomerella tucumanensis); reported from about 69 countries, it has forced the withdrawal of many commercial cane varieties.
• Black heart of potato, the 'Krishnakant' condition, is a non-parasitic physiological disorder caused by oxygen deficiency in waterlogged soil or poorly ventilated storage — the tuber's interior blackens and dies.
• The 'Sehu/Sahu' disease of wheat is ear cockle, caused by the nematode Anguina tritici, with the bacterium Rathayibacter tritici involved in the 'tundu' form. The key files it under 'insect'; the accurate label is nematode, and no List-II option offers that.
• Physiological disorders are distinguished from infectious diseases by not spreading from plant to plant: correcting the environment corrects the disorder.

• Assuming every named plant 'disease' has a microbial cause — black heart is caused by the absence of oxygen, not by an organism
• Reading 'insect' loosely: nematodes and mites are animal pests but they are not insects, and this row is where the key is imprecise
• Picking the option whose pairs run in straight order (A-1, B-2, C-3, D-4); examiners plant it as bait

Both commissions favour the match-list form here — a column of diseases against a column of causal agents — and both mix an odd one in to break a mechanical run. UPPSC leans on crop diseases and vernacular disease names; UPSC has asked the same underlying idea through human diseases and through the agents that spread plant disease.

• practice — not a real PYQ

  Black heart of potato is caused by

  • (a)A soil-borne fungus
  • (b)A bacterial infection carried by seed tubers
  • (c)Deficiency of oxygen in waterlogged soil or poorly ventilated storage
  • (d)A nematode feeding inside the tuber

  Answer(c) Deficiency of oxygen in waterlogged soil or poorly ventilated storage — it is a physiological disorder, not an infection, and so does not spread from tuber to tuber.

• practice — not a real PYQ

  Consider the following pairs of crop disease and causal agent: 1. Red rot of sugarcane — Fungus 2. Citrus canker — Bacterium 3. Ear cockle of wheat — Nematode Which of the pairs given above are correctly matched?

  • (a)1 and 2 only
  • (b)2 and 3 only
  • (c)1 and 3 only
  • (d)1, 2 and 3

  Answer(d) 1, 2 and 3 — Colletotrichum falcatum, Xanthomonas citri and Anguina tritici respectively; note that ear cockle is a nematode disease, which is why an 'insect' label for it is only approximate.