Q.61 Which of the following combinations of plant diseases and the types of their causal
organisms is/are CORRECT?
(A) Late blight of potato − Bacteria
(B) Black rot of crucifer − Bacteria
(C) Tungro disease of rice − Mycoplasma
(D) Root knot of tomato − Nematode
Correct options are (B) and (D).
Late blight of potato is caused by the oomycete Phytophthora infestans, not bacteria, making option (A) incorrect. Black rot of crucifers is caused by the bacterium Xanthomonas campestris pv. campestris, confirming option (B) as correct. Tungro disease of rice results from two viruses—Rice Tungro Bacilliform Virus (RTBV) and Rice Tungro Spherical Virus (RTSV)—transmitted by leafhoppers, not mycoplasma, so option (C) is incorrect. Root-knot of tomato is caused by nematodes like Meloidogyne species, verifying option (D) as correct.
Option Analysis
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(A) Late Blight of Potato: This disease features water-soaked leaf lesions and white mold under humid conditions, caused by P. infestans, a fungus-like oomycete that produces sporangia for spread. It historically devastated Irish potato crops in the 1840s but is managed today with fungicides and resistant varieties.
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(B) Black Rot of Crucifer: Affects brassicas like cabbage with V-shaped yellow lesions and black veins from bacterial entry via wounds or hydathodes. The pathogen survives in seeds, debris, or weeds, thriving in warm, moist weather.
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(C) Tungro Disease of Rice: Shows orange-yellow leaves, stunting, and reduced tillers; RTBV causes main symptoms while RTSV aids transmission by green leafhoppers like Nephotettix virescens. Earlier misattributed to mycoplasma, now confirmed viral.
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(D) Root Knot of Tomato: Results in galls on roots from nematode juveniles feeding on vascular tissue, causing stunting and yield loss; common species include M. incognita.
Plant diseases and causal organisms represent critical knowledge for agriculture students and CSIR NET Life Sciences aspirants tackling competitive exams. Understanding mismatches—like late blight of potato not being bacterial—helps differentiate pathogens accurately. This guide breaks down four major diseases, their true causal agents, symptoms, and control, drawing from verified sources.
Disease Causatives
Each disease links to distinct pathogen types, vital for exam questions on plant pathology:
| Disease | Stated Organism | Actual Causal Organism | Key Notes [web:id] |
|---|---|---|---|
| Late Blight of Potato | Bacteria | Oomycete (Phytophthora infestans) | Sporangia spread in wet conditions |
| Black Rot of Crucifer | Bacteria | Bacterium (Xanthomonas campestris pv. campestris) | Seed/ debris survival |
| Tungro Disease of Rice | Mycoplasma | Viruses (RTBV + RTSV) | Leafhopper vectors |
| Root Knot of Tomato | Nematode | Nematode (Meloidogyne spp.) | Root galls form |
Symptoms and Spread
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Late blight starts with dark lesions on potato leaves/tubers, escalating in cool, moist weather via airborne sporangia.
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Black rot shows V-lesions on crucifer leaves, black veins, spread by rain splash from infected seeds.
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Tungro causes rice stunting and orange mottling, vectored semi-persistently by leafhoppers.
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Root-knot leads to tomato root galls, nutrient uptake block, worsened by soil temperatures above 25°C.
Management Strategies
Control relies on pathogen type:
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Fungicides/oomyceticides for late blight; resistant cultivars like Kufri Jyoti.
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Seed treatment, rotation, copper bactericides for black rot.
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Resistant rice varieties (e.g., TKM6), leafhopper management for tungro.
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Soil solarization, nematicides, marigold rotation for root-knot.
Mastering plant diseases and causal organisms boosts exam scores—focus on viruses vs. mycoplasma distinctions for tungro.
