MPSC 2025 Prelims Paper-I · Question 58 of 100

Correct — option (1), Bacteria. Citrus canker is caused by the bacterium Xanthomonas citri subsp. citri (also referred to in older and regional literature as Xanthomonas axonopodis pv. citri), a rod-shaped, gram-negative bacterium. The pathogen enters citrus leaves, young stems and fruit through natural openings (stomata) or wounds, then multiplies in the intercellular spaces just beneath the surface, producing the disease's characteristic raised, corky, necrotic lesions, often ringed by a yellow (chlorotic) halo, on leaves, twigs and fruit. Severe infection causes defoliation, dieback, blemished and prematurely dropped fruit, and reduced yield, making citrus canker a serious quarantine disease for citrus-growing regions worldwide, actively managed through copper-based bactericide sprays, sanitation, wind-break planting to limit wind-driven spread, and destruction of infected material.

• (2)Fungi — Citrus trees are certainly susceptible to several fungal diseases — for example, citrus scab and various fungal fruit rots — but citrus canker specifically is not one of them; its causal organism is the bacterium Xanthomonas citri subsp. citri, not a fungal pathogen.
• (3)Virus — Citrus is affected by well-known viral diseases too, most notably citrus tristeza virus, which causes a distinct set of symptoms (stem pitting, quick decline of trees on sour-orange rootstock). Citrus canker, however, is a bacterial disease, and its lesion pattern and disease cycle are characteristic of a bacterial, not a viral, pathogen.
• (4)Nematode — Plant-parasitic nematodes such as the citrus nematode (Tylenchulus semipenetrans) do attack citrus, but they infest roots and cause a distinct disease called citrus slow decline, affecting root function and overall tree vigour. They are not the cause of citrus canker's characteristic leaf, stem and fruit lesions, which are produced by bacterial infection.

Citrus canker is caused by the gram-negative, rod-shaped bacterium Xanthomonas citri subsp. citri, which infects citrus plants through stomata or wounds and colonises the intercellular spaces of leaf, stem and fruit tissue, producing raised, corky lesions typically surrounded by a yellow halo. The disease spreads locally via wind-driven rain, and over longer distances through movement of infected plant material, contaminated equipment, or, historically, citrus leaf-miner damage that creates additional wound entry points for the bacterium. Because it can devastate commercial citrus orchards and is easily spread, many countries treat citrus canker as a quarantine pest, with eradication and containment programmes (including destruction of infected and exposed trees) rather than relying on chemical control alone.

MPSC's biology section frequently tests which category of pathogen (bacteria, fungus, virus, nematode) causes a named, economically significant plant disease, since distinguishing these categories by their characteristic disease patterns is a recurring theme in agricultural science questions relevant to Maharashtra's substantial citrus-growing regions.

• Citrus canker is caused by the bacterium Xanthomonas citri subsp. citri (also called Xanthomonas axonopodis pv. citri in some literature).
• The disease produces raised, corky, necrotic lesions with a yellow (chlorotic) halo on citrus leaves, stems and fruit.
• The bacterium enters host tissue through stomata or wounds and spreads locally via wind-driven rain.
• Citrus canker is treated as a quarantine disease in many citrus-growing countries, managed through copper-based bactericides, sanitation and destruction of infected material.

• Confusing citrus canker (bacterial) with citrus greening/Huanglongbing (also bacterial, but caused by a different genus, Candidatus Liberibacter, and spread by a psyllid insect vector) — both are serious bacterial citrus diseases but are distinct
• Assuming all major citrus diseases are fungal, when several of the most economically damaging ones (canker, greening) are bacterial
• Mixing up nematode-caused root diseases (citrus slow decline) with the bacterial leaf/stem/fruit disease that is citrus canker

MPSC's agriculture-adjacent biology questions frequently ask which pathogen category causes a named crop disease, testing precise attribution rather than general awareness that a disease exists.

No directly related past PYQ was found.

• practice — not a real PYQ

  The bacterium responsible for citrus canker belongs to which genus ?

  • (a)Xanthomonas
  • (b)Pseudomonas
  • (c)Ralstonia
  • (d)Erwinia

  Answer(a) Xanthomonas — the specific causal organism is Xanthomonas citri subsp. citri.

• practice — not a real PYQ

  Citrus greening disease (Huanglongbing) is caused by which type of pathogen ?

  • (a)Fungus
  • (b)Virus
  • (c)Bacteria (Candidatus Liberibacter species)
  • (d)Nematode

  Answer(c) Bacteria (Candidatus Liberibacter species) — like citrus canker, citrus greening is bacterial, but caused by a different genus and spread by the Asian citrus psyllid insect vector, not wind-driven rain.