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Citrus canker lesions on leaves and fruit Photo: Plant pests and diseases · CC0 · source ↗

Citrus canker (Xanthomonas citri)

Citrus canker is a bacterial disease of citrus caused by Xanthomonas citri pv. citri (formerly X. axonopodis pv. citri). It is one of the most damaging foliar and fruit diseases of sweet orange in India and is a major constraint on Sathgudi (Sathgudi Sweet Orange) and Mosambi (Citrus Mosambi Sweet Lime) orchards across the YSR Kadapa belt, Vidarbha and Marathwada, particularly during humid south-west monsoon months. The disease affects leaves, twigs and fruit, reducing fruit quality, marketability and tree vigour.

Identification and symptoms

Symptoms begin as small, water-soaked spots on the underside of young leaves which enlarge into raised, corky, brown lesions surrounded by a characteristic yellow halo. Lesions develop on both leaf surfaces and on twigs and fruit. Fruit lesions are scabby and corky, scarring the rind and rendering the fruit unmarketable in the fresh-juice trade. Severe infection causes premature defoliation, twig dieback and reduced yield. Lemons and acid limes are highly susceptible; sweet orange (including Sathgudi and Mosambi) is highly susceptible; mandarin orange is moderately susceptible; pummelo and trifoliate are relatively tolerant.

Hosts and lifecycle

The pathogen survives on infected leaves, twigs and fallen plant debris. Bacteria are dispersed by wind-driven rain over short distances (typically within an orchard) and over longer distances by movement of infected planting material, harvested fruit and contaminated tools. Bacteria enter through stomata, hydathodes and wounds caused by thorns, wind injury or the citrus leaf miner (Phyllocnistis citrella) — leaf miner tunnels are the single most important entry route in young flush. Optimum conditions for spread are 25-30 °C with prolonged leaf wetness, conditions that prevail from June to September across Rayalaseema and Vidarbha.

Damage and economic impact

Direct yield losses of 10-30% are routine in unmanaged orchards, with fruit-quality losses higher because canker-scarred fruit fetches half-price or less in the fresh-juice market. Severe outbreaks accelerate tree decline, particularly when combined with Phytophthora gummosis (Citrus Gummosis) and HLB / greening (Citrus Greening Hlb Candidatus). Canker is a notifiable quarantine disease and presence in an orchard prevents export of fresh fruit.

Management

ICAR-CCRI Nagpur and NHB recommend an integrated programme:

  • Indexed planting material: source budded plants from accredited disease-free nurseries — the single most effective preventive measure.
  • Cultural / sanitation: prune and burn infected twigs in winter (December-January); disinfect pruning tools with 1% sodium hypochlorite between trees; maintain wide tree spacing and pruned canopy to reduce humidity (Citrus Spacing Canopy Collapse).
  • Leaf miner control: as leaf miner injury is the dominant entry point, control flush-time leaf miner with imidacloprid (0.005%) or quinalphos (0.05%) sprays during the May-September flush periods.
  • Copper-based bactericides: prophylactic spray of copper oxychloride 0.3% or Bordeaux mixture 1% at the start of each flush, repeated at 15-day intervals during the high-risk monsoon period. Streptocycline (100 ppm) + copper oxychloride (0.3%) tank-mix is used in outbreaks; rotate to avoid resistance.

See also: Sathgudi Sweet Orange, Citrus Gummosis, Citrus Greening HLB Candidatus.

Sources

  1. Citrus Canker. ICAR-Central Citrus Research Institute, Nagpur.
  2. Citrus Canker. National Horticulture Board.
  3. Citrus Canker. CABI Plantwise Knowledge Bank.