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Mustard Alternaria blight Photo: placeholder pending image-fill pass

Mustard Alternaria blight (Alternaria brassicae / A. brassicicola)

Alternaria blight, also called black spot or dark leaf spot of rapeseed-mustard, is caused chiefly by Alternaria brassicae (Berk.) Sacc. and to a lesser extent A. brassicicola (Schw.) Wiltshire and A. raphani. It is the most widely distributed foliar disease of Indian mustard, ranking second only to mustard aphid in economic importance in Rajasthan, Madhya Pradesh, Uttar Pradesh, Haryana and Punjab. Average yield losses are 15-35 percent on susceptible varieties; in epidemic years on late-sown crops, losses cross 50 percent.

Identification and symptoms

Symptoms appear 35-50 days after sowing on lower leaves as small grey to brown circular spots, 1-3 mm, that enlarge to 1-2 cm with characteristic concentric rings (target-board pattern) and a chlorotic halo. Spots coalesce, leaves dry up and fall prematurely. On stems and pods, lesions are elongated, dark and may bear olivaceous spore masses in humid weather. Pod infection causes shrivelled discoloured seed with reduced oil content; under severe attack, pods crack and shatter at maturity.

Pathogen and lifecycle

A. brassicae produces large multi-celled obclavate conidia (170-300 µm long); A. brassicicola has smaller darker conidia in chains; both are Hyphomycetes. The fungus over-seasons on infected seed (mainly A. brassicicola), on crop debris in the field and on cruciferous weeds. Conidia are dispersed by wind, splash and irrigation water. Disease development requires 18-25 deg C with 8-12 hours of leaf wetness from dew or fog; the latent period is 3-5 days. Late-sown crops that flower under foggy, drizzly January conditions face the most severe epidemics.

Hosts

The pathogens infect a wide range of Brassicaceae — Indian mustard, toria, gobhi sarson, taramira, raya, cabbage, cauliflower, radish, knol-khol — and several cruciferous weeds. There is partial host-specificity: A. brassicae is more aggressive on oilseed Brassicas while A. brassicicola tends to be more aggressive on vegetable Brassicas.

Economic impact

ICAR-DRMR yield-loss studies report 15-35 percent average loss on susceptible varieties, with up to 47-58 percent reduction in seed yield, 2-5 percentage-point reduction in oil content and 15-25 percent reduction in 1000-seed weight in severely infected crops. Pod-stage infection is the main quantitative determinant of loss.

Management

  • Resistant varieties: no commercial variety is fully resistant; DRMR-2035, RH-749, Pusa Mustard 28, RGN-298 and RGN-73 show useful field tolerance.
  • Cultural: timely sowing (10 October-5 November) so flowering escapes peak disease window; use disease-free certified seed; deep summer ploughing to bury crop debris; balanced fertilisation avoiding excess nitrogen; remove cruciferous weeds from field bunds.
  • Seed treatment: thiram + carbendazim at 2:1 ratio @ 3 g/kg seed, or iprodione 50 WP @ 2.5 g/kg seed eliminates seed-borne inoculum.
  • Chemical: prophylactic foliar spray of mancozeb 75 WP at 2.5 g/L or chlorothalonil 75 WP at 2 g/L at 45-50 DAS (rosette stage), repeated at 65-70 DAS (flowering) and 80-85 DAS (pod-filling). For curative action, propiconazole 25 EC at 1 ml/L, tebuconazole 25 EC at 1 ml/L or difenoconazole 25 EC at 0.5 ml/L. Rotate fungicide modes.
  • Integrated: combine resistant variety + clean seed + timely sowing + need-based mancozeb spray for cost-effective control.

See also: Mustard crop, Pusa Bold mustard, Mustard aphid, NMOOP.

Sources

  1. Disease Management in Rapeseed-Mustard. ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur.
  2. Alternaria brassicae datasheet. CABI Compendium.
  3. AICRP-Rapeseed-Mustard PC Report. DRMR, Bharatpur.