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Sunflower Alternaria leaf spot (Alternaria helianthi) Photo: Alex Yu · CC BY-NC 4.0 · source ↗

Sunflower Alternaria leaf spot (Alternaria helianthi)

Alternaria leaf spot, caused by Alternaria helianthi (and to a lesser extent A. alternata, A. helianthinficiens and A. zinniae), is the most widely reported foliar disease of sunflower in India. It is endemic to all sunflower-growing tracts — Karnataka (Bagalkot, Bijapur, Raichur, Bellary), Andhra Pradesh (Anantapur, Kurnool, Mahabubnagar), Telangana, Maharashtra and Tamil Nadu — and causes 20-80 percent yield loss in late-sown rabi or summer crops when conditions favour epidemic development.

Identification and symptoms

Initial symptoms appear on lower leaves as small dark-brown to black necrotic spots with concentric rings (the "target board" pattern characteristic of Alternaria). Spots enlarge to 0.5-3 cm, often coalescing to give blighted patches. Severely infected leaves dry up and drop prematurely. Lesions also develop on stems (elongated dark stripes), petioles, sepals, ray florets and back of the head, where black spore masses become visible in humid weather. Head infection reduces seed-fill and increases head rot.

Hosts and lifecycle

A. helianthi is restricted to Helianthus species but other Alternaria spp. have broader host ranges across Asteraceae. The fungus over-seasons on infected seed, on crop debris in the field and on volunteer plants. Conidia produced on lesions are dispersed by wind and rain splash. Disease development requires 20-26 deg C with 10-12 hours of leaf wetness; the latent period is 5-7 days. Late-sown crops that flower during a wet phase suffer the heaviest disease pressure.

Economic impact

Yield losses are correlated with disease severity at the head-filling stage. AICRP-Sunflower trials across Karnataka and Andhra Pradesh have recorded 25-60 percent reduction in seed yield on susceptible hybrids when severity exceeds 50 percent leaf area. Oil content is also reduced by 2-5 percentage points and 100-seed weight by 15-25 percent. The disease is the single largest reason for the rabi/summer sunflower acreage decline in Anantapur-Kurnool-Raichur belt.

Management

  • Resistant/tolerant hybrids: KBSH-44, KBSH-1, KBSH-41 and DSH-1 show useful field tolerance; older hybrids like MSFH-17 are moderately susceptible.
  • Cultural: clean seed and seed treatment with thiram + carbendazim 2:1 at 3 g/kg seed; crop rotation with non-host cereals; deep ploughing to bury crop residue; avoid late sowing.
  • Chemical: prophylactic foliar spray of mancozeb 75 WP at 2.5 g/L or chlorothalonil 75 WP at 2 g/L at 35-40 DAS (bud initiation), repeated at 50-55 DAS. For curative action use propiconazole 25 EC at 1 ml/L or tebuconazole 25 EC at 1 ml/L or azoxystrobin 23 SC at 1 ml/L; rotate fungicide modes to delay resistance.
  • Integrated: combine resistant hybrid + clean seed + balanced fertilisation (avoid excess N) + need-based fungicide.

See also: KBSH-44 Sunflower, MSFH-17 Sunflower, Sunflower Crop.

Sources

  1. Diseases of sunflower and their management. ICAR-Indian Institute of Oilseeds Research.
  2. Alternaria helianthi. CABI Compendium.
  3. Sunflower production technology. University of Agricultural Sciences, Dharwad.