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Grape downy mildew Photo: Boris Hamer · Pexels License · source ↗

Grape downy mildew (Plasmopara viticola)

Downy mildew, caused by the oomycete Plasmopara viticola, is the single most damaging foliar disease of grapevine in the Indian commercial belts during the south-west monsoon. ICAR-NRC Grapes Pune ranks it as the principal driver of the dense fungicide spray calendar that growers follow during the April-September foundation phase under the double-cycle pruning system.

Identification and symptoms

The disease attacks all young green tissue. On the upper surface of leaves it produces pale-yellow, oily, translucent "oil spots", which on humid nights are covered on the underside by a characteristic white, downy mat of sporangiophores. As the disease progresses, lesions turn necrotic and brown, leaves drop prematurely, shoots and tendrils show oily streaks, and infected inflorescences and young berries shrivel into hard brown "leather berries", with severe yield loss. Older berries become resistant after veraison but rachis infection can still cause bunch collapse.

Hosts and lifecycle

P. viticola is an obligate biotroph of Vitis species. It overwinters as thick-walled oospores in fallen leaves and debris and as systemic mycelium in canes. Primary infection occurs when oospores release zoospores during warm wet weather in June-July, after at least 10 mm of rain on soil at temperatures above 11 deg C. Secondary infection is repeated through the monsoon, with the latent period falling to 4-5 days at 22-25 deg C and high humidity - typical of foundation-phase weather in Nashik and Sangli. Spore dispersal is by rain-splash and wind.

Damage and economic impact

Unmanaged downy mildew can defoliate the canopy before forward-pruning, leading to bud necrosis, poor cane maturity and 30-60% yield loss in the subsequent fruiting cycle. On Thompson Seedless (Grapes Thompson Seedless) and its mutants, which are all highly susceptible, the disease is the dominant economic threat of the monsoon.

Management

ICAR-NRCG recommends an integrated programme:

  • Cultural: prune to open canopy; remove and burn fallen leaves and infected shoots; avoid sprinkler irrigation; install protected-cultivation poly-covers (Grapes Bagging Trellis Protected) on premium blocks.
  • Forecast-guided spraying: NRC Grapes runs a decision-support model based on rain, temperature and leaf wetness; growers spray when the risk index crosses a threshold rather than on a calendar.
  • Contact protectants: Bordeaux mixture 1% or copper oxychloride 0.25-0.30% or mancozeb 0.25% before rain events; rotate.
  • Systemic curatives: phenylamides (metalaxyl + mancozeb), CAA (mandipropamid, dimethomorph), QoI (azoxystrobin), QiI (cyazofamid, amisulbrom), fluopicolide + propamocarb - used in rotation with strict resistance management.
  • Export residue control: respect pre-harvest intervals and EU MRLs (Grapes Export Residue Mrl Europe); maintain spray diary on GrapeNet (Grapenet Apeda Traceability).

See also: Grapes Powdery Mildew Uncinula, Grapes Anthracnose Elsinoe, Grapes Bagging Trellis Protected, Grapes Export Residue Mrl Europe.

Sources

  1. ICAR-NRC Grapes Pune institutional portal.
  2. Downy mildew of grapevine. CABI Plantwise Knowledge Bank.