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Grape powdery mildew on bunch Photo: Wikimedia contributor · CC BY-SA 3.0 · source ↗

Grape powdery mildew (Erysiphe necator)

Grape powdery mildew is caused by the ascomycete Erysiphe necator (older name Uncinula necator). Unlike downy mildew, which is a monsoon-foundation problem, powdery mildew is the principal bunch-quality disease of the dry forward-pruning fruiting cycle in Maharashtra. It attacks Thompson Seedless and its mutants heavily and is the single biggest reason for fungicide use during the marketable cycle.

Identification and symptoms

White to grey, dusty mycelial colonies appear on the upper and lower surfaces of leaves, on shoots, on the rachis and on developing berries. Affected young leaves curl and become leathery. On berries, infection produces a fine grey-white powdery bloom that scars the skin and, in heavy infections, splits the berry as it expands - exposing the pulp to secondary rot. Late-season infection causes brown corky netting on the rind, leading to outright rejection of export bunches.

Hosts and lifecycle

E. necator is biotrophic on Vitis. In Indian conditions it overwinters primarily as mycelium in dormant buds (the "flag-shoot" form) and survives on canes. Conidia germinate at 6-32 deg C with optimum infection at 22-27 deg C under high humidity but - critically - free water on the leaf inhibits the fungus, making powdery mildew a dry-weather problem in contrast to downy mildew. The forward-pruning cycle from October-March, with cool nights and warm dry days, is therefore ideal.

Damage and economic impact

Berry-scarring powdery mildew at full bloom can cause complete loss of marketable export quality; even cosmetic damage drops fruit from export to domestic-grade prices. NRCG records routine 20-40% loss in unsprayed Thompson Seedless (Grapes Thompson Seedless) blocks.

Management

ICAR-NRCG and growers use an integrated programme:

  • Cultural: open the canopy through forward-pruning and shoot positioning; avoid excessive nitrogen which promotes soft susceptible tissue; remove flag-shoots in spring.
  • Sulphur: wettable sulphur 0.2-0.3% or micronised sulphur dust 25-30 kg/ha at the start of forward-pruning growth, repeated through bloom; avoided at temperatures above 33 deg C to prevent scorch.
  • DMI fungicides (triazoles): myclobutanil, hexaconazole, tebuconazole, penconazole - rotate; resistance documented.
  • QoI fungicides: azoxystrobin, kresoxim-methyl, trifloxystrobin used early in the season; high resistance risk.
  • SDHI fungicides: boscalid, fluxapyroxad, fluopyram - newer, but rotate strictly with non-SDHI partners.
  • Bicarbonates and biocontrol: potassium bicarbonate sprays and Ampelomyces quisqualis-based biofungicides are used in MRL-sensitive (Grapes Export Residue Mrl Europe) export blocks close to harvest.

See also: Grapes Downy Mildew Plasmopara, Grapes Anthracnose Elsinoe, Grapes Export Residue Mrl Europe, Grapes Thompson Seedless.

Sources

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