Backyard Grapes Identify & treat grapevine problems
Fungal moderate severity

Botrytis bunch rot (grey mould)

Botrytis cinerea

Fuzzy grey mould that rots ripening clusters, especially after rain near harvest.

Chardonnay grapes with Botrytis infection in the middle of the cluster Photo: Patty Skinkis, Oregon State University Oregon State University Extension Service
Severe Botrytis bunch rot on grape clusters of cv. Vignoles Photo: Wayne Wilcox Cornell University, Appellation Cornell (Research Focus 2016-4)
Typical symptoms of Botrytis bunch rot on a grape cluster Photo: Timothy Miles, MSU Michigan State University Extension

Description

Botrytis (Botrytis cinerea) is an opportunist: it attacks fruit that is already ripe, split, or wounded, and races through tight clusters in damp weather. On wine grapes in the right dry conditions it becomes the prized “noble rot”, but for a home grower it usually just means lost fruit.

Because it exploits injuries, the best control is indirect — keep clusters loose and dry, and stop the powdery mildew, birds, and insects that crack the berries open in the first place.

What to look for

  • Soft, brown, watery rot on ripening berries.
  • A fuzzy grey-brown mould coating that puffs into dust when touched.
  • Rot spreads berry-to-berry through a tight cluster.
  • Splits, wounds, and bird/insect damage are common entry points.

Treatment options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • Open the fruit-zone canopy (leaf removal) so clusters dry quickly.
  • Choose loose-clustered varieties; thin tight clusters.
  • Control powdery mildew and insects that wound berries — the usual entry points.
  • Harvest promptly and discard rotten berries.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • Reduce humidity and injuries first; sprays are secondary for grey mould.

Conventional fungicide / pesticide

For severe cases. Always follow the product label.

  • Botrytis-specific fungicides at bloom, bunch closure, veraison, and pre-harvest.
  • Rotate FRAC groups — Botrytis develops resistance quickly.

Sources

This entry is based on the references below. Read them for full detail and for spray timings and rates valid where you live.

  1. Botrytis Bunch Rot in Commercial Washington Grape Production: Biology and Disease Management — Washington State University Extension
  2. How to prevent Botrytis bunch rot in the vineyard — Oregon State University Extension Service
  3. Botrytis Bunch Rot / Grape / Agriculture: Pest Management Guidelines — UC Statewide IPM Program (UC ANR)
  4. Botrytis Bunch Rot on Grapes in Home Gardens — Penn State Extension