Backyard Grapes Identify & treat grapevine problems
Fungal high severity

Black rot

Guignardia bidwellii

Tan leaf spots ringed with black dots, and berries that shrivel into hard black mummies.

Black rot leaf lesions: tan spots with a dark border and black pycnidia Photo: Cesar Calderon, USDA APHIS PPQ, Bugwood.org Bugwood.org CC BY-NC 3.0 US
Berries at three stages: healthy, soft brown rot, and hard black mummies Photo: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org Wikimedia Commons Public domain
Cluster with about half the berries shrivelled into black rot mummies Photo: University of Georgia Plant Pathology, Bugwood.org Bugwood.org CC BY-NC 3.0 US
Black rot (Guignardia bidwellii) symptoms on grapevine berries and leaf Photo: Daniel Molitor Wikimedia Commons CC BY 2.0 DE

Description

Black rot (Guignardia bidwellii) can destroy an entire crop in a wet year, yet is very manageable with sanitation. The disease survives winter almost entirely in the shrivelled “mummies” left hanging on the vine and on the ground — so removing them is the highest-impact thing a home grower can do.

Infections start on the leaves in spring, but the damage that matters happens on the young berries. Once a berry turns into a hard black mummy it is a spore factory for next season.

What to look for

  • Small tan/brown circular leaf spots with a dark border and tiny black specks in the centre.
  • Berries develop a pale spot, then rot, shrivel, and harden into black "mummies".
  • Mummified berries stay attached to the cluster.
  • Dark sunken lesions can also girdle young shoots.

Treatment options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • Remove and destroy every mummified berry and infected cane — this is the main source.
  • Rake up fallen debris; keep the canopy open and dry.
  • Prune out cane lesions during dormancy.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • Copper or sulphur-based preventive sprays from early shoot growth through fruit set.

Conventional fungicide / pesticide

For severe cases. Always follow the product label.

  • Protectant/systemic fungicides (e.g. mancozeb early, then labelled products) from bud break to veraison.
  • The critical window is bloom to a few weeks after fruit set.

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. Grape Black Rot — Ohio State University Extension
  2. Black Rot of Grape — University of Maine Cooperative Extension
  3. Black rot on grapes (Guignardia bidwellii) — University of Minnesota Extension
  4. Online Guide to Grapevine Diseases: Black rot — Virginia Tech, AHS Agricultural Research and Extension Center