Backyard Grapes Identify & treat grapevine problems
Fungal moderate severity

Bitter rot

Greeneria uvicola

A late-season berry rot that softens ripe fruit and leaves it bitter — often riding in behind other rots.

Bitter rot on a grape cluster Photo: Andy Allen, University of Missouri eXtension (Grapes community of practice)
Grape berries infected with bitter rot Ohio State University Extension (Ohioline)

Description

Bitter rot (Greeneria uvicola) is a warm-climate berry rot that shows up late, as the fruit ripens, and — true to its name — leaves affected berries and any resulting wine tasting bitter. It often travels with the other warm-season rots and takes advantage of berries already weakened by black rot, ripe rot, or feeding injury, so it’s easy to miss as a distinct problem.

The fungus overwinters in mummified berries and dead wood, then spreads in warm rain, so the controls are the familiar ones for the rot complex: sanitation, an open canopy that dries fast, prompt harvest, and preventive sprays timed to the ripening window. Growers in hot, humid regions should watch for it; in cool, dry climates it is rarely a concern.

What to look for

  • Soft, browning rot on ripe and ripening berries, usually starting near the stem end.
  • Whole berries collapse; a bitter taste in affected fruit and any wine made from it.
  • Fine dark fruiting bodies over the rotted surface, sometimes with grey spore masses.
  • Rotted berries dry to mummies that cling to the cluster.

Treatment options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • Remove mummified berries and prune out dead wood, the main overwintering sites.
  • Open the canopy so clusters dry quickly after rain.
  • Harvest promptly and avoid leaving ripe fruit through warm, wet spells.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • Copper-based preventive sprays through ripening in humid climates.

Conventional fungicide / pesticide

For severe cases. Always follow the product label.

  • Late-season protectant fungicides overlap with black-rot and ripe-rot programs; mind pre-harvest intervals.

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. Bitter Rot of Grape — Ohio State University Extension
  2. Bitter Rot of Grapes — University of Kentucky Department of Plant Pathology
  3. North Carolina Winegrape Grower's Guide, Chapter 8: Pest Management — NC State Extension