Why Are My Grapes Going Black and Shrivelling?

Grapes that turn brown or black and then shrivel into hard, raisin-like mummies while still on the vine are almost certainly infected with black rot (Guignardia bidwellii). This fungal disease is particularly destructive because it can affect berries, leaves, and stems simultaneously, and because infected berries that remain on the vine or fall to the ground serve as a persistent source of inoculum for the following season. Early identification and thorough sanitation are the most effective control measures.

Identifying black rot symptoms

On leaves, black rot produces small, roughly circular tan or brown spots with a darker brown or black border. As spots enlarge, tiny black dots (pycnidia — the spore-producing bodies of the fungus) appear within the lesion, giving the spot a speckled appearance. On green berries, a small circular pale spot forms, rapidly spreads, and the berry turns brown, then purple-black, and finally shrivels into a hard black mummy. Infected shoots and tendrils may show dark, lens-shaped cankers.

Conditions that favour spread

Black rot spores require free water (rain, dew, or irrigation) and temperatures between 10–30°C to germinate and infect. The critical infection window is from bud burst through four to five weeks after fruit set — young, rapidly growing tissue is most susceptible. A wet spring and early summer creates ideal conditions for severe outbreaks. Later in summer, berries with toughened skin become more resistant.

Sanitation — the most important step

Remove and destroy all mummified berries, whether still on the vine or fallen on the ground. Do not compost them — they overwinter successfully and release spores the following spring. Prune out and remove any cankered stems during winter dormancy. These sanitation steps dramatically reduce the inoculum load entering next season.

Fungicide treatment

Preventative fungicide applications are necessary in areas where black rot has been a problem. Myclobutanil, tebuconazole, and mancozeb are effective. Begin spraying at bud burst and continue on a 10–14 day schedule through early summer (the key infection period). Copper-based fungicides provide moderate activity and are the organic option. Timing is critical — sprays must be applied before infection occurs.

Choosing resistant varieties

American-derived grape varieties (Vitis labrusca types) and hybrid varieties often carry significantly more resistance to black rot than European wine varieties (Vitis vinifera). If black rot is a persistent problem in your area, choosing a hybrid variety bred in a similar climate significantly reduces the pressure.