Wine Arcana

Concept

Black rot

Black rot is a grapevine disease caused by the fungus Guignardia bidwellii. It can infect leaves, shoots, and fruit, but its most conspicuous economic effect is the loss of berries. Affected fruit can shrivel into hard, dark “mummies” that remain attached to the bunch and help carry the disease into the next season.

From spring infection to damaged fruit

The fungus survives winter in infected vine material, including mummified berries. Rain releases spores, and wet conditions allow infection. Leaf lesions can then produce additional spores that reach the fruit. Warm weather combined with prolonged wetness favours development.

Symptoms on berries may become obvious well after infection. Fruit gradually becomes less susceptible as it develops, with the timing varying by cultivar. This lag explains why a grower may discover damaged clusters after the period when protecting them would have been most effective.

Leaves commonly show round, tan-to-brown spots with darker edges and small black fungal structures. These symptoms differ from the powdery growth of powdery mildew and the yellowish leaf lesions and underside sporulation associated with downy mildew.

Managing the source of infection

Removing infected fruit and old mummies reduces the material from which a new epidemic can start. A vineyard's disease history therefore matters alongside the current weather. Protection during susceptible growth stages and removal of infected material address different parts of the same disease cycle.

For wine production, the clearest consequence is reduced usable yield and additional sorting. University of Minnesota Extension's 2026 account notes that mummified berries are generally excluded from winemaking and that published evidence for detailed effects on wine aroma and fermentation is limited. It is more reliable to explain the loss of healthy fruit than to assign black rot a universal flavour in finished wine.

Sources