Black rot on grapes — symptoms and treatment

Severity: high

What is Black rot

Black rot of grape is caused by Guignardia bidwellii. It can destroy an entire crop if not controlled. The fungus attacks leaves, shoots and, decisively, the berries, turning a full bunch into hard black mummies while the vine itself carries on growing. Vines rarely die of Black rot, so it is easy to underrate; what it takes is the fruit, in the weeks around bloom.

Symptoms

Leaves go first. Infection shows as brown circular lesions with dark margins — tan or reddish-brown patches, distinctly round, ringed by a darker border. Inside the dead tissue, tiny black dots develop in a rough ring; these are the fruiting bodies. Leaf lesions are the early warning, appearing well before the fruit shows anything.

The berries are where the disease is decided. An infected berry first shows a small pale spot, which spreads until the fruit turns black, then shrivels — hard, wrinkled mummies that cling to the bunch rather than dropping. One bunch can hold healthy green berries and finished mummies side by side.

• Early: brown circular leaf lesions with dark margins; tiny black dots within the dead tissue
• Advanced: berries blackening; hard shrivelled mummies hanging on the bunch; lesions on shoots

Leaf lesions alone can be confused with other leaf spots. The mummified berries are unmistakable.

Causes and conditions

The fungus overwinters in the mummies — on the vine and on the ground — and in infected shoot lesions from last season. In spring they release spores onto new growth, which is why a vineyard cleaned of mummies starts with far less inoculum.

Spores need water. Rain splash carries them onto leaves and berries, and they need a period of wetness to infect, so wet springs, prolonged rain, heavy dew and dense shaded canopies turn a small problem into a lost crop. Young berries are highly susceptible; as fruit approaches veraison — the point where berries soften and change colour — susceptibility drops sharply. That is why the calendar matters so much: protection must be on the fruit during its vulnerable window, not applied after symptoms appear.

Treatment

Sanitation and canopy work reduce inoculum and wetness; the fungicide program protects susceptible tissue through the critical window. Neither substitutes for the other.

Canopy management — cultural

Timing: throughout the growing season. Remove mummified berries. Prune and train vines for good air circulation. Remove infected leaves.

Removing mummified berries — off the vine and the ground — takes out the overwintering source. Training the vine so air moves through the canopy lets leaves and fruit dry faster after rain.

Mancozeb + Myclobutanil — chemical

Timing: bud break to veraison. Apply fungicides from bud break to veraison. Critical period is from bloom to 4 weeks after bloom.

Pre-harvest interval: 28 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Strip every mummy off the vine at dormant pruning and clear those on the ground.
• Prune and train for an open canopy so rain dries off quickly.
• Remove infected leaves and shoot lesions during the season.
• Choose a site with air movement and sun rather than a still, shaded corner.
• Do not wet the canopy with irrigation, and protect the fruit through its susceptible period.

Frequently asked questions

Can I eat grapes from a vine that had Black rot? Healthy berries on the bunch are fine; mummies should be discarded. If you applied Mancozeb + Myclobutanil, observe the 28-day pre-harvest interval before picking.

When should I treat? From bud break to veraison, with the critical period from bloom to 4 weeks after bloom. Waiting until you see blackened berries is too late for that crop. Removing mummies is the most valuable cultural step, because they are where the fungus overwinters — but in a wet spring, sanitation alone is usually not enough.