Black Rot on Grape Vines

⚡ The short answer

Small tan spots with a dark border on the leaves, followed by berries that shrivel into hard black mummies still hanging on the bunch, is black rot. The mummies are the key: they carry the fungus through winter and start next season's infection, so removing every one of them is the most effective thing you can do. Treatment is preventive — once a berry is infected there is no recovery.

🔬Confirm it before you treat it

1. 1Look at the leaf spots. Black rot produces tan or reddish-brown circular spots with a distinct dark margin, often with tiny black dots inside them.
2. 2Check the berries. Infection begins as a pale spot that darkens and spreads until the whole berry shrivels into a hard black mummy.
3. 3Look for last year's mummies still hanging in the vine or lying beneath it. Their presence explains the outbreak and predicts the next one.
4. 4Note the timing. Infection happens in warm wet weather from bloom through to when berries begin to soften; a dry spell largely stops it.
5. 5Assess the canopy. Dense unpruned growth stays wet for hours after rain, which is exactly what the fungus needs.

🔍Potential Causes

1. 1Guignardia bidwellii fungus, overwintering in mummified fruit and canes
2. 2Warm, wet spring weather favouring initial infection
3. 3Leftover mummified berries from previous seasons acting as a spore source

✅Solution Steps

1. 1Remove and destroy mummified berries and infected canes at the end of the season, since they carry the fungus into next year
2. 2Apply a protective fungicide starting at bud break through fruit development
3. 3Improve airflow through proper pruning and canopy management
4. 4Remove infected leaves and fruit clusters promptly during the season

🔀What else looks like this

📈Recovery timeline

• NowRemove every mummified berry from the vine and the ground, and bin or burn them. This is the highest-value action available.
• Days 1–7Remove infected leaves and prune to open the canopy so foliage dries quickly after rain.
• This seasonAccept that infected berries are lost. Nothing recovers a berry once the fungus is inside it.
• WinterPrune thoroughly and clear all debris from beneath the vine. Overwintering inoculum is what determines next season's severity.
• Next springIf it recurs, protectant sprays from bud break through to fruit set are the effective window — after that they achieve little.

🚫Common mistakes

• ✕Leaving mummified berries on the vine over winter. This is the single biggest driver of next year's outbreak.
• ✕Spraying after berries are already infected. The protective window is bloom to fruit set; later sprays are largely wasted.
• ✕Composting infected material. The fungus survives and returns to the same vine.
• ✕Leaving the canopy dense. Foliage that stays wet for hours after rain is what allows infection to establish.
• ✕Treating it as a storage problem. Black rot infects on the vine, not after picking.

💬Questions people actually ask

Can infected grapes be eaten?

Mummified berries are inedible and should be removed. Sound berries from the same vine are fine.

What exactly is a mummy?

A berry that has been consumed by the fungus and dried to a hard black shrivelled shell. It holds the spores that start next season's infection.

When should I spray?

From bud break through fruit set, if at all. Outside that window sprays have little effect on this disease.

Will it kill the vine?

No — the vine survives, but it can lose most or all of its crop, and repeatedly so if the mummies are not cleared.

Are some varieties resistant?

Yes, resistance varies considerably between cultivars, and it is worth checking before planting in a wet climate.