Why Are My Blackberries Covered in Grey Mould?

Grey mould caused by Botrytis cinerea is the most destructive disease of ripening blackberries in the UK during wet summers. The fungus infects flowers and developing fruits, causing them to collapse into a brown, water-soaked rot covered in the characteristic grey-brown fluffy fungal growth. In a bad year, with persistent wet and overcast conditions, grey mould can destroy most of a blackberry crop within a week. Understanding how the disease spreads tells you how to limit it.

How grey mould spreads on blackberries

Botrytis primarily enters through flowers — infecting during wet conditions when pollen is present. The infected flower transitions to a latently infected fruitlet that may look healthy for several weeks, then suddenly collapses as the fruit approaches ripeness. A single infected fruit on a cluster releases spores that infect adjacent fruits through direct contact, accelerating the collapse across the whole cluster and to nearby clusters. In wet conditions with poor air circulation, this can progress very rapidly.

Cultural controls

Open cane training — keeping canes well-spaced and not letting them tangle — is the most significant cultural control. A well-trained set of canes allows air to flow through the fruiting zone, drying fruit surfaces quickly after rain and reducing the period of wetness during which Botrytis infects. Remove infected fruit clusters as soon as you identify them — before the grey fuzzy growth develops — to reduce the spore load available to spread further. Do not pick infected berries and leave them on the ground below the plant.

Harvesting promptly

Fully ripe blackberries are more susceptible to Botrytis than slightly underripe fruit. Harvest promptly as fruit reaches ripeness — do not allow overripe berries to remain on the canes. Daily or twice-daily picking during the main harvest period in a wet year prevents the accumulation of ripe fruit that becomes infected and spreads to adjacent fruit.

Fungicide options

In years with severe grey mould pressure, a copper-based or Bacillus subtilis-based fungicide applied at flower opening provides some protection. Conventional thiophanate-methyl products are effective but observe pre-harvest intervals carefully when applying near harvest time.