Why Are My Blackberry Flowers Turning Brown and Rotting?

Blackberry flowers that turn brown, become water-soaked and fail to develop into fruit are most commonly infected by Botrytis cinerea (grey mould). This fungal infection is opportunistic — it exploits wet, still conditions at flowering time (June–July in the UK) and enters through the fragile flower tissue. In a wet flowering period, flower botrytis can prevent a significant proportion of flowers from setting fruit, directly reducing yield.

How flower botrytis develops

Botrytis spores are ubiquitous in the environment and land on flowers continuously. Under dry, warm conditions the infection rate is low and the immune response of the flower tissue is sufficient to resist infection. In cool, wet or overcast conditions with poor air movement, the spores germinate rapidly and infect through the petals. A flower that fails to open fully on a damp day is particularly vulnerable. Infected flowers collapse to a brown mush and do not progress to fruitlet formation — you will see parts of the flower head fruiting successfully alongside collapsed brown sections.

Improving air circulation

The most effective cultural response to flower botrytis is improving the airflow through the plant. Train canes onto a proper support system and remove surplus old canes and any dead or dying material. Reducing the total number of canes per plant to 6–8 well-spaced canes means air can move freely around the flowering laterals. This is the approach with the greatest impact — it is more effective in most years than fungicide treatment.

Removing infected material

Infected flower clusters that show visible brown rot should be removed and disposed of — they are sporulating and will release spores that infect nearby open flowers. Do not leave infected flower material hanging on the plant through the flowering season.

Fungicide at flowering

If grey mould pressure is severe in a wet year, a copper-based or Bacillus subtilis fungicide applied at early flowering provides some protection. Conventional fungicides active against Botrytis (e.g. pyrimethanil) are more effective but should not be applied to open flowers during bee activity — apply in early morning or evening when pollinators are not present.