Surviving Botrytis cinerea attacks on your plants

In a damp summer, it’s inevitable that grey mould will raise its ugly head. It is caused by the fungus Botrytis cinerea and infects many plants, especially those grown under glass where conditions are humid.

Botrytis cinerea normally enters through a wound or infects plants under stress. However, healthy plants are at risk in humid conditions – no time of the year is ‘safe’ from it.

Apples, grapes, strawberries, blackberries, raspberries, gooseberries, beans, cucurbits, lettuce, tomatoes, Chrysanthemum, Cyclamen, Pelargonium (geranium), and Primula are, particularly at risk.

Grey mould
Grey mould on a cut-back geranium in a humid environment under glass

Unlike powdery mildew, which has a very specific host, Botrytis has a very wide range. There are closely related species that are much more specific, including those infecting snowdrops, peonies, onions and broad beans.

One of the most annoying abilities of grey mould is how it can cause delayed infections. Plants can be infected but don’t show any symptoms for a long time. In strawberries and some other fruit, the fungus enters flowers and the embryo fruit. However, the mould doesn’t break out until the fruit ripens and its sugar content goes up.

A similar thing happens with primulas. The seed-borne fungus develops with the plant, only appearing after a considerable amount of growth.

The mouldy garlic Extra Early Wight bulb
Mouldy garlic Extra Early Wight bulb

Grey mould symptoms

  • Under humid conditions, fuzzy grey mould grows on buds, leaves, flowers or fruit.
  • Buds and flowers shrivel and die.
  • Small black seed-like structures form in the infected material.
  • On soft fruit, particularly gooseberries, Botrytis infection kills branches, but the fuzzy mould is seldom seen.
  • On strawberries and grapes, infection leads to soft brown rot, often as the fruit is ripening.
Grey mould
Grey mould on a sweet pea flower

Organic control

  • Hygiene is vital, especially under glass. Remove dead and dying leaves, buds and flowers immediately.
  • Don’t leave dead plant material lying around.
  • Improve ventilation in glasshouses.
  • Do not overcrowd plants, either under glass or in outdoors in beds.
  • Remove soaked flowers after torrential rain, especially Pelargoniums.

Grey mould updated August 2021