Powdery mildew is a fungal disease caused by a number of closely related fungi that together affect a wide range of plants. Chrysanthemums, delphiniums, Michaelmas daisies, climbing roses and gooseberries are particularly susceptible.

The symptoms are a white powdery coating that forms on leaves and stems during summer.

As a prevention,

• Avoid wetting leaves or overcrowding the plants.

• Humid conditions encourage powdery mildew, so keep the greenhouse well ventilated.

At the first sign of attack,

• Prune out affected shoots and spray the plant with a copper fungicide.

• Any parts of a plant that become infected should be destroyed in autumn so that the spores cannot overwinter.