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.