Is a fungus which spreads a white or ash-grey film over the upper and lower surfaces of the leaves of plants (usually the older leaves).
It favours high humidity and dry conditions; water splash and air currents spread the spores. Powdery mildew is a common disease of sweet peas, pumpkins, cucumber, watermelons and vine crops like tomatoes.
Symptoms include the appearance of a whitish film on leaves and buds. New growth is distorted and older leaves become blackened. Flower buds may fail to open.
If you notice this, remove affected leaves and spray the plant weekly with a milk and baking soda spray.