A fungal disease, it grows in cool, moist conditions and appears as a dirty-white fluffy growth under the leaves. It does most damage to young plants especially to members of the brassica family in spring. Other types attack lettuce and onions.
When conditions are favourable transmission is by asexual the spores, usually spring and summer, these develop into the hypae which come together to form the mycelium. Sexual spores develop in less favourable conditions and these can survive on fallen leaves until more suitable conditions return.
Cultural control is mainly by growing plants in conditions which discourage the growth of the fungus. These include good plant separation to allow a good air flow, the removal of debris which might carry spores and weeds which can act as alternate hosts, eg. shepherds purse in the case of brassicas. Try to avoid wetting the leaves when watering
Spraying with mancozeb gives some protection.
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