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.
In 2003 Impatiens Downy Mildew (Plasmopara obducens) was discovered in the stock of busy Lizzies at some nurseries and it has become a major problem since then. In 2008 the disease affected many of the plants grown outside and they had to be destroyed. The wet summers experienced in recent ears have favoured its spread. The plants show stunted growth, lose their leaves and flowers, and only the fleshy stems remain until they eventually rot away. There is no control approved for amateur use so affected plants should be destroyed and busy Lizzies should not be grown in the same soil for a few years as the spores can linger.
There is a guide on recognizing and treating the disease at this link Good Horticultural Practice for the Prevention and Control of Impatiens Downy Mildew, on the British Protected Ornamentals Association site.When conditions are favourable transmission is by asexual spores, usually in 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 that 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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