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A fungal disease, it grows in cool, moist conditions and appears as a dirty-white fluffy growth under the leaves, causing them to turn yellow and fall off. 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 (Impatiens walleriana)at some nurseries and it has become a major problem since then. It may have arrived in imported cuttings. In 2008 the disease affected many of the plants grown outside and they had to be destroyed. The wet summers experienced in recent years 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 as the spores can linger for up to ten years, Busy Lizzies should not be grown in the same soil for at least that length of time. Commercial growers have been trying to control it using a fungicide containing Metalaxyl, but resistant strains of the fungus have emerged so the future of Busy Lizzies looks bleak - a relief to some who are not so keen on garish displays of summer bedding.
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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