A new and very annoying fungal infection has been striking Busy Lizzies (Impatiens) all over Hampstead. It starts as a tiny bit of white fluff on the leaves and leaf stalks, and quickly causes wilting and loss of all or most leaves and flowers. It spreads out from one single plant to affect a whole group, bed or pot, and it spreads quickly.
The fungus is called Impatiens Downy Mildew, caused by Plasmopara Obducens. It was first recorded in the UK in 2003 – possibly arriving on material imported from the US. It has become established very quickly. It is spread by air borne spores and infection is favoured by wet weather conditions. The leaves rapidly turn yellow and fall from the plant, and in favourable weather conditions the plant can be defoliated in a matter of days. It is sometimes possible to see a fine white fungal growth on the underside of the leaves just before they fall.
Unfortunately, there are no fungicides available for the control of this particular problem. The disease is specific to Busy Lizzies and will not affect other plants in your garden. It is thought that the fungus may be able to produce a resting spore that could potentially contaminate the soil. It would therefore be advisable to rest the area in which we planted Busy Lizzies for a year or two.
However, because the disease is so easily spread by air borne spores, there are no guarantees that any Busy Lizzies you plant next year will not be affected. There appears to be some difference in the susceptibility of various Impatiens varieties to the disease, so if it continues to be a problem, it is likely that the seed companies will drop those varieties which have experienced severe attacks.
Up to now, the Busy Lizzy has been a mainstay of our summer planting, mainly because of their shade tolerance and resistance to slug and snail attack.
Next Spring we will have to be a bit inventive in our plant selection. Begonias of various types will be a good replacement in many situations, and for sunny places, with a bit of snail control, some of the old fashioned summer annuals look set to make a comeback!