How to recognise it
Leaves become mottled and yellow, then fall. You may find a fine, white fungal growth on the underside of the leaves, however they may decay so quickly the fungus doesn’t have time to develop into this form. Flowers are often shed so the stems are left bare except for the odd flower bud. Often the plant will die completely.
Why it’s a problem
This serious disease of Impatiens will weaken and, often, kill plants. It spreads from plant to plant quickly and easily. Entire bedding displays can be destroyed within a week and the disease will persist in the soil for up to 10 years.
Where you are likely to find it
Found on Impatiens bedding plants (busy Lizzies). It mostly occurs in spring within glasshouses and in summer outdoors. Most likely to be found during/after periods of wet weather.
To date the New Guinea impatiens, Impatiens × hawkeri, appears to be immune to the disease.
How to prevent it
Raising your Impatiens bedding from seed will reduce the risk of an infection from bought plants, although there is still a risk that airborne spores will reach your seed grown plants. Currently no resistant cultivars are available.
Ensure that plants aren’t grouped too close together, to ensure good airflow between them.
How to get rid of it
Remove any affected plants and destroy them (eg by burning), do not put them into compost. Do not re-plant an affected area with Impatiens bedding for at least a year, if possible more (the spores may survive in the soil for up to 10 years).
Currently there are no chemical controls available for this disease, many strains of which are fungicide resistant.
Is it good for anything?!
Other useful information
Other plants can be grown in soil that has been infected, it appears that the disease only infects Impatiens plants. Bedding begonias, for example, aren’t affected.
This is just one of many types of downy mildew which can affect a wide range of plants. Bremiella sphaerosperma is another form of downy mildew which can affect Impatiens plants.