Impatiens downy mildew is a disease caused by the fungus-like (Oomycete) organism Plasmopara obducens.

It was found for the first time in the UK in 2003 and is likely to have arrived on imported commercial propagation material (seed or cuttings). Statutory action was at first undertaken by the plant health authorities against confirmed outbreaks of the disease, but this soon ceased. After the wet summer of 2008, damage was much reduced by improved control practices at commercial nurseries. However in 2011 control failed, probably due to resistance to the commercial fungicides used. Infected plants were inadvertently sold widely. This led to a very widespread outbreak of the disease, with many gardens, nurseries and local authority displays affected. From that point on, both the popularity and availability of bedding busy Lizzies declined dramatically. However, the work of plant breeders has led to the recent introduction of disease-resistant cultivars, which should see an upsurge once more in sales of this popular bedding plant.

Outbreaks of downy mildew in the UK have been confined to Impatiens walleriana, the common bedding busy Lizzie. No cases have been found on New Guinea impatiens (Impatiens 'New Guinea Group'), or on the few species of Impatiens found growing in the wild in the UK including Himalayan balsam (Impatiens glandulifera), although the disease has been found occasionally on Himalayan balsam in some other countries.

You may see the following symptoms:

• On leaves: Affected leaves turn yellow and are rapidly shed from the plant. A fine white fungal growth may be visible on the lower leaf surface, but affected leaves decay rapidly
• On flowers: Flowers are also commonly shed, and the plant is often reduced to bare branches with a small tuft of yellow leaves and flower buds at the tip. Severely affected plants will eventually die

Non-chemical control

Affected plants should be disposed of as soon as possible. Do not compost them. Ideally burn them or bury them deeper than 50cm (20in).  Although the spores should not survive the commercial composting used for council green waste collections, it is best to deal with contaminated material within the garden.

Because of the risk of soil contamination, rest affected areas from Impatiens for at least a year (some species of Plasmopara affecting other plants produce resting spores that can survive for several years).

Where infected plants have been grown in containers, replace the compost and wash using a garden disinfectant, as directed by the manufacturer, to cleanse the container if you intend to grow Impatiens in it again the following year. The disease is specific to Impatiens, so any other bedding plants can be grown without risk.  Semperflorens-Cultorum begonias and bedding fuchsias perform well in the shaded areas for which busy Lizzies have been invaluable.

In 2018, 'Imara' became the first busy Lizzie cultivar available in the UK with resistance to downy mildew (it is said to be highly resistant, but not totally immune). A second resistant cultivar, 'Beacon', is also now available.

Susceptible cultivars are still available, and with these cultivars raising plants from seed will eliminate the risk of purchasing infected plants.  This type of disease often has a lengthy ‘latent period’, when plants are already infected but not yet showing obvious symptoms. The advent of fungicide-tolerant strains of this disease increases the risk of introducing disease when buying plants.

Unfortunately, growing susceptible Impatiens cultivars, even ones that have been raised at home, in another part of the garden will not guarantee freedom from infection, as the disease may well arrive again as airborne spores from infected plants growing elsewhere.

Chemical control

There are no fungicides available to gardeners for the control of this disease.

Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.

The disease is spread by spores produced on the underside of infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.

The airborne spores remain viable for just a short time, but the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores (oospores) are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for an extended period.