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, 'Beacon', is available on a limited scale in 2019, with a more widespread distribution from 2020.
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.
There are no fungicides available to gardeners for the control of this disease.