Advice
RHS Help & Advice
Powdery mildews
Symptoms
A dry whitish powder coating leaves, shoot tips and often flowers is especially visible in summer.
Other symptoms include stunted and distorted growth, and reduced flowering. On rhododendrons (see picture right) a pale beige felt appears on the undersides of the leaves with a corresponding yellow or reddish-purple area above. On Euonymus and Rhamnus affected foliage is black or discoloured.
Plants affected
Many garden plants are affected, both woody and herbaceous, particularly apple trees (see picture bottom right), roses and sweet peas and those growing in containers. Vegetable foliage is also prone, including beetroot, parsnip and spinach.
Cause
Powdery mildews are caused by a range of closely related fungal species. As each has a limited host range the powdery mildew affecting one plant is unlikely to be responsible for mildew on another species. However some mildews can spread to cultivated plants from closely related weed hosts, so weed control is an important part of disease limitation.
Prevention
The following measures will reduce susceptibility to the disease:
- Keep plants well watered, so they are not dry at the roots
- Mulch to preserve soil moisture.
- Improve air-flow around plants to reduce humidity. For woody plants such as roses this can be achieved by pruning to establish an open branch structure. Avoid overcrowding smaller plants. Thin vegetable crops to recommended spacing.
- Avoid high nitrogen fertilisers, as these encourage soft sappy growth that is more easily colonised by fungi.
- Ensure plants are in their ideal position. For example, a sun-lover will struggle in shade and be at greater risk of infection.
- Check catalogues and grow resistant varieties wherever possible.
Non-chemical treatment
Prune out infected areas as soon as seen. Collect and burn or otherwise dispose of all infected debris and prunings.
Chemical
treatment
Fungicides available to the home gardener for powdery mildew control are myclobutanil (available as various products and formulations, including Systhane Fungus Fighter (liquid concentrate), Fungus Fighter (ready-to-use spray) and Roseclear 3 (which also contains an insecticide for pest control), penconazole (Fungus Clear), and sulphur as a dust (Vitax Green or Yellow Sulphur) or as a spray with fatty acids (Nature’s Answer Natural Pest and Disease Control).
The choice is complicated because pesticides may only legally be used on the range of plants specified on the label.
- For ornamental plants, including roses, use myclobutanil, penconazole or sulphur.
- For some specified fruits including apple and gooseberry, use myclobutanil (only as the liquid concentrate Systhane Fungus Fighter) or sulphur dust.
- Only sulphur dusts may be used on vines. It may also be possible to treat certain vegetable crops with sulphur dusts (check labels).
Check the labels carefully before choosing a product. Occasionally, the label will also list certain cultivars that may be damaged by the application of a specific fungicide; e.g. certain gooseberry and apple cultivars may be damaged by sulphur dusts.
If no pest problems are present on the plants to be treated, avoid the unnecessary use of a product which also contains an insecticide.