Powdery mildews are a group of related fungi which attack a wide range of plants, causing a white, dusty coating on leaves, stems and flowers.
What is powdery mildew?
Powdery mildew is a fungal disease of the foliage, stems and occasionally flowers and fruit where a superficial fungal growth covers the surface of the plant.
Very many common edible and ornamental garden plants are affected including apple, blackcurrant, gooseberry, grapes, crucifers, courgettes, marrows, cucumbers, peas, grasses (the powdery mildew fungi are major pathogens of cereal crops), Acanthus, delphiniums, phlox, many ornamentals in the daisy family, Lonicera (honeysuckle), rhododendrons and azaleas, roses and Quercus robur (English oak).
Powdery mildews usually have narrow host ranges comprising of just a few related plants. For example, the powdery mildew affecting peas is a different species from the one attacking apples.
You may see the following symptoms:
- White, powdery spreading patches of fungus on upper or lower leaf surfaces, flowers and fruit
- Tissues sometimes become stunted or distorted, such as leaves affected by rose powdery mildew
- In many cases the infected tissues show little reaction to infection in the early stages, but in a few specific cases, for example on Rhamnus, the infection provokes a strong colour change in the infected parts, which turn dark brown
- Sometimes the fungal growth is light and difficult to see despite discolouration of the plant tissues, e.g. on the undersurface of rhododendron leaves
Destroying fallen infected leaves in autumn will reduce the amount of infectious spores next spring. Mulching and watering reduces water stress and helps make plants less prone to infection. Promptly pruning out infected shoots will reduce subsequent infection.
Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.
Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogues for details.
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.
Edibles and ornamentals: Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate) can be used on ornamentals, apples, pears, gooseberries and blackcurrants. Difenoconazole (Westland Plant Rescue Control concentrate) can be used on ornamentals, pome fruits and grape vine.
Ornamentals only: Myclobutanil (Westland Rose Rescue and various other products, all as ready-to-use sprays) Difenoconazole (Westland Plant Rescue Control ready-to-use spray), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra) can be used on ornamentals. Some formulations of myclobutanil, tebuconazole and triticonazole contain insecticides to control pests. Avoid these unless an insect pest problem is specifically identified.
Plant and fish oil blends (Vitax Organic 2 in 1) may be used on all plants.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Powdery mildew fungi produce microscopic air-borne dispersal spores from the fungal growth on the plant surface. These have an unusually high water content, enabling them to infect under drier conditions than most other fungal pathogens. Powdery mildews therefore tend to be associated with water stress.
The majority of the growth of most powdery mildews is found on the plant surface. The fungus sends feeding structures into the surface cells, greatly reducing the vigour of the plant. The growth of a few powdery mildew species (e.g. that affecting hazel) is found deeper in the plant tissues.
Powdery mildews either spend the winter as dormant infections on green tissues, or as resting structures on fallen leaves which then release spores the following spring.