Powdery mildews

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.

Powdery Mildew

Powdery Mildew

Quick facts

Common name: Powdery mildews
Scientific name: Various
Plants affected: Many plants
Main symptoms: White, dusty coating on leaves, stems and flowers
Caused by: Fungus
Timing: Spring onwards

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.

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.

Symptoms

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

Control

Non-chemical control

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.

Chemical control

Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.

Edibles: Tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus) can be used on glasshouse crops of cucumber, courgette and summer squash. 

Ornamentals: The fungicides tebuconazole (Bayer Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are approved for the control of powdery mildews on ornamental plants.

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Bayer MultiRose 2, Doff Rose Shield, Vitax Rosegarde, Westland Rose Rescue); tebuconazole containing deltamethrin (Bayer Multirose Concentrate 2), and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

Any crop: The products SB Plant Invigorator, Resolva Natural Power Bug and Mildew Control, and Ecofective Plant Defender contain a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. They have a physical mode of action and may be used against powdery mildews, as well as a range of pests such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

Biology

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.


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