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

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.


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


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.


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


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


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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  • Bogbean

    By Bogbean on 01/06/2014

    I have seen powdery mildew occur on many plants but most commonly on roses. If there is a dry period, you can bet that some of the roses are going to get it, and once they do it can spread rapidly, ruining the appearance of the entire plant as well as weakening it. Thankfully if you catch it early it is not a serious problem and can be easily treated and prevented in the future.

    0 replies

  • Bogbean

    By Bogbean on 01/06/2014

    I have seen powdery mildew occur on many plants but most commonly on roses. If there is a dry period, you can bet that some of the roses are going to get it, and once they do it can spread rapidly, ruining the appearance of the entire plant as well as weakening it. Thankfully if you catch it early it is not a serious problem and can be easily treated and prevented in the future.

    0 replies

  • Craigmill

    By Craigmill on 02/10/2014

    My amelanchier has had powdery mildew for two years now. Last year, we removed other shrubs to give it more air and cleaned up as many of the fallen leaves as we could. This year all the leaves fell off in late September. Last year was a very wet season here and this year has been unusually dry, so not sure if more watering / mulching would help. It's 24 years old and too big to spray. Lost cause?

    0 replies

  • Briony

    By Briony on 02/10/2014

    To Craigmill - Sounds like you are doing the right things. Watering in dry spells and mulching should help. If it's congested you could try thinning out some branches this winter? Age could be a factor, but one should never give up on a precious tree until it gives up!

    0 replies

  • Clive Turner

    By Clive Turner on 03/01/2015

    The leaves on our photinia have brown scabs. What should be done to get rid of these?

  • dg10

    By dg10 on 05/07/2015

    I have powdery mildew on my cucumber leaves, I haven't sprayed with anything yet but have removed the infected leaves. Is it OK to do that?

  • Mark Dykes

    By Mark Dykes on 08/07/2015

    We suspect powdery mildew distorting the leaves on our crab apple and Bramley trees (some 20 metres apart). It seems all the chemical control agents you list are being withdrawn! The crab is surrounded by other plants and the Bramley has ivy directly beneath it, so in both cases, removal of all dropped leaves is difficult. We can put in the effort to clear up leaves and/or clear the spaces around the trees IF it is likely to be successful. Or are these trees doomed?!

  • John Grill

    By John Grill on 24/08/2015

    Mildew on and off. I have two gardens, one at 1300 feet, and one at 900. In the previous dry years the former has had terrible problems with mildew on all five acanthus plants, pretty well wiping them out. Never done anything about it. This wet year, not a problem. Not a sign. But the garden at 900 feet now has the problem, also on courgettes. Spraying makes no difference.

  • Netts16

    By Netts16 on 13/09/2015

    I have recently planted several shrubs - Amelanchier, Virburnham Burkwoodii and Mahonia Media Winter Sun and all of them - especially the Amelanchier are showing signs of white powder on the leaves. What can I do to stop this? Also is it likely to affect nearby plants?

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