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.

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.

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 and ornamentals: Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate*) can be used on ornamentals, apples, pears, gooseberries and blackcurrants.

Ornamentals only: Myclobutanil (Bayer Garden Fungus Fighter Disease Control*, Doff Systemic Fungus Control and various other products, all as ready-to-use sprays); tebuconazole (Bayer Garden Multirose Concentrate 2 and Bayer Garden Fungus Fighter); Tebuconazole and Trifloxystrobin (Bayer Garden Fungus Fighter Plus) 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.

*NB: The following products are being withdrawn:

Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate and Bayer Garden Fungus Fighter Disease Control). These products cannot be sold after the 30th November 2015 and remaining stocks must be used or disposed of before the 30th November 2016.

Plant and fish oil blends (Vitax Organic 2 in 1). These products are being withdrawn but as they are not registered fungicides there is no date by which stocks have to be used up before.

Download

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

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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  • Clive Turner avatar

    By Clive Turner on 03/01/2015

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


  • Briony avatar

    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!


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  • Craigmill avatar

    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?


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  • Bogbean avatar

    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.


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  • Bogbean avatar

    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.


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