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Powdery MildewPowdery Mildew

 What is Mildew?
Powdery mildew is a fungal disease of the foliage, stems and occasionally flowers and in very severe cases even the fruit are effect by a superficial fungal growth that covers the surface of the plant.  But what can we do about this disease?

What to Look for:

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 leaves

Vegetable plants most often affected:

  • Cucumbers.
  • Squashes, marrows, courgettes.
  • Fruit trees: apples, pears, gooseberries, blackcurrants, grape vines.
  • Peas.

About the Powery Mildew

The fungus likes warm dry conditions, especially because we have to water frequently, which creates local high humidity around the plants. Rainy weather is not the problem, but rather hot, still, dry conditions followed by a blast of high humidity, such as... when we water. 

Control Measures

Since powdery mildew thrives in hot, dry conditions with high local humidity around the plant, the easiest way to control it is really to prevent it in the first place by depriving it of what it needs.

How yo water your plants can have a huge say in whether your plants develope the disease or not, if you use a watering can remove the rose and do your best to water the soil, not the plant leaves, thus cutting down the enviroment that the fungus needs to thrieve.

Give your plants room the breath, plants spaced out have better air circulating around them making less localised humity zones.


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.
  • 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.  Potassium Bicarbonate is particularly effective and easily purchased relatively cheaply from eBay etc

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

Potassium bicarbonate Fungicide - Also great as a Blight preventative
  • Mix 4 teaspoons (about 1 rounded tablespoon) of potassium bicarbonate into one gallon of water.
  • Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases.
  • Potassium bicarbonate is a good substitute for baking soda.
  • There are commercial EPA registered as well as generic products available.
Bicarbonate of Soda Fungicide
  • Mix 4 teaspoons (about 1 rounded tablespoon) of baking soda and 1 tablespoon of vegetable oil into one gallon of water.
  • Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases.
  • Avoid over-using or pouring on the soil.
  • Potassium bicarbonate is a good substitute for Bicarbonate of Soda.


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