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Powdery mildew

Baking soda, sulphur or milk! The finest weapons in the fight against powdery mildew.

Powdery mildew on leaves Powdery mildew is a fungal disease that is particularly prevalent in summer and autumn when the weather is warm and dry (or in very mild winters). Plants in conservatories and greenhouses, however, can be infected all year round.

  • Infected plants are covered in a greyish-white mould, typically on upper leaf surfaces and young shoots.
  • Powdery mildews affect a number of plants: many vegetables (beans, cucumbers, courgettes, pumpkins, etc), fruit crop (such as apples, grapes and quinces), sweet peas, violas/pansies, dahlias, roses, hydrangeas and African violets, to name a few.
  • There are many species of powdery mildew and most are host specific. For example, the one on roses will not affect cucumbers, or the one on peas will not affect grapes.
  • You may have seen those pretty black and yellow ladybirds around, particularly if you've got powdery mildew. These ladybirds are the only species in NZ that are not predatory — they don't feed on aphids. What they do feed on is fungi. But if you think these beetles will help control powdery mildew, think again. They actually carry spores of the disease under their wings from plant to plant.


The important thing to remember here is that powdery mildew does not like wet conditions — it's at its worst when the weather's dry. The fact that it doesn't like water on leaves when it's developing means there are a number of methods of controlling it that do not involve fungicide chemicals. Anything that has a detergent type of action, or surfactants, things that will wet the leaves, are very effective at reducing the severity of powdery mildew.

That's why you hear people talking about using old dishwater. Anything with a detergent in it will actually suppress powdery mildew development.

Baking soda
One particular method that works well, particularly for rose mildew, is baking soda. Mix up a solution of baking soda and water and spray onto the infected plant. About half a teaspoon of baking soda to one litre of water. Too much baking soda will burn the leaves.

Sulphur spray
A sulphur spray is also effective against powdery mildew. Spray fortnightly, beginning as soon as the very first sign of powdery mildew appears, or even earlier if you know that the particular plant gets powdery mildew every year.

Milk spray
Or you could head for the refrigerator and try a milk spray! A weekly spray of skim milk (1 part milk, 9 parts water — the low-fat content means there is less chance of odour) will reduce the severity of powdery mildew by up to 90%. You see, milk is believed to be a natural germicide; it contains a certain amount of salts and amino acids which powdery mildew is sensitive to, and also acts as a foliar fertiliser, boosting the plant's immune system. A word of warning, though. If the milk concentration is above 30% (that is, 3 parts milk), a very different fungus (albeit harmless) may begin to grow on your plants.

Jane Wrigglesworth

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Last updated: June 2, 2004