Powdery Mildews (notice the plural – there are hundreds of them) form a large group of related fungi that cover the leaves, stems, flowers and sometimes fruit of infected plants with a familiar – though superficial – white powdery dust-like coating.
They are among the most common of all garden diseases, affecting a wide range of plants, though each affected plant species will have its own species of powdery mildew. For example, the same powdery mildew that affects crab apples in the wild can grow on Granny Smiths in your garden but will leave your Euonymus alone.
Powdery mildew is most active during the spring, summer and pre-frost autumn.
Most Susceptible Plants
Acanthus, Apple, Azaleas, Blackcurrant, Cherry Laurel, Courgettes, Crucifers, Cucumbers, Daisies, Delphiniums, English Oak, Euonymus fortunei, Gooseberry, Grapes, Grasses, Hornbeam, Lonicera, Marrows, Peas, Phlox, Rhododendrons, Roses,
Prevention and Cure
• The spores of these annoying fungi have an exceptionally high water content which enables them to infect plants in much drier conditions than other fungal diseases. Their appearance, therefore, is often a sign that the infected plant is not receiving sufficient water.
The most basic preventative measure is therefore to ensure all your plants – be they hedging or other – are being appropriately watered. Mulching will help reduce any chance of water stress
• Powdery mildews are dormant during winter so it is imperative to clear fallen, infected leaves in autumn. If you don’t, it will come bouncing back in spring
• Any infected stems or branches should be cut out immediately and disposed of – don’t put them on the compost heap
• There are many proprietary brands of effective fungicide available to treat Powdery mildew, generally containing one of the following active ingredients: Myclobutanil, tebuconazole or triticonazole. If you are treating edible plants, check the label as some formulations are only suitable for ornamental plants. Also, avoid those that also contain an insecticide unless you have an insect problem
• Less chemically invasive treatments based on blends of fish and plant oils are also available