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 apples, blackcurrants, gooseberries, 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).
- 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 a purple-brown colour
- Sometimes the fungal growth is sparse and difficult to see, despite discolouration of the plant tissues, e.g. on the undersurface of rhododendron leaves
How to prevent:
- Choose plants for your garden that are resistant to powdery mildew. Many mildew-resistant varieties of cucurbits (melons, cucumbers, squash, etc.) have been developed and can be bought from major seed suppliers.
- Plant in sunnier spots, as powdery mildew tends to develop more often in shady areas.
- Selectively prune overcrowded areas to increase air circulation around your plants; this helps to reduce relative humidity.
- Watering from overhead can help to wash spores off leaves. Note, however, that wet foliage can often contribute to the development of other common diseases, so it’s best not to rely on this as a prevention tactic.
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