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Powdery Mildew - fact sheet
Powdery mildew is probably the most common, conspicuous, widespread and easily recognisable plant disease, causing serious damage to a wide range of plants. It is common on cereals, cucurbits - especially melon, squash and cucumber - strawberries, many ornamentals e.g. roses, begonias and azaleas, crops such as grapes, and many trees e.g. apples, crepe myrtles and oaks.
Powdery mildew is a fungal disease. There are many species that have been identified as capable of causing infection, most of which are in the genus Oidium. Powdery mildew attacks a wide range of plants, with some species having a wide host range, while others are restricted to one host or a group of related plants.
Powdery mildew is an obligate parasite, i.e. it attacks and can live only on living host tissues. Its spread is favoured by moderately dry conditions when nightly dews or irrigation are sufficient for spore germination and infection. The fungus spreads very quickly; it can go from a single infected leaf to an epidemic within 3-4 days. The spores are spread over short distances by wind and rain. Summer and autumn are the prime times for infection and high relative humidity, crowded conditions and poor air circulation favour disease spread. Spores can be released, germinate and cause infection, even when there is no film of water on the plant surface, so long as the relative humidity is fairly high. Wet weather and very hot, dry weather do not favour the disease.
The powdery mildew fungus grows on the surface of the plant tissues and never invades the tissue itself. However it penetrates the leaf surface with specialised cells called haustoria. These gather nutrients and energy from the host leaf. Spores are produced on the leaf surface within as little as 60 hours after initial infection, and are carried by air currents to another leaf where they can directly infect by germinating, particularly on young green tissue, if the temperature and relative humidity is sufficiently high. The fungus can also overwinter in more robust tissues such as buds.
Circular, white to grey, powdery spots develop on young shoots and stems, leaves, buds, flowers and young fruit. The spots spread and coalesce, finally covering the whole surface with a powdery mass of spores. Leaves can become distorted; ultimately they wither and die.
Reducing the likelihood of disease outbreak is more effective than trying to control the disease once it is established. Growing tolerant cultivars whenever possible, and avoiding conditions of high humidity, are important steps. For example, avoid planting in shady areas and prune away all vegetation that produces shade. Don’t over-crowd the plants and do not use overhead irrigation late in the day. Finally, all debris should be removed to reduce the amount of fungus available to infect host plants.