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Oak Powdery Mildew



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Oak Powdery Mildew

Usually appear in

Spring to Autumn.


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Caused By:-     The Fungus Erysiphe alphitoides

Description

Powdery mildews are one of the most widespread plant diseases. They affect virtually all kinds of plants from cereals and grasses, to fruit trees, and broad-leaved shade trees.
Powdery Mildew of oak is caused by the fungus Erysiphe alphitoides (also known as Microsphaera alphitoides) and it is a common mildew of oak in the Northern Hemisphere. It as first found in the UK in 1908 and was considered a contributory factor in the oak dieback in the 1920s. The symptoms are usually the same or very similar regardless of the specie of fungus and plant host and the fungi needs living tissue in order to grow.
It attacks young leaves and soft shoots, especially on young oak seedlings, covering them with a felty-white mycelium, which takes nutrients from the host, causing them to shrivel and blacken. If it is sprayed in the early stages only some of the tissues will die and the leaves will be scarred. The youngest leaves and shoots are especially susceptible to the fungus. The white growth on the leaves are the spores which are carried by the wind or plashes from watering. Although it usually is not usually fatal, the infected plant may become extremely unsightly and is weakened. The fungus overwinters as growth in buds, on fallen leaves or as spores inside a chasmothecia which eventually breaks down to release the spores.
Powdery mildews are usually host specific, which means they do not spread to other types of plants. There are many types of Powdery Mildew fungi and they all produce similar symptoms on stems, leaves and buds. Infected leaves may become distorted, turn yellow with patches of green, and fall prematurely, stems and twigs may collapse and die and infected buds may fail to open.
The disease is usually observed on the upper sides of the leaves with a whitish fungal growth developing on the leaf surfaces in patches which eventually coalesce to cover the leaves, stems or buds. The infected areas dies or may become soggy.
The severity of the disease depends on several factors such as variety of the host plant, age and condition of the plant, and is especially severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface to infect it. However, if the relative humidity is high the problem will be worse, but it does not tend occur when leaf surfaces are wet. Therefore, Crowded plantings where air circulation is poor damp and in shaded areas makes the problem worse. Young growth usually is more susceptible than older plant tissues.
Picture by Andrej Kunca, National Forest Centre, Slovakia, Bugwood.org
See also these other Powdery Mildews Apple, Begonia, Hydrangea, Pansy, Pea, Phlox, Powdery, Quince, Rose.
See also Downy Mildews Brassica, Downy, Hebe, Lettuce, Onion, Pansy, Pea.
See also other Mildews American Gooseberry, European Gooseberry, Gooseberry.


Control

The major factors that are believed to influence the spreading of the disease are humidity and temperature, and cultural practices like high nitrogen fertilisation.
Damage by mildew is more likely to kill the tree when they are exposed to other pathogens or herbivores prior to or after mildew infection.
Spraying with fungicides when the first symptoms appear, and then at intervals throughout growing season before the spores are formed will control this pest.
Care must be exercised as spraying of non-specific fungicides can affect the community of soil microorganisms which ultimately might have adverse effects on the growth and health of the trees and recurring preventive spraying may lead to resistant strains of pathogens being created.
A winter fungicidal wash may be beneficial.
Spraying with a mixture of baking soda(sodium bicarbonate) or potassium bicarbonate combined with a lightweight horticultural oil is said to cure and prevent the problem.


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