Powdery and Downy Mildew
The below info is edited from Maximum Yield Magazine. Readers should check out above link for full article.
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Powdery mildew is often confused with 'downy mildew'.
The first symptom of downy mildew is often the appearance of light green or yellow spots on the upper surface of the older leaves, with the spores forming on the underside of the foliage. Later symptoms include whitish clumps of fluffy spores on the leaf surface. Downy mildew requires moisture (rH of at least 95%) to develop. Keeping the plants dry and the growing area well ventilated when conditions are cool will help prevent the disease.
Powdery mildew is much more common. Powdery mildew covers much of both the lower and upper sides of the leaf. Areas affected enlarge in a circular patten, spreading a white dusty growth over the whole leaf surface. As the disease advances, the leaves become brown and dried and will drop off. Infected foliage and shoots tend to become discolored, distorted and completely covered in a powdery white growth as the disease progresses through a crop. Later fruits will often fail to mature and will be small and often misshapen.
Powdery mildew is favored by dry atmospheric and growing conditions, moderate temps, reduced light intensity, good nutrition and succulent plant growth. Spores are spread via wind or air movement, and certain insects can also carry the disease. Powdery mildew spores don't require a film of water to be present on the leaf surface to germinate and infect the plant tissue. In fact, if the powdery mildew spores are in contact with water they are inhibited to a certain extent.
Powdery mildew spores germinate best in a temp range of around 72-88F, and in shaded areas of the crop, so it is more severe in closely planted crops. If the spores, entering a growing area, make contact with a plant under conditions of reduced light intensity, a temperature of 72-88F, and absence of moisture then germination will occur within two hours, and infection will be two days later.
Silica based dusts and sprays give a protective coat over the leaf surface and thus prevent the mildew spores from germinating.
It has also been proven that baking soda, mixed with a high quality detergent, can give good control of mildew on a number of plant species. It is thought that the high pH of the spray which coats the leaves inhibits the growth of the mildew disease. Current recommendations are for two teaspoons of baking soda per litre of water, with a good 'squirt' of a high quality liquid dish washing detergent (or other wetting agent such as 'coco-wet'), applied as a preventative spray will give reasonable control of mildew disease.
There are numerous fungicidal products for the control of mildew disease.