Powdery mildew is a fungus disease that affects plants on a world wide scale. On crops such as the cucurbits (cucumber,, melons, and pumpkins), powdery mildew is the principal disease causing yield and quality reductions in food crops. While powdery mildew does reduce growth and yields, its main effect tends to be in the reduction of quality, particularly of greenhouse crops such as cantaloupe.

Signs and Symptoms
As with any disease, the first step to control is accurate identification. Powdery mildew is often confused with `downy mildew' another disease which produces whitish clumps of fluffy spores on the leaf surface. However the two can be distinguished from each other by the symptoms. Downy mildew is also common on cucurbits and other crops such as lettuce, grapes, peas, roses and other cut flowers, where it appears as greyish white downy patches on the undersides of leaves. 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. On roses, downy mildew may appear as irregular reddish purple spots on sepals of flowers and leaves, the leaves may then develop burnt margins and drop off. If the entire plant becomes heavily infected, then total crop loss can occur.

Powdery mildew, is much more common than Downy mildew and many vegetables, bedding and fruiting plants, shrubs and even trees may be infected. Powdery mildew covers much of both the lower and upper sides of the leaf. Areas affected with powdery mildew enlarge in a circular pattern, 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. Fruit of cucumber, cantaloupe and squash are usually free of visible infection, even when the foliage becomes white with the spread of the fungal spores, however the fruits will ripen prematurely and will lack flavor under these conditions. Later fruits will often fail to mature and will be small and often misshapen.

Control of Mildew Diseases

Prevention
Powdery mildew is favored by dry atmospheric and growing conditions, moderate temperatures, reduced light intensity, good nutrition and succulent plant growth. For this reason it can be a problem in protected growing areas such as greenhouses, conservatories and grow rooms, on a year round scale. All mildew spores are spread via wind or air movement, and certain insects can also carry the disease. Unlike other fungal diseases, such as downy mildew, the powdery mildew spores don't require a film of water to be present on the leaf surface to germinate and infect the plant tissue. Powdery mildew spores germinate best in a temperature range of around 22 - 31 C (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 22 - 31 C (72-88F), and absence of moisture then germination will occur within two hours, and infection will be two days later.

Cultural Control
With downy mildew which requires moisture (relative humidity 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. Since powdery mildew can infect the crop under dry conditions, it's more important to select resistant varieties, and make sure plants are well spaced. Often a major source of mildew infection can be from plants growers buy in from a nursery - these should be carefully checked for mildew, and treated before they enter the greenhouse. Between crops, all surfaces of the growing area, beds and gullies should be disinfected with a strong bleach solution or with an anti-mildew fungicide to prevent the carry over of disease to the next crop.

Low Chemical Control
Powdery mildew is not difficult to control, so often the `low chemical' or `organic' methods are the best line of defense. The powdery mildew fungus is vulnerable to the action of sulphur throughout most of its life cycle. Sulphur works by `selective toxicity' that is, it is more toxic to the parasite than to the host. Sulphur should not be applied to cantaloupes because they are sulphur sensitive. Cucumbers are somewhat sensitive to sulphur, gourds, pumpkins, squashes, watermelons and most other plant species are sulphur-tolerant. Sulphur dusts are less injurious than sprays, but also less effective than sprays since they do not result in as effective leaf coverage. Sulphur offers long lasting protection and is effective in the absence of moisture. The warmer the temperature, the greater the vaporization of sulphur. One of the up and coming products on the market is a "Sulphur Evaporator". That uses sulphur pellets and are evaporated with the heating element found inside the "Sulphur Evaporator".  In greenhouses and other growing areas, sprays, dusts or aerosol bombs may be used. For the control of downy mildew, copper compounds are effective, but they should be used with caution, as repeated applications can cause crop damage

Many rose and cucumber growers achieve control or prevention of mildew disease with application of silica based dusts or sprays. Silica gives a protective coat over the leaf surface and thus prevents the mildew spores from germinating and infecting the plant tissue.

** Fungicides for Mildew Control
There are numerous fungicidal products for the control of mildew disease. The problem with `chemical' control is that many of the fungicides have caused the development of resistant strains of mildew disease. Please check your local Hydroponic gardening stores for up in coming products.

**NOTE: it is important to check with your local department of agriculture to see which fungicide products are registered for use on food crops in your area. FHD