Mildew Diseases

Powdery mildew is a fungus disease that affects plants on a world -wide scale. On crops such as the cucubits (cucumbers, 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 by the symptoms. Downy mildew is also common on cucurbits and other crops such as lettuce, grapes, peas, roses and all other cut flowers, where it appears as grayish white downy patches on the under sides of leaves. The first symptoms of downy mildew is often the appearance of light green or yellow spots on the upper surface of the older leaves forming on the under side 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 enlarged 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 theses conditions. Later fruits will often fail to mature and will be small.

Prevention

Powdery mildew is favored by dry atmospheric and growing conditions, moderate temperature, reduced light intensity, good nutrition and succulent plant growth. For this reason it can be a problem in protected growing areas such as greenhouses and indoor grow rooms on a year round scale. All mildew spores are spread via wind of 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 72- 88F and in shaded areas of the crop, so it is more severe in closely planted crops. If the spores entering a new grow area make contact with a plant under conditions of reduced light intensity, a temperature of 72-88 F 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 is 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.

Chemical Control

Researchers have found that powdery mildew fungus is vulnerable to sulfur. Sulfur works by ‘selective toxicity’ that is, it is more toxic to the parasite than to the host. Sulfur should not be applied to cantaloupes because they are sulfur sensitive. Cucumbers are somewhat sensitive to sulfur, gourds, pumpkins, squashes, watermelons and most other plant species are sulfur –tolerant. New on the market to combat sulfur is the “sulfur evaporator”. This item uses sulfur pellets which are evaporated with the heating element found inside the sulfur evaporator. 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. Sulfur sprays and dusts are also used, Special not: Don not use Sulfur 2 weeks before harvest on most fruit bearing plants. Sulfur can leave a residue on the fruit, making the fruit taste undesirable. For the control of downy mildew, copper compounds are effective. Use only approved products by your local department of agriculture.

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