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Mildew Diseases

Author: Dr Lynette Morgan

Powdery mildew is a fungus disease that affects plants on a world wide scale. On crops such as the cucurbits (cucumber, gerkins, melons, courgette and pumpkins), powdery mildew is the principal disease causing yield and quality reductions in hydroponic culture. 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. Fortunately, for hydroponic growers, mildew disease is easy to identify and there are a number of both `low chemical' and fungicidal products which can be used to prevent and control disease outbreaks.

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 and time of year at which they infect the crop. Downy mildew is also common on cucurbits and other crops such as lettuce, grapes, peas, roses and other cut flowers, where it appears as grayish 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. On crops such as hydroponic lettuce, with a mild infection of downy mildew, the outer, wrapper leaves are usually removed at harvest and little damage occurs to the harvestable portion of the plant. 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. As with downy mildew, the cucurbit crops are the most susceptible, but under hydroponic cultivation, lettuce is also highly prone to infection under warm, drier conditions (downy mildew being favored by cool, moist conditions). While downy mildew tends to produce the whitish spores on the undersides of the leaves, 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 is a very common disease in later summer and Autumn (Fall), but 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. In fact, if the powdery mildew spores are in contact with water they are inhibited to a certain extent. Smaller growers can achieve some control over powdery mildew by simply spraying the leaves with a garden hose in the afternoon to help prevent infection. 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.

One of the best ways we have of preventing mildew disease is through the use of `resistant varieties'. Resistant types of cucumbers and cantaloupes are available and should be selected for use in Summer-Autumn grown crops. Resistance to both downy and powdery mildew has been bred into a number of plant species, so it's important to check with your seed company to see what is available.

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. An effective sulphur application rate would be 110-220ml per 100 litres of water of a `suspension or wettable powder' product, when the fungus is first observed, followed by repeat applications two weeks later. 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 and since sulphur acts as a vapor one way of using it in a greenhouse is by painting it on greenhouse heating pipes during colder growing periods. During warmer growing periods, 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. Another low chemical method of control which has achieved some success for many growers is the use of sodium bicarbonate sprays (common baking soda). It has been proven that baking soda, mixed with a high quality detergent (as a sticking agent and surfactant), 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. The current recommendations are for 2 teaspoons of baking soda per litre of water, with a good `squirt' of a high quality liquid dishwashing detergent (or other wetting agent such as `coco-wet'), applied as a preventative spray will give reasonable control of mildew disease.

** 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. Sulphur dust or flowers of sulphur poses little or no risk of inducing resistant mildew stains. Others such as Benomyl can no longer be used because resistant strains are now very widespread. One of the chemical fungicides which is still effective against powdery mildew is `triforine', sold under the name of `Saprol'. Saprol is a systemic fungicide with both curative and protectant action against a range of fungal disease. This should be sprayed at the fist sign of infection (at a range of 150ml/100 litres of water), and repeated 10 days later. Powdery mildew can develop resistance to Saprol, so its use should be limited to 4 applications per season, and rotated with use of another fungicide such as sulphur based products. Other fungicide products which pose less risk to inducing resistance are those containing pyrazophos, and dinocap (these may go by various trade names including Afugan or Sabithane).

**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.

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