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Two varieties
There are two families of fungi that are parasitic on living plants. The two families are Peronosporaceae, comprising the downy mildews, and Erysiphaceae, comprising the powdery mildews.
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Powdery mildew
The powdery mildews are called so because of the powdery, cobwebbed pattern that their numerous, white spores produce on the leaves of green plants. They attack green plants, usually those grown in the shade in humid regions. They attack nearly 1,500 varieties of flowering plants including gooseberry, pea, peach, rose, apple, cherry and grape, lilac and hackberry. Attacks of powdery mildew cause curling and withering of leaves and often prevent new shoots on the plant. |
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Powdery mildew |
Downy mildew
Downy mildew is a disease that attacks plants in cool humid regions. The fungus grows from a single
fertilized cell called a spore. On the upper surface, pale-green to yellow or brown areas usually develop. When the mildew attacks the top part
of a leaf, small spores come out of the breathing pores on the bottom of the leaf. Downy patches of mildew that are white, grey, bluish, or violet generally form on the undersides of leaves in damp weather.
Leaves of plants that are affected by downy mildew often wilt, wither, and die early. Sometimes, stems, flowers and fruits are also are infected. Seedlings may wilt and collapse. Garden plants, bush fruits, vegetables, and certain trees, shrubs, field crops, and weeds are susceptible.
Downy plants attack plants such as grape, cucumber, cabbage, lettuce and onion. It can be avoided by rotating annual flowers and vegetables and by avoiding
over watering, overcrowding, and poorly drained soil. Other measures include growing resistant varieties, sowing disease-free seed, removing diseased parts and crop refuse, eliminating weeds, and maintaining balanced soil fertility. The application of copper is effective
against many downy mildews, but the amount of residue on vegetables must be taken into consideration. Sometimes plants are sprayed with a solution of copper sulphate and lime in water.
Clothes and books too
Mildew is a serious problem in damp tropical countries. It can attack clothes, unless they are kept dry. In temperate countries, clothes, if kept wet for too long, are susceptible to being attacked by mildew. Books kept in damp or poorly ventilated places also fall prey to attacks of mildew. There are chemical solutions that may be used to prevent mildew from attacking bookbinding, but many of the solutions are poisonous and should be used with caution and care. |
