Mildew is the name of many kinds of fungi and protists that attack plants and some products made from plants and animals. The name comes from a Middle English word, mealdew, meaning spoiled meal. Two main groups of mildews damage living plants: fungi called powdery mildews and protists called downy mildews. The term mildew also can be applied generally to any fungus that grows in a damp place.
Powdery mildews attack hundreds of flowering plants, including apples, cherries, gooseberries, grapes, peaches, peas, roses, and squash. Many of these mildews attack one type of plant, while some can attack several different types.
A powdery mildew usually grows on the surface of the leaves, but it may also grow on the stems and fruits. The mildew forms white blotches that consist of many hyphae (threads). These hyphae send out structures called haustoria that absorb nutrients from the cells of leaves, stems, or fruits. Hyphae also produce cells called spores by which the mildew reproduces. People can use certain fungicides (chemical solutions that kill fungi) to protect their plants.
Downy mildews produce yellow spots or watery blotches on the upper surfaces of the leaves or young fruits. After mildew attacks the top of a leaf, branchlike hyphae grow from the stomata (breathing pores) on the bottom of the leaf. Tiny, sporelike structures called sporangia soon develop on the hyphae. These sporangia may produce even tinier spores that swim in dewdrops on the leaf’s surface. Infections occur as the spores develop hyphae that grow into the cells of leaf or fruit tissue. In warmer conditions, sporangia may develop hyphae themselves and directly infect the plant tissue.
Downy mildews attack many plants, including grapes, cucumbers, cabbages, onions, and lettuce. In the 1840’s, a fungus related to downy mildew almost destroyed the Irish potato crop, and a terrible famine followed. One way to protect plants from downy mildew is to spray them with such copper-based fungicides as Bordeaux mixture. Scientists also have developed disease-resistant plants that can withstand mildews.
Mildew creates a serious problem in moist, tropical countries because it attacks damp clothes. Even in climates with less humidity, people should not allow their clothing to remain wet for too long.
Mildew also may attack bookbindings in damp or poorly ventilated places. To protect books from mildew, keep the volumes in an enclosed, dry bookcase. If books must be stored in an open place, good air circulation will help prevent mildew. Applying certain chemical solutions to bookbindings may also prevent fungal growth. These solutions, as well as the fungicides used to protect plants from mildews, may be highly poisonous and should be used with extreme care.
