John Eyler, age 12, of Greenville, miss., for his question:

WHAT IS MILDEW?

Mildew is a fungus which attacks plants and some products made from plants and animals. Its name comes from the Middle English word "mealdew" which means "spoiled meal."

There are two main classes which damage useful plants: powdery mildew and downy mildew.

There are about 50 different kinds of powdery mildews and some of them can attack several different plants. Powdery mildews generally attack green plants and about 1,500 different kinds of flowering plants may be infected.

The mildew fungus usually grows on the outside of the leaves. Some of the plants that can be hit include pea, peach, rose, apple, cherry and grape. Sometimes the mildew also forms flowerlike blotches on the stems and fruits of the plants.

Mildew blotches consist of many fungus threads that send out short branches with sucking organs into the stems or fruit. Copper sprays and sulfur dust protect plants from powdery mildew.

Downy mildews produce yellow spots on the upper surfaces of the leaves or young fruits. The fungus grows from a single fertilized cell called a spore. When the mildew attacks the top of a leaf, small spores come out of the breathing pores on the bottom of the leaf. These spores produce even tinier spores which swim in the dewdrops on the surface of the leaf. The spores start new infections by sending tiny threads into the leaf.

Downy mildews attack many plants, including the grape, cucumber, cabbage, onion and lettuce. One way to protect plants from downy mildew is to spray them with a Bordeaux mixture.

Mildew is a serious problem in damp tropical countries because it also attacks clothes unless they are kept dry. Even in temperate regions, clothing should not be allowed to remain wet long.

Mildew can also attack a book's binding in damp climates. Books as well as bookbindings are subject to mildew if they are kept in damp or poorly ventilated places.

If books must be stored in an open place, good air conditioning will help prevent mildew. Once it has formed, dusting or wiping may remove it from the outside of the book. However, this will not stop mildew from continuing to grow.

Several chemical solutions will prevent mildew when applied to the bookbindings. Most of the solutions contain mercurials, such as mercuric chloride. They are highly poisonous and should be used with great care.

In 1845 and 1846, downy mildew almost destroyed the entire Irish potato crop. A terrible famine followed.

Mold also belongs to the fungi group and it is closely related to mildew.

Molds have no chlorophyll, or green coloring matter, and therefore cannot manufacture their own food. They must live on food that is made by other plants or animals, or on decaying matter.