Ellen Wharton, age 13, of Norristown, Penn., for her question:
WHY IS MILDEW BAD?
Mildew is a fungus which attacks plants and some products made from plants and animals. It is a serious problem in damp tropical countries because it also attacks clothes unless they are kept dry. Even in temperate regions mildew can cause problems, especially with plants.
Mildew's names comes from a Middle English word "mealdew," which means "spoiled meal." There are two main classes which damage useful plants: powdery mildew and downy mildew.
Powdery mildews attack green plants. There are about 50 different kinds of powdery mildews, and some of them can attack several different plants.
Actually, about 1,500 different kinds of flowering plants may be infected by powdery mildews. These include such common plants as the pea, peach, rose, apple, cherry and grape.
The mildew fungus usually grows on the outside of the leaves. Sometimes it also forms flowerlike blotches on the stems and fruits. These blotches consist of many fungus threads that send out short branches with sucking organs into the stem or fruit.
Copper sprays and sulfur dusts protect many 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 lettuce, onion, grape, cucumber and cabbage. In 1845 and 1846, 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 a Bordeaux mixture.
A Bordeaux mixture is made by mixing four pounds each of copper sulfate with hydrated lime in 50 gallons of water. Each chemical is mixed separately in a small amount of water before it is added to the remaining water. The mixture can be used on vegetables, fruits and flowers.
Mildew often attacks bookbindings in damp climates. Books kept in damp or poorly ventilated places also are subject to mildew.
To protect books from mildew, some volumes are kept in an enclosed bookcase along with a form of paraformaldehyde powder. This powder evaporates to form a protective atmosphere.
If books must be stored in an open place, good air circulation will help prevent mildew. Once mildew has formed, dusting or wiping may remove it from the outside of the book, but this will not stop the mildew from continuing to grow.
Several chemical solutions can be used to completely eliminate mildew from book bindings. Most contain mercurials, such as mercuric chloride. Great care must be used since mercurials are highly poisonous.