KEEPING plants healthy depends in large part on recognizing their enemies. You can do this by learning the patterns made on your trees, shrubs, flowers, and vegetables by the various types of animal pests, by fungi, bacteria, and viruses, by unfavorable weather and soil conditions. You may never see the insect which causes a particular leaf pattern, and if you should capture it you would probably not, as an amateur gardener, have a chance to examine it under a binocular. You are even less likely to identify fungi under a compound microscope and in pure culture. You can, however, with a little practice in close observation learn to be a pretty good detective with out being a laboratory scientist. And you've got to be such a detective nowadays before you can choose the right medicine from the nearly 50,000 trade-marked preparations now on the market.

More new chemicals will be announced before this book gets into print, but diagnosis remains fundamental. Leafhoppers and red spiders both take the color out of leaves by sucking from the underside. The former are readily controlled by DDT, the latter vastly increased by it. You must know the problem before you can choose the right solution. When you see birch trees "blighted" through the countryside, your instinct is to use bordeaux mixture, a fungicide, but it would not do any good because those big brown blotches in leaves are caused by leaf-mining insects. Nor is it a fungus disease when sugar maples have scorched foliage in summer and beech leaves turn reddish. These are weather reactions, and spraying will not have the slightest effect.

Animal pests in gardens include insects, mites, millipedes, sowbugs, mammals, and a few birds, although most birds are, of course, very helpful.

Insects belong to the animal phylum Arthropoda, which means jointed legs, and they differ from other arthropods in never having more than three pairs of legs. They have three main body divisions-head, thorax, and abdomen-and usually two pairs of wings in the adult stage. They breathe by means of pores (spiracles) along the body, which open into a system of air tubes (tracheae). They do not have bones but an exo skeleton, outer shell, hardened in sections with chitin and with joints in between. Insects grow by a series of molts, casting off their old skeletons until they reach the adult form.

Some insects have a gradual metamorphosis, adults resembling young nymphs except for possession of wings. Others have a complete metamor- phosis, the adult-butterfly, moth, fly, or beetle-looking totally unlike the larva-caterpillar, maggot, or grub. The transition from caterpillar to moth is made in a cocoon; that from grub to beetle, in a pupa.