Yet in each case here mentioned, as well as in most of the other of our common and destructive diseases, cheap and effective means of control are within the reach of every grower.

The value and efficiency of these means have been established beyond doubt. Their profitable application requires only intelligence and practice on the part of the grower.

Symptoms of disease in plants are so varied in character as to make an attempt at wholly satisfactory grouping for practical purposes of doubtful value. Mention of some of the more common types, however, may be useful. The grower must learn by study and experience the more striking symptoms characteristic of those diseases peculiar to the crops that he grows.

Disease may be exhibited in malformations of the leaf, stem, root or fruit, as for example, knots, galls, tubercles, curling, wrinkling or other distortions. There are such symptoms in crown-gall of trees, black-knot of plums and cherries and leaf-curl of the peach (Fig. 1279). Another type are cankers, dead sunken or roughened areas in the bark of trees or the outer rind of herbaceous stems, as for example in the New York apple-tree canker, the brown-rot canker of peaches, frost cankers of many trees, and anthracnose of beans, melons, and others. The blight type of lesion is also very common. Here are the more or less sudden death of leaves, stems, shoots or blossoms, usually turning dark and drying up. Such symptoms characterize fire-blight of fruit trees, potato-blight (Fig. 1280, from Vt. Sta.), alternaria blight of ginseng and similar diseases, especially in their last stages. The leaf- or fruit-spot type is also very common. Brown or black spots appear in foliage or fruit. They may be brown dead or rotted areas, or spots due to the growth of the parasite on or under the surface.

Bordeaux-injury spots on apple foliage, shot-hole leaf injury of stone fruits, leaf-spot of the currant (Fig. 1281), celery or alfalfa, the tar-spot of the maple, the black-spot of the rose and the apple-scab are of this type. Another not uncommon type is that exhibited in certain bacterial and fungous diseases, where the pathogen infests the sap-tube regions of the stems or petioles, resulting in a sudden wilting of leaves and shoots. The wilt diseases of cotton, cucumber, ginseng, watermelon and cowpeas are characterized by this symptom. The yellowing of the foliage, either suffused or localized as spots, rings, and blotches and often accompanied by dwarfing and wrinkling of the affected organs is a common symptom of certain so-called physiological diseases like the peach yellows (Figs. 1282, 1283), little-peach, mosaic disease of tobacco, infectious chlorosis and nitrogen-poisoning of greenhouse cucumbers (Fig. 1284) and other plants.

Effects of the leaf curl fungus on peach foliage. (X1/2)

Fig. 1279. Effects of the leaf-curl fungus on peach foliage. (X1/2)

The causes of disease in plants.

Etiology, or the cause of disease, has been more generally and carefully investigated than any other phase of the subject, so that we now know much regarding the agents primarily responsible for most plant diseases. These agents may be grouped as follows:

Slime molds, lowly organisms having characters of both plants and animals (see article Fungi). The club-root of cabbage, cauliflower and other crucifers, is the best known slime-mold disease.

Bacteria, microscopic unicellular plants which multiply very rapidly by simple fusion (see article Fungi). While most species are harmless scavengers of dead organic matter, and a few are known to cause diseases of men and animals, not less than 150 different diseases of plants are now known to be due to the attacks of parasitic bacteria. Some of the commonest bacterial diseases of plants are, fire-blight, crown-gall, olive-knot, soft-rot of vegetables, potato-scab, cucumber-wilt and black-leg of potatoes.

Fungi (see Vol. Ill) are perhaps responsible for far the greater number of the diseases of plants. They are the causal agents in such well-known diseases as apple-scab, brown-rot of plums and peaches (Fig. 1285), black-rot of grapes, (Fig. 1286) bitter-rot of apples, brown-rot of lemons, late blight of potatoes, peachleaf-curl, heart-rot and canker of trees, mildew of many plants, rusts and smuts of cereals (Figs. 1287, 1288, Kansas Experiment Station); in fact the mere enumeration of the more common fungous diseases of plants would fill many columns in this volume.

Early blight of potato.

Fig. 1280. Early blight of potato.

Algae, low forms of green plants, most of them living in water or very damp places. Few are known to produce disease in plants. The red rust of tea is one of the best known algal diseases.

Currant foliage attacked by the leaf spot fungus. (X 1/2)

Fig. 1281. Currant foliage attacked by the leaf-spot fungus. (X 1/2)

Parasitic angiosperms, - flowering plants, of which there is no inconsiderable number, causing more or less injury to the plants upon which they live. These parasites are usually markedly degenerate in one or more respects, as a result of their parasitism, being often without true roots, or without leaves and frequently without chlorophyl green. As examples we may mention the mistletoes, dodders and broom rapes.

Insects (see page 1034) cause such diseases as galls and similar malformations.

Nematode worms, - minute all but microscopic in size and multiplying rapidly, they constitute one of the greatest crop pests, especially in warm or tropical countries. They usually infest the roots, causing galls or swellings. Some species injure the plants by destroying the fine feeding roots as in the case of the nematode parasites of oats so destructive in certain countries of northern Europe. Over 400 different plants are known to be subject to the nematode root-gall disease. (See pp. 1041-2.)