THE MEANS OF CONTROLLING PLANT DISEASES Almost every farm, garden and orchard crop is open to the attack or influence of one or more kinds of infectious disease. As farming, garden ing, or fruit-producing districts age under cultiva tion, the soil ages, and the conditions and materials that are favorable to the development of disease accumulate. Each crop, or type of cultivated plant, unless properly handled, becomes more and more susceptible to disease, and is more liable to •be attacked by disease-producers that are natural to the habits and growth conditions of that particular kind of crop.
Practically every known cultivated plant and crop, including hothouse-grown plants, vegetables, fruit and shade trees, grasses and cereals, is thus attacked and the yield and quality are often greatly reduced. It is to be expected that the warfare will continue.
Parts of the plant attacked.
Many plant diseases may be said to be systematic or constitutional in the same sense as observed in animal troubles. Though certain parts may be pri marily the chief source of attack, as, for example, leaves in the case of rust, yet the effect on the physiology of the plant finally becomes general. All such diseases reduce the vitality of the plant body as a whole. The points of first injury are various, according to the kind of plant attacked and the nature of the organism which brings about the dis ease. There are "root diseases," "leaf diseases," "diseases of the stem" and "diseases of the fruiting parts," but, as indicated, these terms are so applied largely because the disease first appears on certain parts or is finally most destructive to these parts.
The destruction or damage depends largely on the part that is thus attacked, but also varies greatly according to the kind of organism that produces the disease, the period in the life of the crop when the disease appears, and almost directly according to the environment of weather and soil conditions.
The cause of disease and the effects produced.
The effects produced by disease on the individual plant and on a crop depend on the character of the plants attacked, the nature of the organism that causes the trouble and, as just indicated, on the life conditions, such as heat, light, moisture, fertil ity of soil, drainage, soil texture, and the like.
Some diseases are of parasitic character and are directly infectious, as, for example, fire-blight of apple, wheat-smut, or wheat-rust. Others are im perfect parasites, or merely decay-producers, which become materially destructive only under special conditions of the soil or atmosphere. Some of these last-named types at times become exceedingly de structive, as in the case of numerous decay bacteria and molds on vegetation under conditions of ex cessive moisture. The work of the various damp ing-off fungi is a good example.
Some plant diseases are more or less local in action and temporary in results, depending on the character of the plant and the part attacked or on sudden changes in the weather. There are many others, such as plum-pocket, black-knot and potato scab, that are perennial or persistent, year after year, dependent on special peculiarities of the life history of the organism that causes the trouble, peculiarities of the life of the plant attacked, on some method of cultivation and handling of the crop or soil, or on soil characters that allow of persist ence from year to year in the soil ; or, again, the disease may be transmitted on the parts of the plants that are necessary to continued yearly propagation.
These numerous peculiarities as to conditions, types of disease, modes of attack, differences in types of plant affected, and so on, allow us to contrive as to methods of combating or controlling crop diseases. Such features, closely studied, often make means of complete prevention possible. In some of the most destructive diseases of farm crops, such as potato-blight, stinking smut of wheat, and grape-rot, methods of prevention have been found quite practicable and have come into general use. One cannot estimate accurately the value of the results obtained, but the writer be lieves that from the smuts of cereal grains alone the people of the United States, through practices of seed disinfection, save annually in crop yields in values approximating $20,000,000 to $30,000,000. There are yet other plant diseases, such as wheat rust and apple-blight, in which the natural condi tions influencing their development are so compli cated that means of prevention or control, as yet recommended, have given slight results.