In order to arrive at proper control or reason ably complete prevention of plant diseases, farmers and gardeners must study all characteristic fea tures of the soil, climate, and conditions of plant growth, that affect the development of the indi vidual plants or crops attacked, as well as those conditions that affect, further, or prevent the de velopment of the disease. In this connection, it must be remembered that the development of disease in the crop is associated directly with the conditions that favor the propagation and dis tribution of the disease-engendering organisms. Therefore, close attention should be given to all features affecting the relationship of soil, air, seed and individual plants to crop development. All conditions should be sanitary.
Soil considerations.
In this connection the soil is a factor of great importance, and one should consider such features as texture, drainage, chemical nature, fertility and position, that is, the kind or type of soil and location of the field for the particular crop which it is intended to produce. It must be such as to furnish the properly balanced food supply for the crop or plant growth, so that there may be a regular proper growth and evenness of ma turing. Soil drainage must be right, for it greatly affects many features and conditions that gov ern plant growth. It directly influences such fea tures as soil texture, soil and atmospheric mois ture, and temperature ; and it has a particular bearing on the dissemination or distribution and life of plant diseases in the soil. Surface waters not only cause a souring of the soil and a general sickening of plant growth, but they also serve as a means of rapid distribution of the spores of disease from plant to plant and from soil area to soil area, until, in such soil diseases as cotton root rot, potato-scab or flax-wilt, all flooded areas are quickly overrun or permeated by the disease-pro ducing organism. Poorly drained farm lands not only directly distribute certain diseases but also, through evaporation, directly affect the air con ditions, causing heavy fogs and dews. In the case of such diseases as the rusts of cereal grains, these conditions result in the greatest possible crop destruction. If soil drainage is not proper, it must be made so before one may hope for best results in the control of some of the plant diseases.
Treatment of the soil is a phase of work not evenly developed. There are numerous types of dis ease, especially those which find permanence in the soil, that may be controlled to a large degree through proper culture, rotation of crops, soil resting, and soil weathering. In certain types of troubles, chemical applications have been found to be efficacious. All such methods and treatments de pend for their basis on the nature of the particular disease-producing organism. Proper crop rotation
rests the land, keeps up an equable plant-food ration, and lessens the possibility of disease accumulation, because each plant disease is special in its wants and cannot increase in the absence of its host.
Soil disinfection by means of chemical substances directly applied does not yet give great promise. The disease-producers are usually possessed of greater powers of resistance than the delicate roots of cultivated plants. Careful study of the soil constituents and physical condition often allows of soil treatment that is beneficial in reducing the effects of disease. Some diseases, such as potato scab and flax-wilt, caused by soil fungi, are found to develop with much greater damage on markedly alkaline soils than on soils of neutrality. This is comparatively easy of correction through the use of barnyard manures, the growth of grasses, and the like. Soils of poor texture often result in such weak growths that ordinary infectious diseases become more destructive than under proper tillage. Such features must be remedied by proper methods of handling the soil preparatory to cropping. To this end, plowing and cultivating at the proper time agate the soil, allow it to weather and become a large factor in destroying germs of disease that hold over in the soil from year to year. This, we are rapidly learning, is one of the real truths back of proper crop rotation. [Another discussion of this subject will be found in Vol. I, pages 450-453.] Climatic conditions.
When considering possibilities of controlling plant diseases, the matter of prevailing climatic conditions, to which the crop must be subjected, is of much importance. It decides, primarily, whether or not one should attempt to produce the crop under question, and indicates what variety of the particular crop or type of plant should be selected. While prevailing climatic features can not be directly controlled, one may often avoid the difficulties which they bring about. This matter of climate governs the time of planting, mode of har vest, the types of cultivation, and all such features. To escape the worst effects of disease on farm crops, one must take such features into consider ation, avoiding, if possible, those types of work and methods which allow natural climatic condi tions to favor disease development. For example, in the case of spraying to prevent diseases such as apple-scab and potato-blight, one must consider carefully the time when the work will prove most effective. This will depend almost wholly on the prevailing atmospheric and weather conditions, which account for the spread of the various types of disease-producing parasites and for their vary ing stages and destructiveness of development.