The three fundamental factors, temperature, humidity and light are, in the open field, to a large extent beyond control. Neverthe less, it is often possible, in agricultural practice, materially to modify their action. By suitable methods of cultivation—draining, subsoiling, ridging, manuring—the amount of soil water can be regulated. By the provision of shade trees—as is usual in many tropical cultivations—or by suitable spacing of the crop itself or by pruning, the intensity of light or humidity of the air can be adjusted so as to approximate to the needs of the plant.
Reference has been made to soil moisture as a factor of importance in connection with plant diseases. This is merely one of the many soil factors which influence plant growth and which therefore require to be considered in dealing with the liability of plants to disease. The physical texture of the soil and the presence or absence of certain chemicals are of importance mainly inasmuch as they influence soil humidity. But there are in addition a number of soil factors which act otherwise. Thus the amount of certain constituents present in the soil (e.g., nitrogen, potash, phosphate) influences not only the quantity of plant growth but also, to an appreciable degree, its quality—and in particular its susceptibility to disease. The pres ence or absence of lime, by controlling the reaction of the soil, is in some cases the factor which determines whether a particular disease will develop or not. Soil aeration is a factor in itself, though in practice it is often difficult to separate its effects from those due to soil humidity.
So far the discussion of the factors which influence plant disease has been limited to the environment. The latter, however, merely modifies the result which is in the main conditioned by the structure and physiology of the host plant itself. These "internal" factors are more elusive than those already discussed and in their fundamental aspects are practically un mapped. A few examples of internal factors and of their manner of functioning will be given in the following section. Nevertheless, though very little is known of the underlying factors which deter mine immunity or susceptibility, certain phases of the problem are open to direct observation and experiment.
From the foregoing sketch it is clear that the complete investiga tion of a plant disease may ramify into various branches of agricultural science. The plant pathologist is thus much more than a mycologist, or student of fungi, though in practice the two names are often used as equivalent. Plant disease investigations may call for the joint work of the mycologist (or entomologist), the soil chemist, the physicist and the practical agriculturalist, and lastly—as will be shown more clearly later—the co-operation is required of the geneticist for the breeding of resistant varieties, and of the pure chemist for the preparation of chemical means of combating the disease.