METHODS OF CONTROLLING PLANT DISEASE The checking of a plant disease, so that if not actually elim inated it is reduced to manageable proportions, is spoken of as its "control." This may be complete or partial. The methods of control may be divided into two main groups, first those which are put into practice by the grower himself, and second those which are enforced upon him by legislation. In the following dis cussion these will be considered separately.
Resistance.—For every plant there is a certain range of en vironmental conditions within which it grows best. If the condi tions vary widely in any important respect from those which are ideal, the plant grows badly. A weakly growing plant is in gen eral more susceptible to parasitic attack than is a strongly grow ing one, and the abnormal condition of the plant resulting from the unfavourable environment may in itself be so pronounced as to be called diseased. Good cultivation, therefore, which aims at giving the plant the optimal conditions for its growth, is the first line of treatment to be adopted with a view to lessening the incidence of disease.
The most approved methods of husbandry will not however guarantee freedom from plant diseases, nor are they always practicable. More special methods must therefore be adopted. In recent years it has become increasingly recognized that the main line of defence against many important diseases consists in the development of immune or resistant varieties of the host plant.
To obtain varieties of a crop plant which are resistant to a particular disease is in many cases not difficult. These may in
fact exist among the varieties already in cultivation. If the re sistant forms possess high merits in other respects—e.g., in yield ing power, in quality of product, etc.—then the problem of the particular disease is in large measure solved. Often, however, the resistant varieties which are already known to occur show cer tain disadvantageous features, and these may be so great as to outweigh the benefits conferred by the high disease resistance shown.
One method of obtaining a resistant strain from a commercial variety consists in growing the plants under conditions in which they are exposed to infection and selecting out for propagation those individuals which stand up successfully against the disease. This method has been adopted with great success in the develop ment of strains resistant to some of the important wilt diseases (cotton, cabbage, etc.).
A more elaborate method consists in the artificial crossing of parents of known characteristics, followed by the sorting out of the different types which occur among the progeny. (See PLANT BREEDING.) This is the application to problems of disease of the principles of Mendelism (q.v.). Resistance to disease behaves as a single or multiple Mendelian factor, and is usually dominant in the immediate offspring.
Destruction of the Parasite.—The control of plant diseases by the adoption of measures against the fungus is best con sidered with reference to the life-history of the fungus and in particular to the manner in which it reinfects the growing crop after each period of winter dormancy.
If the fungus is a soil parasite, it may be possible to starve it out by ensuring that it does not come in contact with the particular crops on which it grows and multiplies. This in volves a system of rotation of crops. The practice of crop rotation is justified for other agricultural reasons and is of great value in that it tends to check the multiplication of specific soil (and other) parasites. Some soil parasites however are able to live indefinitely on the humus material of the soil so that they cannot be starved out (e.g., some wilt diseases), others may persist so long in the dormant form that the interval required between successive susceptible crops is too long to be economically practicable (e.g., wart disease of potato).