THE MEANS OP CONTROLLING PLANT DISEASES 



47 



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 fijst 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. 



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- 



