48 



THE MEANS OP CONTROLLING PLANT DISEASES 



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 



Fig. 72. Showing difference in growtli of wheat from rusted 

 and unrusted mother plants of the same crop (alternating) . 

 Seeds planted s^me day and plants same age, 



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 spil 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 

 aerate 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-blig'ht, 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. 



Seeds and seed treatment. 



Of all the features of cropping which allow of 

 direct efilort toward controlling or avoiding disease, 

 the seed is open to the easiest and most effective 

 study. It is an old saying that the seed-time de- 

 cides the harvest. It might as truly be said that 

 the type of seed, how it is cared for and handled 

 and prepared for the soil, decides what the harvest 

 shall be. This is particularly true with types of 

 plants that are subjected to certain crop diseases. 



In handling the seed preparatory to the greatest 

 possible control of plant disease, one should always 

 have in mind a number of very important factors. 

 The introduction of new varieties into standard 

 cropping regions is often attended with troubles 

 arising from disease introduction. Some varieties 

 may not only prove worthless because of lack of 



M\|J|J|| £/ 



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Fig. 73. Wheat from treated and untreated seed. Two bun- 

 dles of wheat heads cut at the same distance from ground 

 from two plots of wheat (the actual area two square feet). 

 1. From very smutty untreated seed; 76 per cent of smutty 

 heads in this sample. 2. Grown from same seed but 

 treated by the formaldehyde method to prevent smut. 



disease-resisting powers, but also may often prove to 

 be great disseminators of disease to the standard 

 crop of the locality. This feature may be noticed 

 in all types of plants, but is markedly noticeable 

 among cereals with reference to rust, as in differ- 

 ent varieties of oats and of wheat. For example, 

 it is very probable that the introduction of winter 

 varieties of wheat into noted spring-wheat areas is 

 alone sufficient to account for the rapid disappear- 



