CHAPTER XX 



FUNGOUS AND BACTERIAL DISEASES OF PLANTS 



Estimates which have been placed upon the damage caused by preva- 

 lent plant diseases during a single season amount frequently to a very con- 

 siderable per cent of the total value of the crops. In the United States 

 alone the destruction wrought by fungous diseases is sometimes not far from 

 half a billion dollars. DUGGAR, "Fungous Diseases of Plants," pp. 7-8 



Civic aspects. Line fences of farm or city lots offer no 

 barriers to clouds of fungus spores in the air. So the spores 



FIG. 99. Mummied plums destroyed by brown rot (Sclerotinia fructigena). 

 At left, tumor on branch, caused by black knot (Plowrightia morbosd) 



of rusts and smuts of grains may sweep over the fields from 

 Texas to Manitoba, or they may live unseen on seeds and 

 thus be distributed the world over. The spores or myce- 

 lium, as is the case with smut of corn and onion, scab and 

 rot of potato, and clubroot of cabbage and turnip, may re- 

 main alive in the soil from year to year. Such fungi can 

 be controlled only by strict rotation of crops. We thus be- 

 gin to realize the size of our problem in its world-wide 



207 



