ciiapti:k XX 



FUNGOUS AND BACTERIAL I)ISEASP:S 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 l)y fungous diseases is sometimes not far from 

 half a billion dollars. — Dlugak, "Fungous Diseases of Tlants," 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 



V 





r 



/ 



Fig. 99. Mummied plums destroyed by brown rot (Sclerotinia f)'uctigcna). 

 At left, tumor on branch, caused by black knot {Plowr'Kjhtia morbosa) 



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

 Texas to ^Manitoba, or tliev mav 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 dubroot of cabbage and turnip, may re- 

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

 be controlled only l)y strict rotation of cro[)s. \\'i' thus be- 

 gin to realize the size of <>ur prol)lem in its world-wide 



207 



