PLANT DISEASES 



571 



[Chap. XLV 



has been accumulated in recent years regarding the direct relationship 

 between the prevalence or scarcity of some diseases and certain environ- 

 mental factors. A few illustrations will be cited. It should be noted also 

 that environment alone may cause injuries, some of which, such as 

 bitter pit of apples and blossom-end rot of tomatoes, have been re- 

 ferred to as "diseases" (Fig. 258). 



Fig. 258. Injuries to fruit caused by environmental conditions such as tem- 

 perature and drought. A, apple soft scald; B, apple brown core; C, tomato blossom 

 end rot; D, freezing injury of orange. Photos from U. S. Department of Agriculture. 



Potato scab, an infectious disease, is likely to be more severe in an 

 alkaline soil, whereas the growth of the organism causing clubroot of 

 cabbage is favored by an acid soil. The organism causing tobacco 

 root rot is practically eliminated from soils of very high acidity. 



The weather influences the abundance or scarcity of plant diseases 

 largely through the effects of temperature and moisture either on the 

 hosts or on the parasitic organisms. 



