428 



General Botany 



Causes of diseases. Bacteria and fungi are the most common 

 causes of plant diseases. Certain insects are almost equally 

 injurious, and not infrequently act as carriers of disease-producing 

 bacteria and fungi. Insects also are the commonest cause of 

 galls. Nematode worms are sources of injury to the roots of 

 many plants, particularly in greenhouses and in tropical and sub- 

 tropical countries. 



Just how these various parasites affect the plants is not thor- 

 oughly known. Some of them certainly withdraw sufficient 

 food from the host plant to cause injury. Others seem to produce 

 poisonous substances that injure the tissues or kill them. Still 

 others in some way stimulate the tissues and cause abnormal 

 growths. 



Weather influences. The temperature and moisture con- 

 ditions frequently influence the prevalence of diseases. Water 

 is necessary for the germination of spores. Severe epidemics of 

 potato blight, brown rot of stone fruits, and rots of grapes and 



A . B. StoiU 



Fig. 266. Cucumber plants attacked by bacterial wilt. The plants wilt because the vessels 

 are plugged by the bacteria that cause the disease. 



I 



