DISEASES OF PLANTS 



235 



crowded, as in seed beds, they are apt in damp weather to 

 "damp off." The cause is a fungus similar to the one just 

 mentioned. When it attacks the plants they rot and turn 

 black near the surface of the ground. 



Keeping the plants dry, stirring the soil, letting in air 

 and sunshine, help to keep it in check. Seed beds should be 

 made of fresh soil free from the fungus. (California Ex- 

 periment Station, Bulletin 175, 1906.) 



POWDERY MILDEWS 



A whitish appearance is often seen on leaves, espe- 

 cially those of roses (figure 134), lilacs, hops, peas, beans, 

 peaches, apples, currants, cucumbers, and grapes. It 

 is caused by 

 the growth 

 of fine white 

 threads of 

 fungi which 

 are known 

 as powdery 

 mildews. 

 They look a 

 great deal 



FIG. 134. Powdery mildew on rose leaf, causing a 

 whitish appearance. 



like the 



molds and, 



like them, grow on the surface, not penetrating into 



the plant as some fungi do. For this reason it is 



easy to kill the mildews by spraying the surface of 



