PLANT DISEASES 447 



produce wilt diseases. Among the other wilt-producing 

 forms are the Fusariums. Under this head come three dis- 

 eases of great importance in the south, namely the wilts of 

 cotton, cowpea, and watermelon. The Fusarium is a soil 

 fungus, so that the infection is probably what is called a 

 " soil-infection," the most difficult kind to guard against. 



FIG. 96. Black rot of cabbage: c, healthy plant; s, diseased plant. After HARDING. 



It means that the soil of a field becomes infected, and that 

 continued planting on that area simply increases the in- 

 fected area every year. Another notable Fusarium wilt is 

 the flax wilt, which is the great enemy to the raising of flax. 

 Fusarium-infected soils are often spoken of as " sick soils," 

 as " cotton sick," " flax sick," etc. 



Mushroom wilts. These are our most important tree 

 diseases, and since they are wood-destroying diseases, they 

 are of great importance to the forester. The invading 



