The Dry Root-Rot of the Bean 1013 



root-rot fungus. The taproot, being the first to develop, soon shows 

 symptoms of the disease. This is frequently a week or two after the 

 host plant is above ground. It has been determined by experiment that 

 further infections may take place at any time during the growing season 

 of the bean plant. 



The exact mode of infection is not known. The fungus, however, 

 may penetrate the healthy epidermis. After the mycelium has once 



Fig. 135. cross section of the cortex and epidermis of the 

 taproot of a bean affected with dry root-rot 



An early stage in the progress of the disease, showing the intercellular and parallel 

 hyphae. Camera lucida drawing, X 514 



gained entrance to the host, it is for a short time intercellular in the 

 cortical tissue. In the main the hyphae are found extending longitu- 

 dinally with the taproot. Occasionally as many as a dozen hyphae may 

 be observed growing parallel in the intercellular spaces (fig. 135). This 

 tendency on the part of the fungus to form rhizomorph-like structures 

 is further developed on the outside of the host, where strands 75 n in 

 diameter frequently occur over the diseased areas. Likewise, in culture, 



