346 Diseases of Greenhouse Crops 



croconidia soon lose their protoplasm or let it break 

 up until it presents a granular appearance. In young 

 cultures, the outer wall of the chlamydospore is 

 smooth, but in old cultures it becomes slightly warty 

 or covered with minute points. No perfect stage has 

 been found to accompany this fungus either in pure 

 culture or on the host. This disease may be con- 

 trolled by the steam method of sterilization. 



Root Rot 



Caused by Rhizoctonia solani Kuhn. 



Symptoms. This trouble is of considerable impor- 

 tance to greenhouse men. The disease may destroy 

 the entire stand, or cause it to be uneven, thus ne- 

 cessitating several resowings. Severely infected 

 plants have practically no root system (fig. 71, b.). 

 In less infected plants only one or two rootlets may 

 be destroyed. The fungus produces a browning ef- 

 fect of the root before total destruction sets in. In 

 very early stages of infection the seedlings are seen 

 to have a wilted appearance; as the disease pro- 

 gresses the infected seedlings fall over and collapse. 

 The fungus is not confined to the roots alone. It is 

 often seen to work its way up the stem and produce 

 a constricted area marking it off from the healthy 

 part. As the fungus is a soil organism, it is usually 

 introduced with manure, infection taking place at 

 any part of the roots or stems. In the latter case 

 reddish sunken spots are observed at the base. Root 

 rot is primarily a seedling disease, although older 



