ii6 Diseases of Greenhouse Crops 



Symptoms. The symptoms of the disease on beets 

 do not differ from those of the Irish potato. Occa- 

 sionally, the scabs which arise before the beet is full 

 grown disappear entirely, leaving merely a small 

 scar. This is somewhat sunken and has a definite 

 outline. In normal cases of infection, the scabby 

 areas on the beet are rough; while the corky layer 

 of the spots decidedly bulge out. Immediate- 

 ly below them, the tissue is a discolored reddish 

 brown. 



The Organism. The cause of beet scab is the 

 same as that of the scab of the white potato. The 

 parasite is a soil organism, and thrives best under 

 alkaline conditions. 



Control. The disease is introduced with infected 

 soil, or with the compost. Care should be taken 

 that no infected potato peelings find their way to 

 the manure pile. Soil sterilization with steam, or 

 formaldehyde (see pp. 32-43) is recommended. 



Damping Off and Root Rot 



Caused by Pythium de Baryanum Hess. 



Symptoms. Damping off very commonly occurs 

 just as the seedlings emerge from the ground. These 

 topple over and die in the characteristic way so fa- 

 miliar to truckers. The greatest damage follows 

 from overwaterings, when a hard crust is formed on 

 the surface, a condition which prevents the seedlings 

 from emerging normally. On old and mature roots, 

 Pythium de Baryanum may cause a rot. A pecu- 



