432 General Botany 



In greenhouses and small seedbeds the sterilization is 

 frequently accomplished by forcing live steam from a boiler 

 through the soil by means of an inverted galvanized iron pan. 

 The sterilization of soil not only kills the damping-off fungi but 

 many other disease-producing organisms, both plant and animal. 

 Moreover, it kills weeds and has a beneficial effect upon the soil. 



Clubroot of cabbage. Many plants belonging to the mustard 

 family, particularly cabbage, turnip, and cauliflower, are subject 

 to a disease that causes swellings on the roots and impairs the 

 efficiency of the roots. When young seedlings are infected 

 many die, and those that continue to live never produce normal 

 plants that can be marketed. The fungus belongs to one of the 

 lowest groups of fungi, the slime molds, and produces enormous 

 numbers of spores that winter over in the soil. In Europe the 

 disease is a constant menace to the cabbage industry. In 

 America it is not so prevalent. The disease may be partly con- 

 trolled by liming the soil and by changing the location of the 



Erwin F. Smith, U.S.D.A. 



Fig. 269. Clubroot of cabbage, caused by infection with a slime mold, Plasmodia phor a. 



