GENERAL DISEASES 61 



are of similar nature and due to similar causes, they are 

 placed under the same caption. 



Damping off may l;e caused by any one of several species 

 of fungi, prominent among them being Pythium, Rhizoc- 

 tonia, Botrytis, Sclerotinia, Phoma, Volutella, Phytoph- 

 tliora, Colletotrichum, Gloeosporium. The fungus which 

 causes this condition may often be seen as a weft of myce- 

 lium around the base of the diseased plant, or even creeping 

 over the ground to some distance. From its original 

 jioints of attack it may spread rapidly to adjacent plants, 

 often sweeping the whole seed bed. 



Since the growth of these fungi is favored by moisture 

 and warmth, the trouble may often be checked or pre- 

 vented by keeping the beds cool and withholding any excess 

 of water; also by means of adequate ventilation, preventing 

 the accumulation of a vapor-laden atmosphere around the 

 bases of the plants. Frequent stirring of the top soil 

 around the plants aids in drying it and may stop an in- 

 cipient attack of damping off. 



Organic matter in the soil favors the growth of da'mping- 

 off fungi, and should be avoided. Fresh, clean sand is best 

 for most purposes of the seed bed and cutting bench. 



Soil known to have once sustained damping off may be 

 regarded as infected with a fungus which caused it. Such 

 soils should not l)e used for seedlings or cuttings without 

 disinfection. If disinfection is impracticable, the soil 

 should be removed, the containers thoroughly disinfected 

 with a spray of Bordeaux, bluestone, or formalin, and new 

 uninfected soil introduced. 



Rhizoctoniose (Rhizoctonia). — In America this dis- 

 ease is known to affect potato, beet, cotton, lettuce, bean, 



