120 DISEASES OF THE SWEET PEA 



able to the host this fungus may attack a 

 variety of plants in the open. That this 

 fungus has not been found to attack sweet 

 peas out of doors does not preclude its ap- 

 pearance in the field at any time in the fu- 

 ture. 



Symptoms. Collar rot is usually a 

 seedling disease which causes "damping 

 off." It is most severe in poorly venti- 

 lated houses or in beds which are over- 

 watered and lack the proper drainage. 

 The disease does its work quickly. Af- 

 fected plants first show a wilting of the 

 tip and flagging of the leaves, and then the 

 seedling falls over and collapses (fig. 17) . 

 The fungus, Sclerotinia libertiana, al- 

 though a soil organism, cannot attack the 

 roots of its host. The parasite penetrates 

 the collar of the stem, completely invad- 

 ing the inner vessels, thus cutting off the 

 upward flow of water from the roots to the 



