340 Diseases of Greenhouse Crops 



sickly yellow, finally white, shrivel and die, show- 

 ing but very little of the fruitings of the causal 

 organism on the surfaces of the affected areas. Un- 

 der very moist conditions, however, the spots become 

 covered with a delicate grayish lilac colored moldy 

 growth. The latter consists of the summer spores 

 of the fungus. The winter or resting spores are 

 found imbedded in the dead tissue of the host. Mas- 

 see * claims that downy mildew is a serious disease 

 of outdoor sweet peas in England. Although the 

 fungus Peronospora trifoliorum is wtry common on 

 other legumes in the United States, it has not yet 

 made its appearance on the sweet pea. Mr. Massee 

 says that Peronospora vici(Z Berk, also attacks sweet 

 peas in England. 



Stem or Collar Rot 



Caused by Sclerotinia lihertiana Fckl. 



Symptoms. This is usually a seedling disease, al- 

 thtDugh it may attack plants of all ages. Like the 

 Rhizoctonia rot, it attacks many different kinds of 

 seedlings. The trouble is most severe in poorly ven- 

 tilated houses in beds overwatered or lacking proper 

 drainage, and in damp places out of doors. The 

 disease spreads very quickly and is soon fatal. Af- 

 fected plants first show a wilting of the tip and a 

 flagging of the leaves, and then the seedlings fall 

 over and collapse (fig. 69, b.). The causal fun- 

 gus does not seem to attack the roots, but penetrates 

 the collar of the stem and completely invades the 



♦Massee, G., Sweet Pea Annual: 20-21, 1906, London 



