198 



FUNGOUS DISEASES OF PLANTS 



conditions the perfect form from sclerotia of the Botrytis. It would 

 seem that Istvanfn has now secured substantial proof that these 

 are pleomorphic stages of a single fungus. 



Much interesting biological work has been done upon this 

 fungus. Infection results most readily from sclerotia or from 

 a mycelium which has been growing saprophytically. Infection 

 frequently fails when conidia germinate directly upon the sur- 

 faces of delicate parts. Upon penetrating a plant there is, first, 

 a direct poisoning effect, supposedly due to oxalic acid, resulting 

 in the death of adjacent cells ; and, second, there is more or less 

 digestion of the cell contents and membranes. 



Fig. 75. Sclerotinia Libertiana. (After R. E. Smith) 

 a, sclerotia and apothecium ; b, penetration of hyphse 



Control. In the case of this fungus, as well as the species of 

 Sclerotinia next discussed, good sanitation is important. Never- 

 theless, in the greenhouse it may be necessary to sterilize the 

 soil in order to control the disease effectively when it becomes 

 virulent. 



IX. LETTUCE DROP 



Sclerotinia Libertiana Fuckel 



Humphrey, J. E. Diseases of the Cucumber Plant. A Sclerotium Disease. 

 Mass. Agl. Exp. Sta. Rept. 10: 212-224. pis- 1-2. 1892. 



Smith, R. E. Botrytis and Sclerotinia : Their Relation to Certain Plant Dis- 

 eases and to Each Other. Bot. Gaz. 29 r 369-407. ph. 25-2-/. 



Stone, G. E., and Smith, R. E. " Drop " of Lettuce. Mass. (Hatch) Exp. 

 Sta. Rept. 9: 79- g i- 1S97. (Compare, also, 10: 55-58,1898; and 11 : 

 149-151, 1899.) 



