\>i OMYCETES 199 



Symptoms, effects, and hosts. It is difficult to determine how 

 many of the reported sclerotial diseases of greenhouse and garden 

 crops may be due to this fungus. It is unquestionably, however, 

 one of the most disastrous of the sclerotium-producing fungi, and 

 it is, moreover, widely distributed and not readily controlled.' So 

 far as can be judged from the studies and experiments which 

 have been made, it is the cause of the worst type of the lettuce 

 "drop," a disease of great importance in the greenhouses of the 

 eastern states. 



As this disease commonly occurs there is little or no evidence 

 of the incipient stages in the form of definite spots or ulcers. 

 The host plants may show some evidences of flagging, in a short 

 time there are indications of water-soaked areas over the stem 

 and basal portions of leaves, and finally the whole plant collapses 

 and melts into a formless mass. 



Kven from early evidences of the disease fungus threads may 

 appear upon the surface of the leaf, and this mycelium may be- 

 come superficially conspicuous, even resulting in the development 

 of small sclerotia. These appear first as white specks and later 

 take the form of deep black, rather irregular sclerotia, as shown 

 in Fig. 75, a. This fungus quickly spreads from plant to plant 

 through the soil, and furthermore, in its relation to healthy 

 plants, results almost invariably in the production of the disease. 

 De Bary showed that ascospores are commonly ineffective in pro- 

 ducing direct infection, but sclerotia or bits of the mycelium may 

 serve for inoculation purposes. The Sclerotinia Libertiana type 

 of sclerotium will yield almost invariably the apothecia of the 

 Sclerotinia. 



This fungus is apparently widespread. It has been reported by 

 various observers as a cause of destructive diseases of hemp, rape, 

 cucumbers, and of many forced vegetables and bulbous plants. A 

 disease of the tobacco, discussed by Clinton, 1 has also been attrib- 

 uted to this fungus. 



The life cycle. In no case has it been possible positively to iden- 

 tify a conidial stage in the life cycle of this species, although Botrytis 

 cinerea has frequently been found upon plants unquestionably 



1 Clinton, G. P. Tobacco Diseases. Stem Rot. Conn. Agl. Exp. Sta. Kept 

 (1906) : 326-329. pis. 20 a, b ; 21 a. 



