72 



fungus threads, or myeelimn, are parasitic on the lettuce leaves and stem, 

 and soon cause a total collapse. As the plant decays there are formed 

 small black bodies called Sclerotia, composed of densely packed threads of 

 the fungus. These are very resistant bodies and may live for a considerable 

 time under very unfavourable conditions. 



Conidia or summer spores have not yet been positively identified in 

 the life-cycle of this fungus. The disease spreads mainly by the produc- 

 tion by the Sclerotia of a mycelium, the threads of which may inoculate 

 healthy plants. By this means the fungus may spread rapidly from 

 plant to plant through the soil. 



There is another form of reproduction, the development of a cup form 

 of fruiting body from large Sclerotia. Sacs containing spores are borne 

 on the upper surface of the cup. These sac or ascopores, it has been shown, 

 .seldom inoculate the plant directly. 



Treatment. — No really simple, effective method of prevention has been 

 devised, but good sanitation in the form of good ventilation and good 

 drainage aids materially in holding the fungus in check. Sterilization of 

 the soil is perhaps the most effective method of stamping out the disease. 

 "A coating of ^ or ^ inch of sterilized sand or earth will materially re- 

 duce the effect of the drop, while four inches has in certain experiments 

 completely destroyed all of the disease." Another form of partial steriliz- 

 ation is to treat the soil with hot water so as to raise the temperature of 

 the surface to .176°— 186° F. 



Spraying with ordinary fungicides has little effect, since the Sclerotia 

 are not destroyed. 



Lettuce Drop is sometimes caused by another fungus of the same 

 genus, Sclerotinia Fuckeliana De Bary, but the injury is not nearly so 

 marked as is the ease just described. The conidal stage, known as Botrytis 

 vulgaris, Fr., is a very common grayish mould on fallen and decaying 

 leaves and stems. As a parasite, however, on greenhouse plants it often 

 does considerable injury. 



As in the case of the other lettuce drop (Sclerotinia libertiana), in- 

 fection occurs most frequently by means of threads sent out from Scler- 

 otia, or from threads which are growing saprophytically on dead and 

 decaying organic matters on the soil. 



