34 



which would not occur if low temperatures would control the trouble. 

 In our house, also, no such effect has been evident except with the 

 Botrytis disease. With low temperatures lettuce will develop much 

 more slowly, as will the Drop, and in the end little would be gained. 

 Practical men prefer not to wait twelve weeks for the development 

 of lettuce when a better article at greater profit can be produced in 

 one-half this time. 



The maximum temperature which the mycelium will stand appears 

 to be not far from 130° F., while the sclerotia are not killed until 

 the temperature reaches a much higher point. Numerous experi- 

 ments were made with sclerotia buried in the soil, the temperatures • 

 to which they were subjected being brief in duration. The temper- 

 ature of 160° F. in most cases killed the sclerotia, though in some 

 experiments made with sclerotia of a larger type it was found that it 

 took considerable more heat to kill them. Somewhat lower tem- 

 perature for a longer time would doubtless be equally effective. 

 Sclerotia subjected for a brief period of time to a considerably high 

 temperature not sufficient to kill them, were greatly accelerated in 

 their germination. 



The minimum temperature which the Drop will stand has not been 

 ascertained either for sclerotia or mycelium, but it may be stated 

 that we have never observed any detriment from exposing it to a 

 temperature below zero. Some of the large sclerotia and also bread 

 cultures of the usual Drop fungus were left out of doors for about a 

 month in midwinter. The temperature "went down to at least' — 12° F. 

 on two occasions and was below freezing almost all the time. When 

 brought into the laboratory all the material at once showed strong 

 stimulation by the cold, producing a much more abundant mycelium 

 growth than in similar lots not frozen. From what we know in 

 regard to the behavior of other reproductive bodies, there is reason 

 to believe that the sclerotia are capable of standing exceedingly low 

 temperatures without interfering in the least with their vitality. 



Period of Greatest Loss to Lettuce Crops by Disease. 



From carefully dated records made of the Drop and other dis- 

 eases, we are able to ascertain at what period in the life-history of 

 thj lettuce plants they succumb to disease. The curve shown in 

 Diagram VII, represents the average amount of Drop recorded each 



