53 



living plants. For the absolute control of lettuce diseases, none 

 of the substances experimented upon have any value, and there 

 appears to be very little probability that any similar treatment will 

 be successful. However powerful the toxic effect of the gaS' may be, 

 the indispensable penetration of the soil from the surface seems to 

 be entirely lacking. 



Effects of Temperature Upon the Drop. 



The following shows the rate of development of the Drop myce- 

 lium at different temperatures. Pure cultures were used in prune 

 agar slant tubes and the development of the mycelium with which 

 eachtube was inoculated was noted. 



TABLE SHOWING DEVELOPMENT OF THE DROP FUNGUS AT DIFFER- 

 ENT TEMPERATURES. 



Average Temperature, 48' F. 54° F. 60° F. 76° F. 101° F. 



Degree of Development,' Mere trace gj^ g^^wl^"'^^ ^tf"^ gia. 



These experiments lasted ten days, the readings being made three 

 times each day, and the temperatures given represent only the aver- 

 age temperature for the whole period. From these experiments it is 

 clear that the optimum or best temperature for the development of 

 the mycelium of the Drop is not far from 76° F., and that an aver- 

 age temperature of 101° F. is too high for much growth. 



The temperature at times went higher than 101° F., otherwise 

 greater growth would have taken place. On the other hand the 

 mycelium will develop at 48° F. and probably at a temperature 

 somewhat lower than this, but this point retards growth and would 

 require a longer ti'me than ten days for the culture to show well 

 developed mycelium growth. 



A greenhouse kept at a low temperature is held by some to be 

 less subject on this account to disease. This appears to be true in 

 the case of the Botrytis disease, which is not of a strictly parasitic 

 nature, but with the real Drop the evidence at our disposal shows 

 no decided lessening of the disease on account of low temperatures. 

 Experienced growers, who run their houses at what are considered 

 proper temperatures, often suffer serious loss from this disease, 



3 



