June 3, 1918 Blackleg of Potato and Temperature and Precipitation 513 



Heretofore it has been shown that there was a great difference in the 

 air temperature during the two summers of 191 5 and 191 6. In figure i 

 is shown a thermograph record of the soil temperature as recorded for the 

 month of August in 191 5 and 191 6. These records were obtained by 

 burying the thermograph bulb at the depth at which potatoes are planted, 

 about 5 inches, at practically the same spot in both 191 5 and 191 6. An ex- 

 amination of this figure shows that in 191 5, during the month of August, 

 the soil temperature fluctuated between 60° and 70° and dropped below 

 60° twice during the latter half of the month. In 191 6 the temperature 

 was more fluctuating than for the same month the previous year and 

 varied from 60° to 70° the early part of the month, reaching a maximum 

 of 70° to 80° the last part of the month. 



A close correlation therefore appears to exist between high tempera- 

 ture and little rainfall with a small percentage of blackleg, and vice versa. 



SUMMARY 



(i) No evidence could be obtained to indicate that the blackleg or- 

 ganism, under the winter conditions that existed during 1915-16 and 

 1 91 6-1 7 in Aroostook County, Me., and during 191 6-1 7 at Norfolk, 

 Va., can live over in the soil or in diseased tubers that may remain there. 



(2) Weather records show that the winter of 191 5-16 was not an 

 unusual one for Aroostook County. 



(3) The severity of the disease during the growing season is closely 

 correlated with temperature and precipitation and is dependent upon 

 them. A high temperature and low precipitation tend to diminish the 

 disease, while a low temperature and high precipitation produce condi- 

 tions favorable for it. 



