June 3. 1918 Blackleg of Potato and Temperature and Precipitation 511 



ing tne two seasons. Aside from the counts as given, careful watch was 

 kept of a large number of fields throughout the county and in all cases 

 there was a surprisingly small amount of blackleg in 191 6 as compared 

 to 1 91 5. This was especially true during the month of August. This 

 also explains why there is a greater difference shown in the Spaulding 

 Rose than in the Irish Cobbler. The former, being a late variety, gen- 

 erally shows a greater number of plants diseased with the blackleg in 

 August than the latter. In 191 5 the Spaulding Rose showed a number of 

 affected plants during August, but in 191 6 few or none were seen during 

 the same month. With the Irish Cobbler, on the other hand, blackleg 

 makes its appearance in the latter part of June and early July and usually 

 continues in evidence up into the first part of August. By this time 

 potatoes begin to approach maturity, and very few additional plants 

 show symptoms of the blackleg. 



The writers wish to call attention to one exception found in 191 6. 

 On a one-six-acre plot of Irish Cobbler tubers used in connection with 

 another experiment, a portion showed approximately 8 per cent of 

 blackleg during August. This part of the plot was very low and wet, 

 compared with the adjoining plots, and, as will be seen later, this ex- 

 plains the large percentage of diseased plants. 



TEMPERATURE AND PRECIPITATION DURING I9I4, 1915, AND I9I6. 



Morse ^ makes the following statement regarding the influence of 

 weather conditions on blackleg: 



The progress of the disease is markedly influenced by weather conditions 



More blackleg is observed in wet than in dry seasons. 



and further — 



Soil conditions also are factors which influence outbreaks of blackleg. All other 

 things being equal, the disease is more likely to occur in wet than in dry soil, and is 

 more prevalent when the early part of the growing season is characterized by abundant 

 rainfall. 



The great difference in the severity of the disease found in 1914, 1915, 

 and 1916 can be explained by differences in temperature and precipita- 

 tion found during the three growing months, June, July, and August, 

 during these three years, especially as it existed in August. In Table 

 III are given the mean maximum and the mean minimum temperature, 

 and the precipitation. Averaging the three months, it is seen that both 

 the mean maximum and the mean miminum temperatures in 191 6 were 

 higher than in 1915 or 1914, while the precipitation for the same year 

 was less than in the two preceding years. During the month of August, 

 which showed the greatest differences in the percentages of the disease, 

 the mean maximum in 1916 was 79° F., while in 1915 it was 72.7°, a 

 difference of 6.3°; the mean minimum in 191 6 was 64.8° as compared to 

 50.9° in 1915, a difference of 13.9°. The higher maximum as well as the 



•Morse, W.J. op. cit., p. 84-85. 



