56 Journal of Agricultural Research voi. vi, no. i 



during the latter part of June and early part of July. This was doubtless 

 due to the fact that during this period the stomata were closed against 

 the fungus the greater part of the day on account of the excessively high 

 temperature, which was generally above 100° F., and the excessively 

 low humidity, which at one time fell to 10 and which was only above 

 60 from 6 to 15 hours a day, and also to the fact that a temperature as 

 high as 95° inhibits the growth of the fungus and kills it after a few days. 



After July 5 the temperature was lower and the humidity higher than 

 during the period above mentioned. As a result, numerous conidia were 

 produced from July 9 to 12, and at the end of the period of incubation — 

 July 21 to 25 — there was a slight increase in the number of spots (Table 

 VIII). Rains between July 19 and 35 and the resulting high humidity 

 caused a rather marked increase in the number of spots in late July and 

 early August, these spots appearing on many leaves hitherto uninfected 

 (Table VI). Prior to this period the number of leaves showing spots 

 were comparatively few, but after July 30 the majority showed spots, 

 and the proportion of infected to uninfected leaves gradually increased 

 until August 28, after which it decreased. During the period from July 

 30 to August 28 the humidity was comparatively high, remaining above 

 60 on an average of 14.6 hours on all except three days, on which it 

 remained above 60 for 10 hours; and the maximum and minimum 

 temperatures generally were not above 90 nor below 55, respectively. 

 The increased proportion of infected to uninfected leaves during this 

 period, however, was not necessarily due to increasingly favorable 

 climatic conditions but to the cumulative effect of the organism, the 

 amount of viable conidia and consequent new infections increasing as 

 the number of spots increased, as shown by the enormous increase of 

 3,776 spots per plant on August 27 and 28. After September 3 the 

 increase in infection was considerably less (fig. 9) . This was due appar 

 ently to the fall in temperature, the maximum being rarely above 76° F 

 and the minimum seldom above 50° after September 8, while the humidit} 

 was comparatively favorable. 



The increase in infection through the season was considerably higher in 

 the early than in the medium-early sugar-beet field, as shown by the 

 total amount of the disease (Tables VI and VII) and the actual increase 

 (Table VIII and fig. 9). This was due to the fact that the foliage was 

 heavier in the early than in the medium-early field (Tables VI and VII, 

 functional leaves) , and consequently the humidity was higher and the 

 infection greater in the former than in the latter (Table VIII and fig. 9). 



The maximum increase in spots was reached on August 1 1 in the early 

 field and on August 28 in the medium-early sugar-beet field. The period 

 of greatest increase in the disease is not its period of greatest destruc- 

 tiveness, however, as the plant is not immediately affected by the dis- 

 ease, some time being required for the leaves to be killed. 



