30 Journal of Agricultural Research voi. vi, No. x 



Low temperatures are not entirely inhibitive, as was shown by thermal 

 tests of artificial cultures. After such cultures had been exposed to 

 temperatures averaging 0.9° C. for 48 days and then kept at 28° C, 

 numerous colonies developed. Also, heavily infected leaves kept at 

 0.9° C. for 97 days yielded good growth when cultures were made and held 

 at favorable temperatures. Had the cultures been exposed to freezing 

 temperatures or to extreme variations in temperature, the effect would 

 doubtless have been more pronounced. 



Although the temperature variations and the amount of soil moisture 

 at Rocky Ford and Madison differed greatly, the effect on the life of the 

 fungus was apparently the same at both places. It may be concluded 

 that conditions of the soil which favor the process of disintegration are 

 the most important factors in the control of the disease, and these ex- 

 periments indicate that these processes are most active at a depth of 



6 to 8 inches. 



SUMMER CLIMATIC CONDITIONS 



The summer climatic conditions here considered were recorded during 

 1 91 3 in fields of first-year sugar beets grown at Rocky Ford, these fields 

 being an example of the usual progress of the disease where neither ro- 

 tation nor sanitation at the preceding harvest time had been practiced. 



A study of the temperature and humidity records taken at different 

 places in a beet field at Rocky Ford and at the Weather Bureau station 

 3 miles from the field was made to determine their comparative values 

 in making important correlations. The records made in the sugar-beet 

 field were taken by means of hydrothermographs kept in meteorological 

 instrument shelters 5 feet above the ground (PI. IV, fig. i) and among 

 the plants (PI. IV, fig. 2) . These were checked at frequent intervals with a 

 sling and cog psychrometer (Shaw, 1914), respectively, and under Col- 

 orado conditions were found to be accurate. The records of the Weather 

 Bureau station were taken by means of maximum and minimum ther- 

 mometers kept in an instrument case about 5 feet above the ground in 

 an open space (fig. 3). 



The daily maximum and minimum temperatures and humidities, 

 together with the total number of hours the humidity was above 60 

 from noon of the preceding day to noon of the given day, are used in 

 the present interpretations. It has been found that when a high relative 

 humidity prevails, the stomata of the sugar-beet leaves are usually open; 

 and as the fungus enters the leaves only through the open stomata, the 

 length of time they remain open is a fundamental factor in determining 

 the possible occurrence of infection (Pool and McKay, 191 6). 



AIR TEMPERATURE AND RELATIVE HUMIDITY 



The temperature and relative humidity taken with hygrothermographs 

 placed near the ground among the plants varied widely from those taken 

 with hygrothermographs in the air above the field and also from those 



