36 Journal of Agricultural Research voi. vi. no. i 



SUMMER INFECTION CYCLES 



The thermal relations of the fungus are closely linked with the effect 

 of various climatic factors on the production and dissemination of co- 

 nidia and on infection cycles. With a view to determining these relations 

 the fungus was grown in Petri-dish cultures in thermostats at different 

 and varied temperatures. At first the moisture was probably more or 

 less constant, but as time went on it became relatively low. The effect 

 of different temperatures, however, was comparable with that observed 

 under existing field conditions. 



THERMAL RELATIONS OF THE FUNGUS IN CULTURES 



Tests of the fungus on string-bean agar were made at Washington 

 during November and December, 191 3, and January, 191 4. The cul- 

 tures were obtained from isolations made at the time of the tests from 

 infected sugar-beet leaves collected at Rocky Ford during the preceding 

 September. One colony of the first isolations was macerated in 10 c. c. 

 of sterile water, and one platinum loop of this suspension was used for 

 each tube of medium. Three poured plates were used for each single 

 test. The cultures were exposed to different constant temperatures and 

 to varied constant temperatures (high and low changed to low and high, 

 respectively) . Exposures were also made for 8 hours at the higher tem- 

 peratures and then for 16 hours at lower temperatures, and, after a short 

 interval of exposure in a certain number of these tests, both temperatures 

 were lowered, it being possible in this way to approximate night and 

 day temperatures in the field under normal conditions. 



Series A (different constant temperatures). — When the cultures 

 were held at different constant temperatures, the abundance and size of 

 the individual colonies gradually increased, while the time necessary for 

 development decreased with the temperatures 12.5°, 17.3°, 19.2°, 20°, 

 and 30.8° C. The best growth was made at a temperature of 30.8°, but 

 this in all probability was slightly above the optimum constant tempera- 

 ture, as no growth took place in cultures held for 9 days at 34.7°, 35.8°, 

 and 40.6°, respectively (Table III, series A). 



Series B (varied constant temperatures). — ^Although no growth 

 of the fungus took place in cultures held at constant temperatures of 34.7° 

 and 35.5° C, a small percentage of normal colonies developed in cultures 

 exposed for three days to these temperatures and then for several days 

 to a temperature of 30.8°, while in cultures exposed for three days to 

 40.5° no growth occurred when subsequently held at 30.8°. On the 

 other hand, in cultures exposed for three days to a temperature of 30.8° 

 there was almost a normal development of the colonies for three days after 

 they were exposed to 34.7°, but at the end of five days the inhibitive 

 effect of the latter temperature became manifest. In the case of cultures 



