8o 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
From this table it appears that the optimum soil temperature for the 
development of the disease is 28° to 31 0 C. (82° to 88° F.), the minimum 
22 0 C. (71 0 F.), and the maximum 33 0 C. (91 0 F.). In this series one 
infection occurred at 19 0 C. but none was found at 21 0 C., a temperature at 
which one isolated infection had been secured in a previous experiment. The 
temperatures from 22 0 to 32 0 C., inclusive, resulted in the infection of about 
the same number of plants, the most favorable temperature for infection 
being distinctly marked, however, by the quickness and virulence of the 
attack. At 33 0 C. only one plant was infected, and at 35 0 C. none were 
infected. 
The growth of the check plants is illustrated in Plate IX, C. The tops 
there shown are representative of the condition of the check plants one 
month after they had been transplanted to the temperature pots. At 35 0 
C. these check plants maintained themselves and increased in size very 
slowly; 33 0 C. was a slightly more favorable temperature, and at 31 0 C. 
the plants grew luxuriantly. The growth at 28° and 24 0 C. was almost as 
great, and these plants appeared to be growing most rapidly at.the time 
the experiment was stopped. The plants were growing thriftily at 22 0 , and 
equally well at 21 0 , while at 19 0 a slight decline was apparent. 
b A3 15 17 19 2 ! 23 25 27 29 3t 33 35 
° 5OIL TEMPERATURE IN DEGREES CENTIGRADE 
Fig. i. The data for graphs A and B were secured in soil-temperature experiments 
II and III. Graph A shows the relation of soil temperature to the percentage of plants 
killed. The plants were exposed to the different temperature conditions for one month. 
Graph B shows the relation of soil temperature to the incubation period in days. Thus, 
it took much longer for the disease to appear with the soil at a temperature of 22° to 23° 
C. than with it at a temperature of 27 0 to 30° C. 
