Observations and Experiments on Cereal Rusts. 105 
Average minimum temp. for May 73°5° F., max. 99:3° F. 
i, $3 . June 72°0° F., max. 862° F. 
Similarly Ward*, Mains} and Fromme} have emphasised the 
influence of higher temperatures on the length of the incubation 
periods of other rusts. It is only fair to conclude, therefore, 
that the duration of the incubation period is exceedingly variable 
and that Eriksson evidently left out of consideration altogether 
the effect of temperature on the growth of the rust mycelium 
inside the host. 
8. CULTURE OF RUSTS AT COMPARATIVELY 
LOW TEMPERATURES. 
During the spring of the year 1922 some further experiments 
were conducted with the object of finding out the comparative 
duration of the incubation period in the case of all the three 
rusts cultivated at temperatures regulated to lie above the 
freezing point but not allowed to go very high. A constant 
record of the temperature of the greenhouse was kept with a 
thermograph and the average of daily temperatures was worked 
out by noting six readings per day. In all these experiments 
seedlings of Red Sudan wheat (which is very susceptible to all 
the three rusts) were used. The germination of spores was in 
every case found to be very good. 
The lowest temperature recorded during the first experiment 
was 32°5° F. and the highest 72° F. In the second experiment 
it was 37° F. and 78° F. respectively. In the third it was 43° F. 
and 77° F. The weather during the course of the fourth experi- 
ment however was very warm and although the experiment was 
conducted in a cool greenhouse the lowest temperature recorded 
was 50° F., the highest being 82° F. 
Ae results of these experiments are shown in the following 
table. 
ee Number of days after which 
for the period. pustules appeared 
Date at which | (The average for the 
the experiment | whole period is shown | Yellow Brown Black 
No. | was started within brackets) rust rust rust 
oF, Days Days Days 
I March 2nd 38-6 to 50:6 (40:0) 12-13 17-18 21-22 
2 April 3rd 43°5 to 58-0 (50°8) II-I2 13-14 I4-I5 
3 op. Lizils} 48-5 to 58-8 (54:1) 8-9 8-9 II-I2 
4 May 18th 59°1 to 71°8 (66-5) I2-I4 8-9 8-9 
ee eee ee eee ee 
* Ward, H. M., Ann. Bot. xv, p. 560 (1901). 
} Mains, E. B., Amer. Journ. Bot. Iv, p. 179 (1917). 
¢ Fromme, F. D., Bull. Torrey Bot. Club, XL, p. 501 (1913). 
