WHITE-PINE BLISTER RUST. 59 
while ice-cooled ones of the same lot gave 25 to 54 per cent germina- 
tion. Marshall found the same stimulating effect from a tempera- 
ture of 23° F. upon urediniospores on Bibes leaves stored outside a 
window, as compared with those on leaves kept at room temperature. 
Posey dried infected leaves of Ribes nigrum in the hot July sun for 
four hours. Urediniospores from these gave germination ranging 
from 3 to 45 per cent, with an average of 17 per cent. Spores from 
other leaves collected at the same time and dried inside in the shade 
gave 3 to 90 per cent germination, an average of 47 per cent. During 
dry, hot weather it has been found that the viability of the uredinio- 
spores out of doors is very low. 
Duff (30), in studying the factors affecting their viability, found 
that exposure to bright sunlight reduces their germination, the ultra- 
violet rays being the destructive agent. Their viability appears to 
him to be low. Three days after collection less than 50 per cent ger- 
minated in distilled water. In about two weeks germination, was 
negligible even when stimulated by cooling to 2° to 5° C. 
Pennington 37 made germination tests of newly-matured uredinio- 
spores of Cronartium ribicola produced naturally in the vicinity of 
Lewis, N. Y., from early June until late in the autumn in hanging 
drops of tap water. During the first two weeks of June (frequent 
rain) the percentage of germination was high. From June 16 to 28 
no tests were made. On June 29 and July 1 (rather dry) only 1 per 
cent germinated. After this time, many tests were made with spores 
from various localities. By July 14 (very hot alternating with some 
rain) less than one- third of 1 per cent germinated, and from July 22 
to 26 (hot and dry) less than one-tenth of 1 per cent. Fresh yellow 
spores kept in an ice box gave no better results. On August 1 (rain 
July 29 and 30) 5 per cent germinated. After this (decidedly more 
rain) from 10 to 40 per cent germinated. The viability of these spores 
seemed to be greatly decreased by hot, dry weather and increased by 
cool rainy spells at the time they were produced. When the number 
of spores produced decreased because of hot, dry weather, their rate 
of germination also decreased and vice versa. 
Doran (28) found that the limiting temperatures for the germina- 
tion of urediniospores are: Minimum 8°, optimum 14°, and maximum 
25° C. He calls attention to the fact that — 
There is apparently a relation between the season when spores occur and their tem- 
peratures for germination. The seciospores of Cronartium ribicola occur i.n the spring 
when the average temperature is lower than in the summer, the season of occurrence 
of the urediniospores of this fungus. The aeciospores of tliis fungus have a minimum 
tempera ture for germination, which is 3° C. lower than that of the urediniospores; an 
optimum 2° C. lower than that of the urediniospores; and a maximum 6° C. lower 
than that of the urediniospores. 
37 Pennington, L. H. Op. cit. 
