44 BULLETIX 957, U. S. DEPARTMENT OF AGRICULTURE. 
asciospores out of doors. The leaves were not wet, but there was 
dew every night. No infection resulted. 
In 1913, Stewart and Eankin (151) made some observations on 
this problem. On May 14 two white-pine trees were found bearing 
abundant open aecia. On May 17, they were cut down and burned. 
About 120 feet from the pines there was a small plantation of Ribes 
nigrum and R, vulgar e. The weather was dry and unfavorable for 
infection of the currants to take place for several days before May 
15. The forenoon of May 15 was damp, but in the afternoon it 
dried off and remained dry until after the trees were destroyed. 
They concluded that the infection of Ribes which developed on June 
10 apparently could have taken place only in the wet forenoon of 
May 15. It appears to the writer that the long incubation period 
indicates that the spores from the pines stuck to the Ribes leaves 
without germinating until a later rainy period long enough for 
infection to occur. 
Studies made by Pennington 26 and Snell (145) in New York in 
1918 on Ribes rotundifoiium show the absolute dependence of the 
spread of this fungus upon moist weather. They found that six 
distinct generations of urediniospores were produced during the 
season, with a slight seventh one the last of the summer. These 
appeared approximately two weeks after spells of rainy weather. 
York working in the White Mountain region for three years has 
made a great many successful field inoculations on various species 
of Ribes without using any form of moist chamber. His work was 
largely carried on, however, in localities naturally moist. 
Temperature also is an important factor. Probably much of the 
efficiency of Hunt's iceless refrigerator inoculating chamber is due 
to the rather low temperature obtained. Cronartium ribicola is 
favored by low temperatures, as is shown by the optimum tempera- 
tures determined for it by Doran (28) . Doran also inoculated plants 
of Ribes which were then kept at 3°, 12°, and about 22° C. No 
infection occurred on the first and last, while the one at 12° C. devel- 
oped uredinia. 
Sunshine is an important factor, indirectly if not directly. Its 
direct influence upon the spores is destructive (30, 88) but it is doubt- 
ful if this action is powerful enough to hinder germination greatly 
if sufficient moisture is present. Indirectly sunshine affects in- 
fection by quickly reducing moisture. It seems that a moderately 
cool temperature is most favorable and that bright sunlight may 
elevate the temperature above the optimum for the fungus. 
The size of the leaves alone seems to have little or no influence 
upon their susceptibility to infection (147). Leaves as small as 
26 Pennington, L. H. Op. cit. 
