44 BULLETIN 9S1, IT. Si DEt^AfitMEIsft OF AGRICULTUEE. 



seciospores out of doors. The leaves were not wet, but there was 

 dew every night. No infection resulted. 



In 1913, Stewart and Rankin (151) made some observations on 

 this problem. On May 14 two white-pine trees were found bearing 

 abundant open secia. 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'*' and Snell (145) in New York in 

 1918 on Ribes rotundifolium 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 riiicola 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. 



