68 BULLETIN 957, U. S. DEPAKTMENT OF AGRICULTURE. 



of 30.5 per cent. In another experiment the sporidia were placed 

 on the periderm of white-pine twigs of the same season's growth and 

 on Hving leaves of Pinus strohus and P. rigida. They were then 

 exposed dry at 66° F. and with a hmnidity of 90 per cent. None 

 survived for 10 minutes. At 72° F. and a humidity of 69 per cent 

 none survived for 10 minutes. They conclude that the sporidia can 

 endure very little desiccation and are short lived under seemingly 

 optimum conditions. Abundant moisture is necessary for infection 

 of pines to occur. 



HETERtECISM OF THE SPORIDIA. 



A number of tests have been made to learn whether Ribes might 

 become infected by sporidia of Cronartium rihicola. Jaczewski (59) 

 states that experiments have shown that they will not infect Ribes 

 leaves. In 1913, Clinton, Stewart, and the writer (151) inoculated 

 Ribes nigrum leaves with teliospores overwintered out of doors, but 

 there were no infections. In 1912 the writer tested fresh teliospores 

 without infection occurring (136). In 1917 Gravatt made several 

 tests of fresh, sporidia-producing telia, but no infection resulted. 



OVERWINTERING OF CRONARTIUM RIBIC OLA. 

 Overwintering on Pines. 



The generally accepted view has been that Cronartium rihicola 

 lives over winter by means of the mycelium in the bark of living 

 infected pines and by this means only (142). A number of writers 

 have mentioned cases where their observations seemed to indicate 

 the possible overwintering on infected Ribes, but nothing that could 

 be accepted as real evidence was offered until the last few years. 

 There is no question that the fungus overwinters chiefly in the 

 infected living pine trees and has been carried in the dormant con- 

 dition from continent to continent in young infected pines. 



It has been discovered, as has been mentioned earlier, that Cro- 

 nartium rihicola may overwinter as myceliu]ii in infected branches 

 cut from diseased trees late in the fall, or during the winter, and 

 allowed to lie until spring. Then, if these cut-off branches lie close 

 to damp soil or with the cut ends in a stream or pool, fresh vigorous 

 secia are produced (89). Still another phase of overwintering was 

 discovered by Dosdall (29) in Minnesota. On April 19, 1918, a 

 dead branch of white pine, bearing an infection which bore secia in 

 1917, was collected. Germination tests in distilled water showed 

 that 1 to 2 per cent of the old seciospores were still viable. 



Overwintering on Ribes Plants. 



Investigations of overwintering of Cronartium rihicola on Ribes 

 plants, in Europe, seem to be limited to field observations. They 



