WHITE-PINE BLISTER RUST. 37 



Pinus strohus trees with aeciospores, but without producing infection. 

 Hennings (53) inoculated P. strobus trees with aeciospores and also 

 with teliospores. No infection resulted from either. In the spring 

 of 1916 the writer made 100 inoculations with wounds into the bark 

 of Pinus strohus trees, out of doors, with fresh aeciospores of Cronar- 

 tium rihicola. No infections have resulted to date. In May, 1917, 

 the writer (146) inoculated white pines by dipping the tips of young 

 twigs in water containing great quantities of newly formed aeciospores 

 of C. rihicola. The needles as well as the twags were covered with 

 spores. Glassine bags, containing wet wads of cotton, were then 

 tied over the inoculated twigs to keep up the humidity of the air. 

 No evidence of infection is yet visible. 



GERMINATION OK THE .ECIOSPORES. ' 



Experience shows that fresh o?ciospores taken from aecia just as 

 they are about to break open, or just at the time of breaking, possess 

 maximum infective power. Doran (28) confirms this opinion. 

 Inoculations made with such spores are sure of results if conditions 

 are at all favorable. Old^r aeciospores are erratic in germination, 

 but some of them retain viability to a marked degree. Cooling on 

 ice stimulates their germination to a decided degree, as is shown by 

 experiments performed by Eriksson (32), Gravatt, and others. 

 Each spore produces from one to five (20, 29) germ tubes, which 

 branch freely. The viability of fresh aeciospores is generally high, 

 as many as 95 per cent germinating under favorable conditions. 

 They require 8 to 10 horn's to germinate (28). 



Doran (28) determined the minimum, optimum, and maximum 

 temperatures for the aeciospores of Cronartium rihicola. Five series 

 of tests were made. It was found that germination in distilled-water 

 drop cultures occurred through a range of 12° C, but the percentage 

 of germination dropped rapidly both above and below the optimum. 

 The minimum temperature for the aeciospores was 5° C, the optimum 

 was 12° C, and the maximum was 19° C. 



LONGEVITY OK THE ^CIOSPOUES. 



Klebahn (70, p. 26) seems to have been the only European investi- 

 gator who has tested the longevity of aeciospores. He found them 

 strongly viable after seven weeks. 



From the beginning of the work of the writer on Cronartium 

 rihicola it has been known that the aeciospores retain their viability 

 for a relatively long time under favorable conditions. In 1910 a 

 single aeciospore wliich had been kept in the laboratory for more 

 than five months in an open lecium on a diseased young tree which 

 was dried and kept as a specimen germinated in water (131, p. 30). 

 In 1915 McCubbin (85) collected on May G a diseased young wliite 



