WHITE-PINE BLISTER RUST. 25 



of 1915, SO that infection was limited to the season of 1914. Of 40 

 infections found, 12 were on 1913 wood and produced aecia in May, 

 1917; 28 were on 1914 wood and produced aecia in May, 1917. The 

 period of incubation for the former cases could not be more than 

 three years and nine months. It may be that infection took place 

 on 1-year-old wood, in which case the incubation period would be 

 only two years and ten months, as was the case with the 28 infections 

 on 1914 wood. 



Study of infected branches which had just borne aecia for the first 

 time was made by Posey and Gravatt in 1917 at Stratham, N. H. 

 Their notes show that more than 99 per cent of these infections 

 might be 3 years and 6 months old, but could be no older. In 

 another locality, they found a number of aecia upon growth of the 

 year 1915, making an incubation period of about 18 months. 



The first successful inoculations of pines with sporidia of Cronartium 

 rihicola are apparently those made by Klebalin on August 27, 1903 

 (71). Rihes nigrum leaves with telia were placed over two young 

 trees of Pinus strohus and the whole covered with bell jars as long as 

 the Ribes leaves remained fresh. On June 19, 1904, these trees had 

 swollen twigs bearing juvenile leaves. Early in July, pycnial drops 

 formed, after a lapse of 10 months. It is probable that in the normal 

 course of events aecia would form the. next May. This woukl make 

 an incubation period of about 19 months. 



The wi^iter (133) in November, 1910, inoculated a number of 

 healthy Pinus strohus trees in the greenhouse with teliospores. 

 These inoculations were made on the young bark. In January, 1912 

 one each of those inoculated with wound and without wound of bark 

 developed marked swelling. A little later pycnial drops formed, 

 but snails ate them aiid the surrounding bark, so that the infections 

 did not develop. Apparently it would have been a matter of but a 

 few months when aecia would have formed. This would give an 

 incubation period of about two years. In May, 1916, the wTiter (145) 

 set out healthy Pinus strohus trees among some experimental Ribes 

 bushes on Block Island, R. I. The Ribes were heavily infected the 

 rest of the season. Telia began to form the latter part of July and 

 were abundant by September. On May 10, 1918, several of these 

 trees were found bearing a3cia. This makes a maximum incubation 

 period of about 21 months. 



In 1917, 10 young plants of Pinus Jlexilis were set out in the experi- 

 mental plat on Block Island. Nine of them have lived. In the spring 

 of 1920 seven of them bore tecia of Cronartium rihicola on the growth 

 of 1918, and the other two were much swollen and discolored, so that 

 if alive they will certainly produce a^cia in 1921. It appears that P. 

 Jlexilis is very susceptible. The experiments on Block Island indicate 

 that it is even more susceptibh^ than is P. strohus. The incubation 



