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 secia in May, 
1917; 28 were on 1914 wood and produced secia 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 secia 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 secia upon growth of the 
year 1915, making an incubation period of about 18 months. 
The first successful inoculations of pines with sporidia of Gronartium 
ribicola are apparently those made by Klebahn on August 27, 1903 
(71). Ribes nigrum leaves with telia were placed over two young 
trees of Pinus strobus 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 ascia would form the next May. This would make 
an incubation period of about 19 months. 
The writer (133) in November, 1910, inoculated a number of 
healthy Pinus strobus 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 and 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 writer (145) 
set out healthy Pinus strobus 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 secia. This makes a maximum incubation 
period of about 21 months. 
In 1917, 10 young plants of Pinus jiexilis 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 secia of Cronartium ribicola on the growth 
of 1918, and the other two were much swollen and discolored, so that 
if alive they will certainly produce aecia in 1921. It appears that P. 
jiexilis is very susceptible. The experiments on Block Island indicate 
that it is even more susceptible than is P. strobus. The incubation 
