26 BULLETIN 957, IT. S. DEPARTMENT OF AGRICULTURE. 
period would be about 20 months. A number of P. strobus trees set 
in 1917 also bore recia in 1920, and many more will do so in 1921. 
Clinton and McCormick (12, 15) found that artificially infected 
trees of Pinus strobus, kept in the greenhouse, developed pycnial 
drops in five to six months after inoculation. 
All of the writer's experience in various outbreaks of this parasite 
shows that most of the newly formed ascia are located on nodes and 
internodes that are 3 years old or over. It is rather exceptional for 
aacia to be borne on needle-bearing wood 2 years old, in which case 
the minimum incubation period is about 18 months. The average 
incubation period out of doors is approximately 3 years and 6 months. 
TIME, PLACE, AND MANNER OF INFECTION OF PINES. 
There is constant danger of infection of pines at any time after 
telia form; that is, after about the 1st of June. The teliospores pro- 
duce sporidia in 6 hours under favorable conditions. 6 The sporidia 
germinate immediately. According to Clinton and Miss McCormick, 
infection of pine leaves may take place within 48 hours (14, 15) after 
the germinating sporidia are placed on the leaves. Any period of 
moist weather of 54 hours or longer after about June 1 may cause 
infection of pines. 
The available evidence indicates that infection of pine twigs takes 
place in or about the bases of the leaf fascicles (71, 84). If this is 
true for most cases, as seems likely, infection of Pinus strobus can 
occur only on wood that is 1 or 2, or exceptionally, 3 or 4 years old. 
This follows from the fact that the needles of this species ordinarily 
live only two seasons, but rather exceptionally they may live three 
or four seasons. 
Tubeuf (174) in 1917 published the results of successful inocula- 
tions with sporidia of Cronartium ribicola on pines. He inoculated 
Pinus strobus, P. lamhertiana, P. excelsa, P. parvijlora, P. peuce, P. 
cembroides, P. jiexilis, P. montezumae, and P. cembra. He got 
yellow spots on the needles of P. lamhertiana but no further results 
were noted. P. strobus became infected readily and bore secia, but 
none of the other species showed definite signs of infection. Infection 
evidently occurred in the needles, many of them having yellow spots. 
They were also present on the stems. Mycelium was abundant in 
the yellow areas. Tubeuf says that infection of the stem from the 
leaves is not common, but that direct infection of the stems is much 
more likely to occur. No pycnia were obtained on leaves, although 
the masses of mycelium in the j^ellow spots seemed to form the base 
for the pycnial spots in the bark. Older plants of Pinus strobus 
became infected less readily than those only 2 years old. This 
6 York, H. H. Field studies of Cronartium ribicola in the White Mountains of New Hampshire. Seen 
in manuscript. To be published in Phytopathology. 
