26 BULLETIN 957, U. S. DEPAETMEl^T OF AGBTCULTUKE, 



period would be about 20 months. A number of P. strohus trees set 

 in 1917 also bore secia in 1920, and many more will do so in 1921. 



Clinton and McCormick (12, 15) found that artificially infected 

 trees of Pinus strohus, 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 secia are located on nodes and 

 internodes that are 3 years old or over. It is rather exceptional for 

 secia 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.*^ 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 strohus 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 rihicola on pines. He inoculated 

 Pinus strohus, P. lamhertiana, P. excelsa, P. parvijlora, P. peuce, P. 

 cemhroides, P. jiexilis, P. montezumae, and P. cemhra. He got 

 yellow spots on the needles of P. lamhertiana but no further results 

 were noted. P. strohus 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 yellow spots seemed to form the base 

 for the pycnial spots in the bark. Older plants of Pinus strohus 

 became infected less readily than, those only 2 years old. This 



6 York, H. H. Field studies of Cronartium ribioola in the White Mountains of New Hampshire. Seen 

 in manuscript. To be published in Phytopathology. 



