After remaining in the inoculation chamber for 1, 2, 

 and 3 days, the pines were placed in a greenhouse 

 until autumn and then moved to outdoor cold 

 frames. 



/?esu/fs. — SeedUngs were examined fre- 

 quently over a period of 2 years. No infections devel- 

 oped on 50 pines reserved as controls. On inoculated 

 seedlings, no positive indication of infection was 

 noted until the summer following inoculation. Then, 

 infections could be identified only by stem swellings 

 and pycnial droplets that exuded in August and Sep- 

 tember. Aecia formed and cast aeciospores in May of 

 the next year (fig. 14). The most abundant infection 

 resulted from inoculations at temperatures of 15° 

 and 20° C; 24 hours in the mist chamber was appar- 

 ently sufficient for the formation and dispersal of 

 basidiospores, and their germination and infection of 

 pine tissues (table 10). Infection also occurred under 

 less favorable conditions but no infection occurred at 

 5° or 25° C. From the position of the infections, it 

 was determined that 13 had entered through current 

 year shoots, 1 1 had entered 1 -year-old tissue (three of 

 these might have entered through current year adven- 

 titious shoots), and none had entered 2-year-old 

 tissue. 



Although no outward signs of the rust were 

 evident in needles, microscopic examination of sec- 

 tions showed rust hyphae in needles attached to stem 

 swellings. Unfortunately it could not be determined if 

 the rust had entered the needles first or had grown 

 out into them from infected bark. However, needles 

 would seem a likely place for infection since direct 

 stem infection by basidiospores of conifer rust fungi 

 has been reported only in primary tissue of current 

 year stems. In the pines tested here, stem elongation 

 had terminated at least a month before inoculation, 

 and periderm tissue had formed. 



Figure 14.-Lodgepole pine 21 months after ex- 

 perimental infection with telia of 

 Cronartium comandrae from Cache 

 National Forest. Ruptured aecia are pres- 

 ent on the upper portion of the swollen 



Table IQ.-Influence of time and temperature on experimental infection oflodgepole pine 



Time in Ratio of infected pines to total pines 



mist chamber Temperature of mist chamber (degrees C.) 



(hours) 5 To 15 20 25 5-1 5 ^ 



24 0/8 0/9 3/11 1/8 0/8 1/8 



48 0/8 0/10 3/12 5/8 0/8 1/8 



72 0/8 1/10 3/12 1/6 0/8 4/12 



Approximation of natural conditions as described in text, footnote 3. 



15 



