in plant communities near rust-infected lodgepole 

 pine for long periods even though heavily infected by 

 comandra rust almost every year. The combination of 

 susceptible comandra shoots and general rainfall 

 during the period aeciospores are being shed makes 

 conditions suitable for primary infection, as the rust 

 is usually common by mid-July on comandra near 

 infected pine stands. The capability of aeciospores to 

 germinate weU at prevailing cool temperatures also 

 enhances primary infection of comandra. By mid- 

 July, aeciospores are reinforced by urediniospores as 

 a source of inoculum. By this time, thundershowers 

 generally prevail and spread becomes more hap- 

 hazard. Because infection of comandra shoots takes 

 only a short time, thundershowers usually result in 

 some secondary spread. In most years infected 

 comandra can commonly be found a few miles from 

 infected pines, but in 1966, which was drier than 

 normal in July and August, the rust in comandra was 

 rare beyond a few hundred yards from infected pines. 

 Apparently little secondary spread occurred in that 

 year. 



Although telia are present from late July 

 until hard frosts in autumn, basidiospores are cast 

 only rarely. Not only were basidiospores rarely 



JULY AUGUST 



Figure 21.~A successful suction trapping of 

 basidiospores; and the corresponding 

 weather data in Beaver Mountain plot 

 1966. 



trapped, but also visual observations indicated that 

 basidiospore production had seldom occurred. Low 

 temperatures prevent any appreciable cast at night 

 during radiation dew periods even though relative 

 humidity near comandra commonly reaches 100 per- 

 cent for 4 to 6 hours. Moisture from thundershowers 

 usually does not last long enough to initiate basidio- 

 spore casting, but when it does, drying on succeeding 

 sunny days greatly reduces chances of pine infection. 

 By such action the potential of the inoculum may be 

 reduced to the point of limiting infection, even if 

 favorable conditions follow. Therefore, it appears that 

 thundershowers are detrimental to the chances for 

 pine infection. 



It seems likely that serious outbreaks of pine 

 infection in the Rocky Mountain States can occur 

 only when large warm storms invade and remain in 

 the area for several days during August or early Sep- 

 tember when telial inoculum is plentiful and highly 

 viable. As shown in figure 21, heavy rainfall is not 

 necessary for abundant telial germination, but satu- 

 rated air and long periods of mild temperatures must 

 prevail. Even during those periods when most basidio- 

 spores were trapped during these studies (figs. 21 and 

 22), casting was soon followed by periods of moder- 



la l« 20 21 22 



AUGUST 



Figure 22.-A successful suction trapping of 

 basidiospores; and the corresponding 

 weather data in Beaver Mountain plot 

 1965. 



22 



