Viability of aeciospores and urediniospores 

 was highest during June and July and tapered off in 

 mid-August and into September (figs. 23 and 24). 

 Thus the chances of additional comandra infection in 

 late summer and autumn are diminished in two 

 ways-the inoculum is reduced in quantity and the 

 spores are less able to infect comandra because of 

 reduced vigor of germination. 



As telia elongate by formation of new telio- 

 spores, their ability to cast basidiospores increases, 

 and they become highly viable in early August. TeHal 

 viabihty then decreases irregularly over the rest of the 

 season. Decreases result from occasional germination 

 of teliospores and possibly from aging and daily expo- 

 sure to near-inhibitory high and low temperatures. 

 New telia and new teUospores continue to form, and 

 the ability of telia to cast basidiospores may again 



increase somewhat. Viable telia are present until 

 death of the host shoot, even though the number of 

 spores cast is reduced late in the season. 



The evidence as to the ability of basidio- 

 spores to germinate is more confusing. In general, ger- 

 mination by germ tubes decreased as the season pro- 

 gressed. Sometimes this was because secondary 

 basidiospores were formed rather than germ tubes. 

 Factors thought to influence the type of germination 

 of basidiospores of the conifer rusts include pretreat- 

 ment of basidiospores (Reed and Crabill 1915 and 

 Spaulding and Rathbun-Gravatt 1926) and environ- 

 mental conditions during germination (Bega 1960 and 

 Hirt 1935); these did not enter into my experimental 

 results, as conditions during germination tests were 

 held constant and there was no pretreatment of 

 basidiospores. 



SUMMARY AND CONCLUSIONS 



The rust fungus Cronartium comandrae was 

 studied in the laboratory and field to determine how 

 various environmental factors influence the rate of 

 spread of comandra bhster rust. C. comandrae over- 

 winters and is perennial in cankers in pines. It dies 

 out annually on comandra in the mountains. Activity 

 of C comandrae is thus limited chiefly to the growing 

 season of the hosts. 



Aeciospores that form on infected pines are 

 disseminated by wind and infect only comandra 

 plants. Aeciospores may survive several days of fairly 

 severe conditions before losing viability; they germi- 

 nate when exposed to mild temperatures on moist 

 surfaces. Temperatures from 8° to 18° C. are best for 

 maximum germination and germ tube growth. On 

 comandra, germ tubes develop appressoria, which 

 attach to stomates. From appressoria, infection pegs 

 penetrate between guard cells to infect the plant. 

 Maximum infection occurred in shoots held at 18° C. 

 in mist chambers for 48 hours. 



In. the field, uredinia develop on comandra 

 leaves and stems about 2 or 3 weeks after infection. 



By wind dispersal, urediniospores are distributed to 

 other comandra shoots. Urediniospores also retain 

 viability for several days after dissemination even 

 under harsh conditions. In the presence of free water 

 and mild temperatures, urediniospores germinate. 

 Temperatures of about 13° to 23° C. are most favor- 

 able for germination and infection of comandra. Like 

 those of aeciospores, germ tubes of urediniospores 

 penetrate through stomates of comandra. 



About 4 to 6 weeks after initial infection in 

 the field, telia form on leaves and stems. The telia 

 remain attached to comandra, and only under proper 

 conditions do their tehospores germinate to form 

 basidiospores that are dispersed by wind to infect 

 pines. Teliospores germinate in either darkness or 

 light after exposure to several hours of saturated air 

 at mild temperatures. In unsaturated air telia may 

 remain viable for several weeks. Prolonged exposure 

 to temperatures above 25° C. reduces telial viability, 

 and daily warm temperatures in the mountains prob- 

 ably reduce viability slowly. Freezing temperatures 

 reduce telial viability. After about 6 hours' exposure 

 to conditions favorable to germination, teliospores 



24 



