100 



3 8 13 



TEMPERATURE 



18 23 

 (Degrees Celsius) 



Figure 4. -Germination of aeciospores after 24 

 hours' incubation at several temper- 

 atures. The darker lines are means of the 

 individual samples. 



An exploratory test also was conducted to 

 determine how quickly aeciospores germinate and 

 germ tubes grow. Methods used to collect and prepare 

 spores were similar to those described above, but only 

 one collection (Cache National Forest) was tested. 

 Germination of spore groups was stopped after incu- 

 bation periods varying from 3 to 48 hours. Results 

 indicate that most viable spores germinate within the 

 first 3 hours of incubation at favorable temperatures 

 and that germ tubes continue to grow for more than 

 24 hours (table 1). 



Effects of Light on Germination 



The tests of temperature effects were run in 

 darkened chambers and probably indicate what 

 would happen at night in field conditions. In other 

 tests, aeciospores germinated equally well in low light 



such as might be present on cloudy days. Germi- 

 nation was nil in tests in direct sunlight, but in the 

 field there would rarely be sufficient moisture for 

 germination while there was bright sunshine. Appar- 

 ently contact with free water is necessary, as evi- 

 denced by failure of spores to germinate when tested 

 on glass surfaces in moist chambers if free water has 

 not contacted spores. 



Effect of Exposure on Viability and Infectivity 



Tests also were run to determine how well 

 the disseminated spores could withstand the kind of 

 waiting period that may pass in the field before mois- 

 ture and temperatures become favorable for germi- 

 nation. Fresh aeciospores were deposited on shoots of 

 live comandra plants in a glasshouse where temper- 

 atures between 10° and 21° C. were maintained. Sur- 

 face temperatures-^ of comandra leaves remained 

 within a few degrees of air temperatures. At intervals, 

 leaves were plucked from shoots, pressed lightly 

 against 2 percent water agar, and removed. Aecio- 

 spores were thereby transferred to the stickier agar 

 surface. Examination showed that aeciospores had 

 not germinated during their exposure on leaves. 



Aeciospores on water agar were then placed 

 in moist chambers, and incubated at 18° C. for 24 

 hours. Examination proved that some aeciospores had 

 remained viable more than 20 days (table 2) at these 

 rather mild conditions. 



Infectivity of aeciospores was tested after 

 the spores were exposed to field conditions in Cache 

 National Forest at about 6,000 feet elevation. During 

 1966, aeciospores were deposited on comandra 

 shoots at various times in an area where natural infec- 

 tion occurs only in unusual years. (None was detected 

 during these tests.) The ability of aeciospores to 

 remain infective was estimated from the degree of 

 development of uredinia on test plants after a period 

 of mild, moist weather that would allow infection. A 

 hygrothermograph in a shelter described by Hunger- 

 ford (1957) and a recording rain gage within 50 feet 

 of all test shoots provided data on time elapsed 

 between deposition and conditions possible for germi- 

 nation. Uredinia developed on a few leaves of one 

 plant; aeciospores had been deposited 3 days before a 

 trace of rain that made conditions favorable for 

 germination. Other plants on which spores had been 

 deposited 6, 10, and 22 days before conditions 



^Measured with thermocouples of overlapped and 

 soldered wires 0.05 mm. in diameter. 



4 



