46 



MASS. EXPERIMENT STATION BULLETIN 202. 



Seven days later the stromata were visible. Ten days after inoculation 

 the uredinia began to appear. One week later the number of sori on the 

 plants was counted, with the results shown in Table 3. 



Table 3. — Effect of Temperahire on the Infective Power of the Uredinio- 

 spores of P. Antirrhini. 



It is thus seen that raising the temperature 5° to 8° C. above the 

 optimum for gennination of the urediniospores causes the amount of 

 infection to fall off more than 90 per cent. 



As a further test of the effect of temperature on infection of snapdragon 

 by P. Antirrhini, plants of a susceptible variety were inoculated in two 

 different greenhouses, having a night temperature of 10° C. in one case, 

 and 15° C. in the other. Fifteen days after inoculation the plants in the 

 greenhouse at 15° C. bore an average number of twelve uredinia, and the 

 plants in the greenhouse at 10° C. bore an average number of one hundred 

 and twenty uredinia. 



As indicated by the results with P. Antirrhini, the rusts are able to ger- 

 minate best at rather low temperatures. A consideration of the literature 

 also supports this view. Erikson (1895) discovered that low temperatures 

 are suitable to the germination of rust spores. He found that the spores 

 of yEcidiiim Berberidis germinate best when cooled for seven hours to 

 3° C, and that the spores of Peridermium strobi germinate best when cooled 

 for twenty-four hours to 6.5° C. He found the optimum temperature for 

 the germination of the spores of Uredo glumarum to be 4.5° C, and that the 

 spores of Uredo coronata germinate best after being cooled for sixteen 

 hours to a temperature of —10° C. In the last case it seems probable 

 that he went below the optimum temperature, and that the spores 

 germinated when the temperature again rose to the optimum. 



Howell (1890) found that the urediniospores of Uromyces TrifoUi (Alb. 

 and Schw.) Wint. germinate best between 11° and 16° C. They do not 

 germinate below 7° C. nor above 21° to 25° C. If the minimum, optimum 

 and maximum temperatures for germination of U. TrifoUi are taken as 

 7° C, 11° C. and 21° C, respectively, this fungus has about the same 

 temperature-germination relation as has P. Antirrhini, the minimum, 

 optimum and maximum temperatures for germination of urediniospores of 

 P. Antirrhini having been found by the writer to be 5° C, 10° C. and 

 20° C, respectively. 



