RUST OF ANTIRRHINUM. 



45 



the longevity of urediniospores, rusted shoots were removed from the 

 plants and placed at temperatures of 0° C, 10° C. and 22° C. Half of this 

 material at each temperature was allowed to dry in open containers, and 

 half of it was kept in closed chambers to prevent its drying out. Spores 

 were removed every seven days and placed at their optimum temperature 

 for germination, with the results shown in Table 2. 



Table 2. — Effect of Temperature and Drying on the Longevity of the 

 Urediniospores of P. Antirrhini. 



Percentage of germination of urediniospores. 



When this experiment was begun, 60 per cent of the spores germinated. 

 Some of the spores in moist air retained the power of germination forty- 

 nine days, and in dry air, forty-two days. Some of the spores at 0° C. and 

 at 10° C. retained the power of germination forty-nine days. It is evident 

 that exposure to freezing temperature does not shorten the life of uredinio- 

 spores. In Massachusetts the snapdragon remains green and lives through 

 mild winters with no protection, and lives through harder winters if 

 protected by a mulch. In January the writer obtained urediniospores 

 from green plants growing outdoors in Massachusetts. These uredinio- 

 spores germinated readily when placed at the optimum temperature for 

 germination, but after these plants had been dried three weeks at room 

 temperature the spores no longer germinated. Temperature is of less 

 importance than drying in shortening the life of urediniospores. 



To determine the effect of temperature on infection, twelve plants were 

 sprayed with fresh spores distributed in distilled water. Four of these 

 plants were placed at a temperature of 10° C, four at 15° C. and four at 

 18° C. The plants used were of a susceptible variety, Carter's Pink, but 

 they were free from disease when selected, and from a disease-free bench. 

 The plants remained in the above-mentioned temperatures twelve hours, 

 after which they were all placed in the greenhouse at the same temperature. 



