WHITE-PINE BLISTER RUST. 59 



wliile ice-cooled ones of the same lot gave 25 to 54 per cent germina- 

 tion. Marshall found the same stimulating effect from a tempera- 

 ture of 23° F. upon urediniospores on Ribes leaves stored outside a 

 window, as compared with those on leaves kept at room temperature. 



Posey dried infected leaves of Ribes nigrum in the hot July sun for 

 four hours. Urediniospores from these gave germination ranging 

 from 3 to 45 per cent, with an average of 17 per cent. Spores from 

 other leaves collected at the same time and dried inside in the shade 

 gave 3 to 90 per cent germination, an average of 47 per cent. During 

 dry, hot weather it has been found that the viability of the uredinio- 

 spores out of doors is very low. 



Duff (30), in studying the factors affecting their viability, found 

 that exposure to bright sunlight reduces theu' germination, the ultra- 

 violet rays being the destructive agent. Their viability appears to 

 him to be low. Three days after collection less than 50 per cent ger- 

 minated in distilled water. In about two weeks germination was 

 ne^lisible even when stimulated by cooling to 2° to 5° C. 



Pennington ^^ made germination tests of newly-matured uredinio- 

 spores of Cronartium rihicola produced naturally in the vicinity of 

 Lewis, N. Y., from early June until late in the autumn in hanging 

 drops of tap water. During the first two weeks of June (frequent 

 rain) the percentage of germination was high. From June 16 to 28 

 no tests were made. On June 29 and July 1 (rather dry) only 1 per 

 cent germinated. After this time, many tests were made with spores 

 from various localities. By July 14 (very hot alternating with some 

 rain) less than one- third of 1 per cent germinated, and from July 22 

 to 26 (hot and dry) less than one-tenth of 1 per cent. Fresh yellow 

 spores kept in an ice box gave no better results. On August 1 (rain 

 July 29 and 30) 5 per cent germinated. After this (decidedly more 

 rain) from 10 to 40 per cent germinated. The viability of these spores 

 seemed to be greatly decreased by hot, dry weather and increased by 

 cool rainy spells at the time they were produced. T\Tien the number 

 of spores produced decreased because of hot, dry weather, theu- rate 

 of germination also decreased and vice versa. 



Doran (28) found that the limiting temperatures for the germina- 

 tion of urediniospores are: Minimum 8°, optimum 14°, and maximum 

 25° C. He calls attention to the fact that — 



There is apparently a relation between the season when spores occur and their tem- 

 peratures for germination. The reciospores of Cronartium ribicola occiu- in the spring 

 when the average temperature is lower than in the summer, the season of occurrence 

 of the urediniospores of this fimgus. The tcciospores of tliis fiuigus have a minimum 

 temjierature for germination, wMch is 3° C. lower than that of the lu-ediniospores; an 

 optimum 2° C. lower than that of the urediniospores; and a maximum 6° 0. lower 

 than that of the lu-ediniospores. 



" Pennington, L. H. ' Op. cit. 



