WHITE-PIISrE BLISTER RUST. 71 



previous winter a single seedling plant of an unknown species bore 

 leaves flat on the soil under similar conditions until March 12, 1917, 

 when it was brought into the greenhouse and inoculated. It 

 promptly took the disease on the overwintered leaves. York ^^ found 

 Rihes glandulosum plants in the spring of 1918 which bore overwin- 

 tered leaves that later became infected naturally. In such cases, it 

 would be easy to understand that late infections in the fall might lie 

 dormant until sprmg and then produce vigorous uredinia. More 

 time is necessary to determine whether this actually occurs. 



As stated previously, infection of petioles by Cronartium rihicola 

 is quite common. Early in 1917 Colley (17) discovered that infected 

 petioles often had telia and masses of active mycelium as well as 

 uredinia in the central pith. This raised the question of the possi- 

 bility of such mycelium remaining active imtil spring and producing 

 new uredinia. 



Whether the fungus can live over winter on dead diseased leaves 

 seemed unlikely in view of the negative results of Arthur and Petry 

 (151) with urediniospores from stems of plants diseased the preceding 

 summer, and the negative results of Stewart (151) with material 

 overwintered out of doors at Geneva, N. Y. Howitt and McCubbin 

 (56) early in 1915 attempted to produce infection by spores which 

 remained over winter out of doors on dead Ribes leaves. All of their 

 attempts were unsuccessful. In the spring of 1918 York (180) and 

 the writer obtained infections with urediniospores overwintered out 

 of doors in Massachusetts on dead Ribes leaves, proving that uredi- 

 niospores may survive the winter. This was repeated in the spring 

 of 1919 by Taylor (157). 



The possibility of infection on Ribes stems was early investigated 

 by the writer but with no success. Many inoculations were made on 

 young Ribes shoots by the writer and later by Gravatt, Doran (28), 

 and York " but without success. However, in the summer of 1917, 

 Posey and Gravatt (112) discovered fruiting uredinia on the young 

 shoots of Rihes Mrtellum at Kittery Point, Me. They inoculated 

 other young shoots with seciospores and secured mature uredinia. 

 Colley found uredinia in the pith of these infected stems. Gravatt 

 later inoculated young seedlings of Rihes fasciadatum in the green- 

 house with jEciospores and secured heavy infection of the cotyledons. 

 In one seedling the fungus also attacked the stem just below the 

 diseased cotyledons and developed several uredinia (PI. V, fig. 2). 

 Later, however, the plant outgrow the disease. Taylor and York 

 have successfully inoculated stems of several species of Ribes. (See 

 p. 50.) 



That Cronartium rihicola overwinters on Ribes is established. 



62 York, H. n. Op. eit. 



