WHITE-PINE BLISTEK KUST. 45 



4 mm. wide have been found bearing groups of uredinia (PI. IV. 

 fig. 2). 



The age and relative maturity of the leaf has much to do with its 

 susceptibility. It has been the general experience that Ribes leaves 

 may be overmature and also may be too young to take the disease. 

 Infection does not occur on the leaves of a given species of Ribes 

 untd they have reached a certain degree of maturity. Leaves 

 produced by buds developing in late summer or faU, even if very 

 small, readily become infected. The different species of Ribes vary 

 much in this regard. Rihes nigrum shows a great range in its age 

 of susceptibility, while resistant species become "infected only on 

 leaves of a certain maturity. The most favorable stage of growth 

 seems to be about when the leaf attains full size but has not become 

 hardened and leathery as it does later. Tests were made by Gravatt 

 in 1915 in the greenhouse on Rihus nigrum. The plant had three 

 shoots of nearly equal size and development. They bore fully 

 mature leaves at the base and had leaves at the tips about half 

 grown. In this case all the leaves became infected except the lower 

 three or four on each shoot. In 1916, McCubbin (ms. report) made 

 several series of inoculations with seciospores upon Ribes nigrum 

 leaves of various ages. The plants were not kept under the best 

 growing conditions, so the results are less pronounced than might 

 otherwise be expected. He produced no infection on the youngest 

 leaves. The older ones took the disease, but the overmature ones 

 took it least of all. York " made greenhouse tests with plants of 

 Rihes nigrum, R. triste, R. glandidosujn, R. hirtellum, and R. lacustre. 

 Leaves of various ages were present on all the plants. The mature 

 ones showed infection first. The degree of infection was heaviest 

 on the first species and decreased in the order named, R. lacustre 

 having but two pustules on a single leaf. Later, the haK-mature 

 leaves of R. nigrum and the leaves of R. triste and R. glandulosum 

 two-thirds mature became infected. The younger leaves did not 

 become infected then, but when reinoculated later they took the 

 disease, except that those of R. lacustre remained healthy. In most 

 inoculation tests made by the ^^^'iter and his associates in the green- 

 house both the oldest and the youngest leaves remained free from 

 disease, although they were treated exactly like the others. York " 

 tested this point extensively in the open in 1918 and found that 

 leaves just unfolding were almost invariabl}' immune to the fungus; 

 older ones took the disease readily; and overmature ones (especially 

 late in the season) were immune. Pennington -^ reached similar 

 conclusions working independently. 



a' York, H. H. Op. cit. 



28 Pennington, L. H. Op. cit. 



