WHITE-PINE BLISTER RUST. 45 
4 mm. wide have been found bearing groups of uredinia (PL IV, 
%. 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 
until they have reached a certain degree of maturity. Leaves 
produced by buds developing in late summer or fall, even if very 
small, readily become infected. The different species of Ribes vary 
much in this regard. Ribes 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 Ribus 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 27 made greenhouse tests with plants of 
Ribes nigrum, R. triste, R. glandulosum, 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 half-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 writer 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 27 
tested this point extensively in the open in 1918 and found that 
leaves just unfolding were almost invariably immune to the fungus; 
older ones took the disease readily; and overmature ones (especially 
late in the season) were immune. Pennington 28 reached similar 
conclusions working independently. 
2v York, H. H. Op. cit. 
28 Pennington, L. H. Op. cit. 
