46 BULLETIN 957, U. S. DEPARTMENT OF -'AGRICULTURE. 
Snell 29 made inoculations out of doors with spores from unopened 
ascia on April 30, 1918. He inoculated opening buds of Ribes glandu- 
losum by carefully inserting a knife so as not to injure the leaflets and 
inserting the spores between the folds of the leaves. The largest 
leaves were 3 cm. broad, the smallest 3 to 5 mm. long. It was very 
rainy, so there was plenty of moisture. No infection was visible on 
May 15, but on May 22 heavy infection was present on all the leaves 
inoculated. The leaves on this date ranged from nearly full size to 
those just opening. The infection was heaviest on the largest leaves 
inoculated and decreased to a light infection on the smallest. The 
check plants were healthy. There are two possible factors which 
might have delayed the infection a week longer than usual. These 
are cool temperature and the immaturity of the leaves. The experi- 
ence of the writer leads to the belief that the latter was the principal 
factor involved in this case. Later Snell found natural infection on 
leaves of Ribes vulgar e that were only 12 mm. wide. 
It has been noted repeatedly that the earliest infections on Ribes 
leaves in the spring are about a month later than the time when the 
first aeciospores are set free. It is a question whether this is due to 
very cool nights or to the immaturity of the Ribes leaves at this time. 
Noninfection of immature leaves in the greenhouse leads the writer 
to suspect that the latter is the main factor involved. European 
writers (63, 101) have stated that Cronartium ribicola is decidedly 
earlier than the native pine-stem Peridermiums. 
Observations made by Gravatt at Block Island show that new in- 
fection in midsummer was present upon the fifth to the eighth leaves 
from the tip, not counting those less than 5 mm. wide; that is, on 
leaves just mature but not hardened. 
Gravatt and York and Overholts made many inoculations of 
leaves, petioles, and stems with wounds, but found no evidence that 
infection was favored by wounds. 
Whether viability of spores of Cronartium ribicola in culture solu- 
tions, water, etc., is a reliable index of their infective power (43) is a 
question which has arisen more or less insistently since inoculation 
experiments began. Klebahn (70) made a definite test with refer- 
ence to this question with aeciospores of Cronartium ribicola. The 
spores were collected on March 20 and kept dry until May 8 when the 
test was made. Some were sown on the leaves of Ribes aureum, some 
were sown on a cover glass coated with a thin layer of sterile Ribes- 
decoction agar, and others were sown on a cover glass moistened with 
water. The cover glasses were kept in a moist chamber to prevent 
drying. The Ribes plants became infected after 12 days on every 
leaf inoculated. The spores on Ribes-decoction agar germinated 
29 Snell, W. H. Period of exposure and size of Ribes leaves infected by the blister-rust fungus. Seen in 
manuscript. To be published in Phytopathology. 
