46 BULLETIN 957, U. S. DEPARTMEISTT, OF AGEICULTUEE. 



Snell ^^ made inoculations out of doors with spores from unopened 

 secia on April 30, 1918. He inoculated opening buds of Rihes 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 vulgare 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 seciospores 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 rihicola 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 rihicola 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 Vvdth seciospores of Cronartium rihicola. 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 Rihes 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. 



