WHITE-PINE BLISTER RUST. 43 



Aderhold placed his experimental plants in these cells and plats 

 on April 16. On May 6 all of the plants were heavily dusted with 

 seciospores, and half of those in each cell and plat were put under 

 conditions opposite to those they were in before inoculation. The 

 plants from the closed rain cell when inoculated and replaced in the 

 same cell took the disease heavily. Those from the closed dry cell 

 when inoculated and placed in the closed rain cell also took the 

 disease heavily. Those transferred from both the rain and dry 

 cells to the dry after inoculation showed no infection. All the plants 

 kept in the open plats failed to take the disease. It is apparent 

 from liis results that infection depends upon an atmosphere that is 

 nearly saturated with moisture. 



Experience in the greenhouse has shown that it is necessary to 

 have abundant moisture on the leaf surface for infection to succeed. 

 The leaf itself must be wet, without having large drops of water 

 collect. This moisture must be retained for some time by keeping 

 the surrounding air saturated with water vapor. Gravatt made a 

 series of parallel tests, part of the inoculated plants being kept under 

 bell jars 2 hours, part of them 7 hours, and another part 24 hours. 

 Infection occmTcd with the 7-hour and the 24-hour plants, but not 

 with the 2-hour tests. The writer made a series of inoculations in 

 the greenhouse with seciospores mthout putting the plants in moist 

 chambers. Not one infection resulted within 14 days, the usual 

 time necessary to reach full fruiting condition. The plants were 

 then put into moist chambers for 48 hours, and fair infection resulted 

 from the spores put on the leaves 14 days before. Ewert (37) per- 

 formed a similar experiment in 1912 with the same results, as did 

 Werth (177) in 1915 and York=^ in 1919: McCubbin in 1916 inocu- 

 lated two leaves on each of seven shoots of a Rihes nigrum plant. 

 Two of the shoots were put in a moist chamber for 48 hours. The 

 remaining five shoots were left uncovered. The leaves of only the 

 two inclosed shoots developed infection. 



It is equally necessary to have plentiful moisture for infection to 

 occur out of doors. Hennings (53) found a severe outbreak of the 

 disease on Ribes during a dry time in the Dalilem Botanical Garden, 

 but he attributes the intensity of the attack to the daily watering 

 (sprinkling) of the bushes. Ewert (35) says, "In the summer of 

 1902, moisture, the important factor for infection, was not lacking, 

 so all conditions were favorable for the sprcatl of the Cronartium. " 

 Schellenberg (123) attempted to inoculate Ribes bushes with oecio- 

 spores from Pinuft ccmhra in the open air. He attributes his failure 

 to produce infections to the bright sunny weather prevailing at the 

 time. In 1910, the writer (131) inoculated Ribes leaves with fresh 



«> York, H. H. Op. cit. 



