42 



BULLETIN 957, U. S. DEPARTMENT OF AGRICULTUEE. 



tried, but is unsatisfactory because the tender leaves of the inoculated 

 plants are liable to scald in hot weather. Hunt (57) tested a form 

 of the iceless refrigerator for this purpose. This is a modification 

 of the field moist chamber described by Keitt (61, p. 540-541), 

 without a continuous water spray. It is essentially a framework 

 large enough to receive several potted plants, on top of which a large 

 pan of water is placed. Around the framework is fitted a loose 

 curtain of heavy cheesecloth completely surrounding the framework 

 on the sides and extending from the water in the pan on top to the 

 ground. In use, the cloth is wet thoroughly and the water in the 

 pan keeps it wet. This keeps the air within the chamber saturated 

 with moisture and cool, which is the desired condition for the plant. 

 This has been very successful even in the hottest summer weather 

 and has the desirable qualities of durability, cheapness, portability, 

 and simplicity. 



Clinton (12) has recently reported the successful inoculation of 

 plucked leaves of Ribes.in moist chambers. This is an old method 

 with the rusts, and was used by Barclay in India as early as 1887 (4) . 

 Clinton has apparently developed this method to a point of maximum 

 efficiency. It has not been used in the investigations by the writer 

 and his associates, the preferred method being to retain natural 

 conditions as far as possible in making susceptibility tests. 



FACTORS CONTROLLING THE INFECTION OF RIBES. 



Among the factors controlling infection of Ribes by Cronartium 

 ribicola may be mentioned moisture, sunlight, age of leaves inocu- 

 lated, and age of inoculum. 



Frequent allusions are made by investigators to the need for 

 abundant moisture in producing the infection of Ribes by ascio- 

 spores and urediniospores of Cronartium ribicola and in spreading 

 the fungus on Ribes. 



In 1904, Aderhold (1) performed a series of experiments to deter- 

 mine the influence of moisture upon the infection of Itihes vulgare 

 by ^ciospores of Cronartium ribicola. He had two inclosed cells, 

 the air in one of which was moistened by artificial rain, while in the 

 other it was kept relatively dry; he had similarly arranged plats 

 open to the free air. The conditions in these cells and plats he 

 summarized, as in Table III. 



Table III. — Conditions in cells and plats of Aderhold' s experiments. 



Experiment. 



Air. 



Temperature. 



Moisture. 



Amount 

 of light. 



1. Rain cell 



Motionless 



High 



Very great 



Small. 



2. Dry cell 



do 





Slight 



Do. 



3. Open rain plat 



Mox'ing 







Great. 



4. Open dry plat . 



do 





Normal 



Do. 













