48 BULLETIN 957^ U. S. DEPARTMENT OF AGRICULTURE. 



Plant 2 was treated exactly like plant 1 except that the inoculations 

 were made on the upper sides of the leaves. No infection resulted. 



Plant 3, on April 10 and 15, was inoculated on one branch on only 

 the lower surfaces of the leaves. Another branch was inoculated on 

 the upper surfaces only. A third branch was left as a check. Weak 

 infection resulted on the first branch, A repetition on April 29 with 

 fresh seciospores gave better results. 



Plant 4, left untreated, was in another inclosure of the greenhouse. 

 It remained healthy, showing that infection had not occurred before 

 the plants were brought into the house. 



On May 26 similar inoculations were made by Ewert with uredinio- 

 spores on the upper sides and lower sides of leaves. Infection 

 resulted in the latter case and also a slight infection of the lower 

 surface of one leaf which was inoculated on the upper surface. 



Another similar series of inoculations made by Ewert on June 6 

 gave infection only on the leaves inoculated on the lower side. He 

 fails to say in all cases that the sori formed only on the lower surface 

 of the leaves, but his language implies that this is the case. Attack- 

 ing the problem in another way, Ewert (37) sprayed Rihes nigrum, JR. 

 aureum, and R. ruhrum, leaves, part on the lower side only, part on 

 the upper side only, and part on both sides, while controls were left 

 unsprayed. The details are presented on pages 77 to 79. Because 

 some leaves which were sprayed on the upper side developed a few 

 sori. beneath, he appears to be a little doubtful whether infection 

 may occur on the upper surface, but he concludes that it " apparently 

 almost exclusively takes place on the lower surface of the leaf." 



The writer and his associates have made hundreds of inoculations 

 on the upper surface of leaves of many species and varieties of Ribes, 

 without a single direct infection occurring there. Numerous in- 

 stances have been noted in these experiments in which infection ap- 

 peared on the lower surfaces of leaves that had been inoculated on 

 the upper side. This is believed to be due to spores reaching the 

 lower surface in some unknown way. In fact it is very difficult if 

 not impossible to guard against this. York and Overholts inoculated 

 leaves of Ribes glandulosum on the upper side, both with and without 

 ring cells, to prevent the spores reaching the lower surface. Slight 

 infection occurred on the lower side in some cases where cells were 

 not used. Where the cells were used no infection occurred. These 

 tests were made on leaves of difi^erent ages on plants of various ages, 

 from young seedlings up to fruiting bushes. Tubeuf (173) inoculated 

 leaves of Ribes nigrum on the upper side by applying the spores in 

 water with a brush. All of the leaves thus inoculated remained 

 healthy except a single one which had a uredinium on the lower sur- 

 face. He was uncertain whether a spore infected it through the lower 

 surface or through a lesion on the upper surface. ■ 



