WHITE-PIXE BLISTER RUST. 69 



relate to instances where diseased Ribes bushes were found widely 

 separated from Pinus strobus or from all 5-leaved pines (6). These 

 necessarily depend for reliability upon the observer's complete and 

 minute knowledge of the Ribes and pines within considerable areas. 

 Hence, such observations are of very uncertain value. Investiga- 

 tions showing that the aeciospores of this fungus are distributed for 

 miles largely invalidate such observations so far as overwintering is 

 concerned. 



In North America, investigations of overwintering on Ribes plants 

 have been along the following lines: (1) By means of spores adhering 

 to dormant Ribes plants, (2) in dormant or partially opened Ribes 

 buds, (3) in living Ribes leaves which themselves lived over ^vinter, 

 (4) on dead Ribes leaves, and (5) on infected Ribes stems. 



Ovenvintering by means of spores adhering to dormant Ribes 

 plants has been investigated in several ways. A great many field 

 observations have been made upon bushes diseased heavily one year 

 and not infected the succeeding year. Cases where bushes were 

 shipped from known diseased localities and have shown the rust the 

 next season in their new locations, have been considered, but the 

 evidence has been too incomplete to be seriously considered except 

 as it might help to confirm or refute other stronger evidence. A 

 great many Ribes plants have been used by the writer in greenhouse 

 experiments ; they shed their leaves and become dormant for several 

 months, yet there has been no hint of the carrying over of the fungus 

 upon them from one season to the next. A cooperative experiment 

 was made with Stewart (151), using 500 plants of Ribes nigrum, which 

 in the summer of 1912 were heavily infected. The leaves dropped 

 normally. They were then dug and most of them heeled in out of 

 doors until February, 1913. They were then brought into green- 

 houses in six widely separated localities and allowed to put forth 

 new leaves. Examination of some of these dormant plants by Arthur 

 and Retry showed that plenty of urediniospores still adhered to the 

 stems and buds. Inoculations with these spores did not give any 

 infection, so that they presumably had lost their viability. The 

 results reported by six different investigators showed no infection 

 appearing on the new leaves. 



Howitt and McCubbin (56) in attempting to solve the overwinter- 

 ing problem, made the following tests : 



(1) In tlie fall of 1914, 16 black and 7 red currant bushes and 1 gooseberry bush, 

 all badly rusted, were stripped of leaves and placed in cold storage, where they 

 remained until March 10, 1915. At this date they were removed and planted in a 

 greenhouse. All grew well and produced healthy leaves and fruit and were entirely 

 free from rust throughout the summer. In addition, 17 black currant bushes, which 

 had been badly rusted in 1914 and which wore wintered in the field, were added to 

 the above on April 21, 1915. These also grew normally and without rust. 



