178 Scientific Proceedings, Royal Dublin Societif. 



Where the disease occurred m 1922 m this experiment it appeared early in 

 the season in the pronounced secondary form. It was clearly visible at the 

 time of the first detailed examination of the plots for leaf -roll on June 20 ; and 

 the contrast between the produce of the infected and the control plants was 

 then striking. This is illustrated in the photographs taken on July 10, repro- 

 duced in figs. 11 and 12, PI. VI, which show one of the plants infected through 

 the agency of eapsid bugs and one of the still healthy controls beside it, the two 

 being reduced to the same extent. The plants infected by means of jassids 

 and their corresponding controls presented an entirely similar contrast at that 

 time. The starch-content of the lower leaves of the diseased plants was com- 

 pared with that of similar leaves from the healthy control plants, and the usual 

 difference was evident. 



While the above results show hardly more than a suspicion that the potato 

 flea-beetle can act as a carrier of leaf-roll, there is little room for doubt that 

 both eapsid bugs and jassids act as efficient transmitters. This is important as 

 showing that there is no exclusive specific relationship between aphides and the 

 actual cause of leaf-roll, which is presumably an organism. This finding might 

 indeed have been expected in the case of capsids from the work of Oortwijn 

 Botjes (8), if it be assumed that the German common name "Wanze" refers to a 

 species of eapsid. This author seems to have been in doubt about his result, 

 because in one experiment with these insects no infection followed, while in the 

 other, in which it did, it was considered equally attributable to aphides, which 

 accidentallj^ found their way into his cages. 



General observations in the plots made in 1921 and 1922 showed the com- 

 parative scarcity of ai^hides in both years; and yet the ease and rapidity with 

 which infection was carried in quantity over comparatively long distances were 

 clear. Hence it appeared that some other and more active carrier must be 

 concei'ned in the matter, and that this in fact was the main problem. It may 

 be noted that in many of the original and later experiments of Quanjer, van der 

 Lek, and Oortwijn Botjes (10 and 8) the removal of healthy plants to a distance 

 of from 25 to 4 metres from diseased individuals frequently protected them to a 

 very large extent, if not entirely, from infection. Similar results were obtained 

 by the writer (5) in Eastern Canada. Under the conditions under which the 

 experiments now being described were carried out such a comparatively small 

 degree of isolation was of little or no avail, and mass infection occurred over very 

 much greater distances. In 1921 it is believed that jassids were the principal 

 carriers. On account of the proximity of a number of elm trees, these insects 

 were so abundant that if a potato plant in the vicinity of the trees were disturbed 

 on a sunny day in July they flew into the air almost as thickly as bees in a 

 swarm. Fortunately they appeared to be entirely absent from the plots in 

 1922, which were in a different situation. Capsid bugs, however, were present 

 in that year in greater numbers than in the preceding one. This was particu- 

 larly so near a hedge bordering on the plots. On a day following a period of 

 heavy rain as many as nineteen of these insects, feeding voraciously, were counted 

 on the exposed portions of leaves of one plant in such a situation. How many 

 more there may have been concealed amongst the foliage was not determined. 



Insect infection through sijrouts. — The presence of aphides feeding in late 

 winter and in spring on the sprouts of seed potato tubers before they are planted 

 is apparently not imcommon in some places iii Ireland as well as in Great Britain, 

 and probably elsewhere. It was known previously that they occurred thus on 

 the farm on which the experiments described here were caried out ; and since 

 the lots of potatoes selected for planting in the plots were stored alongside of 

 each other in small chip baskets ("punnets"), and included healthy tubers as 



