MuuPHY — On the Cause of Rolling in Potato Foliage. 181 



the experiment being brought to a premature close, the net result was that of 

 the seven plants produced by half tubers the sprouts of which had been 

 infested with aphides from diseased tubers, four developed symptoms of leaf- 

 roll within about one month of their appearing above ground." All the controls 

 remained entirely healthy throughout, except, of course, the plant from the half 

 tuber known to have been diseased with mosaic. 



The diagnosis of leaf-roll was in all cases confirmed Ijy making a test of the 

 stai'ch-eontent of the leaves ; and it was found that all the plants which showed 

 sjmiptoms of leaf-roll also showed starch accumulation in the lower leaves, while 

 the upper leaves showed none. Corresponding leaves, both upper and lower, of 

 the plants which showed no signs of leaf-roll, when tested at the same time, 

 reacted normally. 



It would appear, therefore, that the potato may be in danger of infection, 

 certainly with leaf-roll, during its resting period. This is particularly the 

 ease if tubers are stored in houses during the winter, or are sprouted in 

 boxes before planting. In all probability the more delicate and etiolated are 

 the sprouts the better the nidus thej' provide for the insects ; for some cases 

 have come under observation in which improperly sprouted tubers became so 

 iiifested that at planting time they were almost covered with the cast cuticles of 

 aphides, and were slimy with their exudate. On the other hand, sprout- 

 infestation does not appear to occur in the case of pitted potatoes, although, of 

 course, sprouts may develop on the tubers. 



In the older English literati^re concerning "Curl" — a term which would 

 appear of necessity to have included leaf -roll, at any rate so far as Clreat Britain 

 and Ireland are concerned — several observers recorded the fact that potatoes 

 stored in houses suffered more from the disease than similar potatoes stored in 

 pits. Atanasoff (1) refers to one of these cases; and it is possible that under 

 certain conditions the coi'rectness of the observation might be less debatable than 

 would appear on the surface. 



The presence of aphides on the sprouts of seed potatoes may be of importance 

 in another waj", because it was observed that in the course of their development 

 such sprouts can carry the aphides above ground, and thus, no doubt, give rise 

 to the first seasonal infestation of the foliage with these insects. Where this 

 occurs, the assumption made in many quarters that the infestation of potato 

 foliage arises from the migration of aphides during the summer from an 

 alternate host, such as plants of the order Eosaceae, is obviously an unnecessary 

 one. Even when rose bushes infested with aphides are in the immediate vicinity 

 of potato plants, the amount of migration that takes place is sometimes negligible, 

 for the potatoes ma^' remain practically free from aphides. This was the case 

 in 1922 in the writer's garden, where two small patches of potatoes were under 

 observation during June, July, and part of August. A large "rambler" rose 



' As has been stated, tlie disease appeared at oaee in the secondary form, and therefore 

 the dietnm that secondary leaf -roll results only from the planting of tubers from infected 

 plants requires modification. There was another exception of common occurrence in the dry, 

 hot summer of 1921 — namely, the development of the full symptoms of secondary leaf-roll 

 on plants which had previously shown clear primary rolling, and which previous to that had 

 all the appearance of health — all within the course of the same season. The disease took this 

 course apparently when plants were infected through the foliage in June and July. The 

 ToUing and discoloration characteristic of the primary phase appeared first lu the top of the 

 plant and then extended gradually downwards so as to involve all the leaves. At this stage 

 such plants were indistinguishable from secondarily diseased plants produced from infected 

 tubers and carbohydrate translocation from their lower leaves appeared to be similarly 

 affected. The course of the disease here described is evidently different from that following 

 infection through the sprouts. 



