MuKPHY— Oh the Cause oj Rolling in Potato Foliage. 177 



the capacity of the four commonest of the insects listed above to carry leaf -roll 

 were undertaken at once hy transferring individuals taken from diseased plants 

 to healthy plants, the latter being protected by muslin cages which had been 

 placed in position soon after the plants appeared above ground. Each caged 

 healthy plant was provided with a caged control plant, the two being derived 

 from the two halves of one tuber. The caged plants were separated by a 

 distance of four yards of unplanted ground from the nearest potatoes, which 

 were for the most part healthy. It was found that under local conditions, 

 and at least in the alDnormal season of 1921, the caging of a healthy plant with 

 muslin was not a guarantee of protection against leaf -roll infection unless the 

 plant was also placed at a certain distance from diseased plants. This distance 

 need not apparently be very great, probably because a small space of clear 

 ground is a considerable help in preventing infestation with aphides, while the 

 other insects (which, it will be seen, were proved to be carriers also) are com- 

 paratively easily excluded, although much more active. The experimental 

 infestation of the caged plants with the various kinds of insects took place on 

 July 27, 1921, the latter being collected from diseased plants, and one lot of 

 each, from five to twelve in number, being introduced into a cage. No insects 

 were put in the adjacent cages containing the control plants. Owing to circum- 

 stances which were beyond control, it Avas not possible to note the behaviour of 

 the plants during the latter part of the season, but the tubers of each of them 

 were dug and saved separately at the usual time. 



The tubers were all planted in separate lots in the open field in 1922. All 

 those from the control cages produced healthy plants, except those from cage 

 No. 3 (two in number), both of which were diseased with leaf-roll. The plants 

 from tubers from the corresponding cage, to which frog-hoppers had been 

 introduced in 1921 (again two in number), were also both diseased with leaf-roll. 

 Unfortunately there appeared in this exceptional control cage in 1921 a 

 ' ' volunteer ' ' plant from a tuber which survived in the ground from a previous 

 crop of the same variety as the experimental plant (Up-to-Date). This crop 

 was very badly affected with leaf -roll in 1920 ; and although the plant was 

 removed as soon as noticed, under the circumstances it is thought better to 

 exclude this part of the experiment from consideration. 



The remaining results were as follow : — 



1921. 1922. 



Healthy plant experimentally infested \ , ;irii hj- j -uir 



■ ,, \^ -j,"^ iri , ■'■I- produced 5 plants, all diseased with leaf- 



v/im 12 ca])sid bags (Cawcoris bipinic- r . ii 



tatus) taken from affected plants — ) 



Control plant, from half of same tuber, ) , j ^ i t n i ij.u 

 , ■ f , ■] y produced 4 plants, all healthy. 



Healthy plant experimentally infested | ^^^^^^^^^ g j^^^^ ^jj ^j^^^^^^, ^.^j^ j^^j_ 

 with 12 jassids (Ti/phlocyba Uliiii) v '- ,, ^ 



taken from affected plants — ) 



^°not°nfested- "' ^'''" °^ ''''''' *"^"'' j P^'°''"°''^ * P^'^"^'' ^" 1^^^"^^- 



Healtbv plant experimentally infested ] j j , i i r i • i j- 



■lu n a 1, 11 m II- J J- ■ \ \ produced 4 plants, of which one was dis- 

 with 6 flea-beetles (fsiiUwdes alfmis) \ '^ j -ti i ^ n j o i i^i 



,, „ ji7i.jii eased with leaf-roll, and 3 were healthy, 



taken from aflected plants — j ' •' 



Control plant, from half of same tuber, ) , j r i . n i ui 



not infested- produced 5 plants, all healthy. 



