Murphy — On the Cause of Rolling in Potato Foliage. 179 



well as others from plants which had suffered from various diseases, it became a 

 matter of urgency to determine whether aphides could cari'y infection from the 

 sprouts of diseased tuljers to those of healthy ones. This was proved to be the 

 case so far as leaf -roll, at least, is concerned/ 



The details of the exi^eriraent are as follows. The sprouts on a number of 

 tubers which had been kept on a dish in a lobby, and which originally came from 

 England, the past history of the tubers being not known, were found in 

 February, 1922, to be strongly infested with aphides. One of these tubers, 

 which was assumed to be healthy, but which gave rise later to a plant affected 

 with leaf -roll, was selected, and eighteen aphides {Myzus Persicae, Sulzer)' from 

 its sprouts were transferred to the sprouts of one-half of a tuber from a plant 

 known to have been affected with mosaic in 1921. The other half of this mosaic 

 tuber was kept separately, and no aphides were placed on its sprouts. Similarly, 

 eighteen aphides from the same original source were placed on the sprouts of a 

 half tulier derived from a healthy plant which was caged in 1921, while the other 

 conti'ol half received no aphides. On two subsequent occasions further lots of 

 twenty-five and twelve aphides respectively were transferred from the same 

 source to the sprouts of the same two half tubers, because those originally trans- 

 ferred did not multiply with sufficient rapidity. 



From the sprouts of the mosaic half tuber aphides were transferred, as their 

 numbers increased and permitted (about twelve to twenty-four were used in 

 each case) to the sprouts of three half tubers derived from healthy plants caged 

 in 1921. The corresponding three half tubers were kept free from aphides. 

 Similarly, from the sprouts of the healthy half tuber first infested from the 

 original source aphides Avere placed on the sprouts of three further healthy 

 half tubers, the corresponding halves of these receiving no aphides. 



Each of the half tubers was covered almost completely with sterilized soil 

 which half filled a flower-pot, the terminal sprouts, on which the aphides fed, 

 alone projecting. The half tubers and their sprouts were in each case enclosed 

 in a wide glass cylinder, pressed down nearly two inches below the soil-level, and 

 closed on top with a triiDle layer of muslin. The non-infested sprouted half 

 tubers were similarlj' protected, and remained free from aphides. 



The left-hand portion of accompanying figure diagrammatically illustrates 

 the transference of' the aphides from the sprouts of the original tuber (centre) 

 to those of the mosaic and healthj^ half tubers, and from both of the latter to the 

 three healthy half tubers in each case. The controls (not infested) of all these 

 are shown with the cut surfaces facing in the opposite direction in the top and 

 bottom rows. The original state of health of all the tubers is indicated by 

 shading. 



When the aphides had fed on the sprouts for from 13 to 52 days (and still 

 longer in the case of those first infested), the sprouts, including those of the 

 control half tubers, were thoroughly fumigated with a commercial greenhouse 



' Owing to unavoidable circimistanees, the experiment about to be described and another 

 similar one miscarried before any results were secured regarding the transmission of mosaic; 

 but that this disease may also be carried in the same way is only to be expected. This con- 

 ■clusion can perhaps be drawn from an experiment of Schultz and Folsom (13), in which 

 sprouts were infected with leaf -roll and mosaic respectively by means of aphides taken from 

 the foliage of diseased plants. These authors state that "this experiment is probably not 

 duplicated by natural conditions"; but storage conditions in milder climates may be such 

 that the transmission by means of aphides of infection from the sprouts of unplanted 

 <liseased tubers to the sprouts of healthy tubers may occur and be of serious practical im- 

 portance. 



-Thanks are due to Messrs. Rhynehart, Theobald, and Laing for identifying the aphides 

 used. 



SOIENT. PROC. K.D.S., VOL. XVII, NO. 20. 2 I 



