Sept. IS, 1919 Investigations on Mosaic Disease of the Irish Potato 251 



TRANSMISSION BY GRAFTING 



Experiments were carried out in the winter of 191 6-1 7 at Washington, 

 D. C, to see if it were possible to transmit the disease by grafting. In 

 these experiments two methods of grafting were followed, the cleft-graft 

 and the inarch. According to the first method the top of a young, 

 apparently healthy, potato plant was removed, the base sliced down to a 

 thin wedge and grafted in the place of the top of a diseased plant. The 

 scion was held in place by winding with adhesive tape. Of six plants 

 grafted in this way that grew well, all the scions showed evidence of 

 the disease (PI. 26, A). Four of the plants from which the scions were 

 taken remained apparently healthy. The other two showed evidences of 

 the disease. Grafts were made according to the inarch method by placing 

 a healthy and a diseased plant side by side, slicing away a thin layer of 

 the outer tissue of the stem, bringing the cut surfaces in close contact, and 

 fixing them by wrappings of adhesive tape. After the plants had re- 

 mained in contact for several days the stem of the healthy plant was cut 

 below the point of attachment and the top of the diseased plant removed. 

 In three grafts made in this way the scion of one became diseased while 

 the parent plant remained healthy. The other two were doubtful. 

 This last-mentioned method of grafting seemed not to be adapted to 

 potato plants because, unless maintained in a very humid atmosphere 

 the scions wilted. However, Giissow (j) in 191 8 by inarching a mosaic 

 shoot on a healthy one found that no mosaic symptoms formed on the 

 foliage of the sound plant but that tubers from it produced mosaic 

 plants. 



The results obtained in these preliminary experiments were corrobo- 

 rated by a number of experiments in the field in 191 7, the results of which 

 are shown in Table I. In this series no attempt was made to control 

 aphids, noi were any observations made, after grafting, on the plants 

 which supplied the scions. However, these plants were from 4 to 6 

 inches high and free from mottling at the time of grafting. 



Table I. — Grafts of potato vines, Presque Isle, Me., igi'j 



During the summer of 191 8 grafting experiments were continued in 

 northern Maine. Although more than 100 grafts were made, relatively 

 few of these made sufficient growth, 4 to 12 inches, to show distinct 



