164 Maine Agricultural Experiment Station. 1920. 



ietin, that the disease is caused by something in the juice of the 

 tuber or plant and not, as in many potato diseases, by an organ- 

 ism upon or in the surface layers of the tubers. 



Tubers from apparently healthy plants often have produced 

 mosaic plants. In fact, many groups of healthy plants have been 

 selected but in any such lot of stock the disease has always ap- 

 peared the next season unless the selected healthy group had 

 been grown under certain conditions, to be described later in this 

 bulletin. The frequency of the appearance of the disease in stock 

 selected as being healthy was at first puzzling and led to tests 

 of the communicability of the disease. 



Proofs of Infectiousness, 

 grafts. 



in order to disclose whether the disease could be trans- 

 mitted from one plant to another, tests were made with methods 

 that seemed most likely to succeed if such transmission were 

 possible. Grafts were made with plants in the greenhouse, in 

 the field both in the open and under cages, and with tubers in the 

 field. 



In preliminary trials in the greenhouse two methods were 

 used, the cleft-graft and the inarch. Both gave cases of trans- 

 mission, the originally healthy scion becoming mosaic after being 

 grafted upon a diseased stock while its parent remained healthy. 

 The former method proved to be more practical for securing 

 successful grafts. By it the base of the scion was sliced down 

 to a thin wedge, inserted between the parts of the split stock, 

 and held in place with the help of adhesive tape. 



The cleft-graft method was used somewhat more extensive- 

 ly in the field the following season, 191 7, with 62 per cent of 

 the originally healthy scions becoming diseased. In this test the 

 parents of the scions were not observed. 



In 1918 the same method was used again. In only thirty- 

 three grafts of healthy scions on diseased stocks was satisfactory 

 growth obtained and in each of these the scion became mottled 

 while the parent plant remained healthy. In the case of five 

 grafts of diseased scions on healthy stocks, these previously 

 healthy stocks showed mosaic in their later growth. Eleven 



