July I, I&20 Transmission of Mosaic Disease of Irish Potatoes 319 



CONCLUSIONS REGARDING TUBER TRANSMISSION 



Tubers from mosaic hills may be expected to transmit mosaic. In 

 addition, at least part of those from apparently healthy hills growing 

 near diseased plants will transmit the disease; and they tend to do so 

 more when the parent hill contains only two or three tubers, when the 

 relative size of the tuber in the parent hill is greater, and when the 

 seed piece is nearer the bud end. However, hill selection results in dis- 

 carding the hills with few tubers. The relation of relative size to mosaic 

 transmission is not sufficiently marked or consistent to justify attempting 

 tuber selection for the elimination of mosaic. 



TRANSMISSION BY GRAFTING 



TUBER GRAFTS 



Grafting was attempted with a few tubers by bringing into contact the 

 freshly cut surface of half a mosaic tuber and half a tuber from an ap- 

 parently healthy hill. In 14 cases the nongrafted half of the supposedly 

 healthy tuber remained healthy, and in 3 of these 14 cases the corre- 

 sponding grafted half produced mosaic shoots. The three cases of 

 apparent transmission were the only ones of the attempted grafts which 

 established organic union. The failure of transmission in the 1 1 other 

 cases indicates that mere proximity in a hill was not sufficient for trans- 

 mission. Furthermore, the small number of successful grafts apparently 

 was due to the fact that relatively old tubers were used. 



DISEASED SCIONS UPON HEALTHY STOCKS 



Since transmission by grafting had been somewhat effective both in 

 the field with insects uncontrolled and in the greenhouse with insects 

 controlled,* the same method was finally used in the field with insects 

 excluded by means of cages. Three tuber units were used, each con- 

 sisting of three hills. The untreated plants, the first hill of each unit, 

 remained healthy until dug. In each other hill two or three stalks, from 

 14 to 17 inches high, were cut down and split, mosaic scions inserted, 

 and contact established with the help of cord and adhesive tape. Soon 

 after the dates of grafting, June 28 and July 2, 191 9, the scions died 

 because of shading in the cages; but the branches of the stocks made 

 good growth, and by July 28 a branch in each of two grafted hills was 

 mottled. By August 9 a number of shoots in each cage were mottled 

 and were tagged. At the time of han.'est, August 26, these were found 

 to belong to the grafted hills. Healthy stalks also came from these hills 

 but were ungrafted, one even coming from the same seed-piece eye as a 

 grafted stalk. 



As the Irish Cobbler variety had not been used for this kind of grafting, 

 six mosaic scions were grafted upon uncaged stalks when the latter were 



' ScHULTz, E. S., FoLSOM, Donald. Hildebrandt, F. M., and Hawkins, Lon A. op. err. 



