July 1, 1920 Transmission of Mosaic Disease of Irish Potatoes 321 



instruments was used for the controls and another for the virulent juice. 

 In these experiments the Green Mountain, Bliss Triumph, and Irish 

 Cobbler varieties were used. In each case the juice was taken from 

 vines of the same variety. 



The plants of the Green Mountain and IBliss Triumph varieties used in 

 this experiment developed from progeny which in 191 8 showed from 11 

 to 15 per cent of mosaic, eliminated in three roguings. In view of the 

 fact that, with the exception of the Irish Cobbler variety, these were 

 planted by using four seed pieces from a tuber, it was possible to inoculate 

 two of the hills in a tuber unit and have two additional hills of the same 

 tuber unit remaining as uninoculated controls. In each tuber unit the 

 plants in the second and third hills were inoculated — that is, a hill from 

 a stem-end quarter and one from a bud-end quarter. In Table IV are 

 given the results of these inoculations. 



From Table IV it is apparent that plants not infected in 191 8 if treated 

 with juice from healthy vines remained healthy to the end of the season. 

 (See PI. 49-51.) As indicated, the exceptions to this result, where 

 some tuber units produced plants which became mottled with mosaic 

 after being treated with juice from healthy plants, were due to the fact 

 that such units had become infected in 191 8 in the field but did not 

 present any evidence of infection at the time of the first treatment in 

 1 91 9. 



Plants inoculated with juice from mosaic-diseased vines showed the 

 first mosaic mottling upon the newly developed leaves July 14. At this 

 time aphids were just beginning to appear at the rate of a few individuals 

 to a plant, so that those agents of dissemination can be disregarded as a 

 factor in transmission in these open-field inoculations. It will be noted 

 that with virulent juice a certain number of tuber units showed mottling 

 throughout within a few days after the first inoculation, indicating that 

 the tubers had become infected in 191 8. In the remaining inoculated 

 hills every hill, with the exception of one of Bliss Triumph, showed dis- 

 tinct mosaic mottling, while the untreated hills of these same units re- 

 mained healthy to the end of the season. 



In addition to the mosaic mottling, distinct spotting and streaking of 

 the leaves, petioles, and stems obtained by July 25, so that at this time 

 some of the lower leaves began to die. Furthermore, a marked ruftling 

 and dwarfing of the leaves also became apparent, so that many of the 

 plants appeared like those in the medium plus or bad stage, indicating 

 that in a single season plants may develop an aggravated form of this 

 disease if inoculated properly. (Pi. 52.) 



