July 1, 1920 Transmission of Mosaic Disease of Irish Potatoes 317 



The foregoing results indicate that mosaic in northern Maine does 

 not necessarily change much from year to year in any diseased stock 

 after the first appearance of the effects of infection. The conditions 

 which determine the severity of the initial symptoms are not yet 

 understood. 



RELATION TO NUMBER OF TUBERS IN A HILL 



The tubers from 10 Bliss Triumph hills and 130 Green Mountain hills, 

 healthy in 191 8 but grown near to diseased hills, were all planted uncut 

 in hill lots in 191 9. In Table II these hill lots are classified according 

 to the number of tubers in a hill, and the percentage of tubers of each 

 class that transmitted the disease is given. 



Table II. — Relation of the number of tubers in a hill to mosaic transmission 



Number of tubers per hill . . 

 Number of tubers planted . . 

 Percentage of tubers mosaic 



41 



There is a high percentage for the classes with two or three tubers 

 to a hill, but otherwise no consistent relation obtains between number 

 of tubers and percentage of mosaic. The results are not modified 

 appreciably if the Bliss Triumph hill lots are disregarded. It thus 

 seems that the increase of mosaic could be reduced by the selection of 

 hills according to yield only if the hills with very low yields were 

 discarded. 



RELATION TO RELATIVE SIZE OP TUBERS 



In connection with the problem of control, the question has arisen 

 whether the selection of tubers according to size would have any effect 

 in regard to the increase of mosaic. Consequently each of the 140 

 hill lots which are considered in the preceding section was planted in 

 the order of decreasing apparent size of the tubers. With regard to 

 mosaic 69 were mixed — that is, with both mosaic and apparently 

 healthy plants in the same hill lot. In Table III the tubers of mixed 

 lots are classified according to their relative rank. No. i being the largest. 

 In addition, the percentage of tubers of each class that transmitted the 

 disease is indicated. 



Table III. — Relation of the relative size of tubers in a hill to 'mosaic tra?ismission 



Rank of tuber in size j i 



Number of tubers planted j 69 



Percentage of tubers mosaic 1 67 



The percentage is high for the group of tubers consisting of the largest 

 ones in the hills and tends to decrease, being 48 per cent for No. 2, 45 

 per cent on the average for No. 3 to 6, and 36 per cent on the average 

 for No. 7 to 10. 



