Sept.is.i9i9 Investigations on Mosaic Disease of the Irish Potato 267 



in this paper are preliminary in character. It is hardly possible to draw 

 conclusions from so limited an amount of data, though the facts seem 

 worth recording. 



METHODS OF CHECKING NATURAL TRANSMISSION 



The experimental results previously described in this paper suggest at 

 least one way in which transmission of potato mosaic may occur in the 

 field — namely, by aphids. Both species of aphids that were experimented 

 with are commonly found on potatoes, including those in Aroostook 

 County, Me. In 191 8, a year in which aphids were unusually abundant 

 upon potatoes in northern Maine, they began to appear upon the plants 

 about the middle of July. Since in the experiments mottling did not 

 appear after the plants had finished elongating and had produced blos- 

 soms, it is quite probable that aphid transmission in the field occurs too 

 late for the effects to be shown during the same season. The possibility 

 of this was demonstrated in one experiment (p. 262-264) in which 

 after aphid transmission some plants remained unmottled but produced 

 progeny that showed disease the next season. Before evidence had 

 accumulated regarding insect transmission, control of the disease was 

 attempted by means of hill selection and roguing. The results of such 

 attempts, together with notes made at the same time on yields, will 

 now be discussed. 



HILL SELECTION 



A number of hill selections were made in 191 6 and 191 7 in northern 

 Maine in order to ascertain more especially the progress of mosaic from 

 one season to another upon the same strain and stock. Plants in three 

 different stages of the disease as well as healthy checks were included in 

 these selections. The term "slight stage" was used when the plants 

 had just begun to show a few mottled spots on the leaves though the 

 foliage otherwise appeared like that of healthy plants. "Medium 

 stage" was used when the leaves had apparently just begun to become 

 slightly corrugated, had six or more mottled areas, and had begun to 

 show slight dwarfing. "Bad stage" indicated that the leaves were 

 mottled, corrugated, and decidedly dwarfed. The results of the obser- 

 vations on the behavior of the foliage are presented in Table VIII. 



