176 Maine Agricultural Experiment Station. 1920. 



tuber — 200 in all — was cut into four pieces, those from all-mo- 

 saic stock being sliced across and those from the rogued stock 

 being quartered. The cut seed was further mixed in a sack and 

 left for over a day. Then the seed pieces from the all-mosaic 

 stock, having been cut in a different shape from those of the 

 other stock, were sorted out and discarded. The row of 400 

 plants grown from the rest contained 72 mosaic plants. These 

 evidently were diseased because of infection in 1918, since an- 

 other check row of 400 plants, from 100 tubers of the same 

 barrel, contained 85 mosaic hills although no chance had been 

 given since harvesting in 191 8 to get the disease. 



A similar test, but involving contact of vines in the row 

 also, was made with the same stocks. Instead of removing the 

 seed pieces of the all-mosaic stock, all the seed pieces were 

 planted unsorted in two rows. Of the 800 plants 475 were mo- 

 saic, 400 of course coming from the all-mosaic stock. Since the 

 additional 75 is less than the 85 in the check row, no transmis- 

 sion since the harvesting of 1918 was evident, except of course 

 that through diseased tubers. 



In 1918 a number of tuber units, each a group of hills from 

 one tuber, were partly mosaic. It was thought that such partial 

 infection of tuber units might be due to knife transmission. 

 Consequently in 1919, when most of the tuber units were planted, 

 usually three knives were used in rotation, each one being im- 

 mersed in a sterilizing solution, 4-per cent formaldehyde, when 

 not in use. However, the partial infection of tuber units was 

 as common as in 1918 and as in the case of the tuber units 

 planted in 1919 without knife sterilization. 



These various tests indicate that the seed-cutting knife is 

 negligible as a factor of mosaic transmission. 



CONTACT. 



The effect of contact between mosaic and healthy plants 

 was first studied in the greenhouse. As has been described, of 

 nine healthy plants each grown with stems and leaves in contact 

 with those of a mosaic plant, one showed mottling but not until 

 after a few uncontrolled aphids, possibly from mosaic plants, 

 were discovered upon it. At about the same time 8 healthy 

 plants from as many different Green Mountain tubers were 



