Potato Mosaic. 167 



New methods were used in the greenhouse with better re- 

 sults. Juice from crushed mosaic plants was applied to the 

 young leaves of a healthy plant, growing from half of a split 

 tuber, and rubbed in by crushing the leaves somewhat with the 

 fingers. This application was made several times in the course 

 of a month, beginning when the plants were from 2 to 6 inches 

 in height-. The plants thus treated usually became mosaic if 

 new growths were produced. The same results were secured 

 at the same time by rubbing and crushing together the leaves of 

 mosaic and healthy plants. All new growth was healthy which 

 was produced by the untreated plants from the check halves of 

 the seed tubers or by control plants treated with juice from 

 healthy plants. i\ll these greenhouse plants were free from 

 plant lice. > 



The former of these methods was used in the field in 1919, 

 the Irish Cobbler variety being added to those used previously, 

 namely, Bliss Triumph and Green Mountain. Some of the non- 

 inoculated plants became mosaic early, due to infection in I9i8 xi , 

 and an equal number of the inoculated ones also, from the same 

 tubers. This was true also for some plants treated with the 

 juice of healthy plants. Of the other plants in the experiment, 

 all remained healthy except those inoculated with juice from 

 mosaic plants. Of 48 such plants, 47 became mottled — doing so 

 before aphids became numerous — in a series wherein juice was 

 transferred between plants of the same variety. Forty-three of 

 47 such plants became mottled in another series wherein juice 

 was transferred either from Bliss Triumph to Green Mountain, 

 from Green Mountain to Bliss Triumph, from Green Mountain 

 to Irish Cobbler, from Irish Cobbler to Green Mountain, or from 

 Irish Cobbler to Bliss Triumph. The plants that were mosaic 

 from 1918 infection did not show mottling until after the inocu- 

 lations had been begun, so that, in spite of the customary pre- 

 cautions, a chance was offered for accidental transmission when- 

 ever healthy juice was applied to a series of plants that included 

 some of these mosaic plants. This did not occur, thus showing 

 that rubbing and bruising the leaves of a healthy plant subse- 

 quent to such treatment of a diseased plant was not sufficient 



"This assumption seems reasonable as a result of various experi- 

 ments, with plant lice, described later. 



