Potato Mosaic. 177 



grown in the same pots with mosaic plants, roots as well as stems 

 and leaves being intermingled. Opportunity for infection through 

 the roots was increased by the transplanting of the healthy plant 

 into the potful of soil containing the diseased plant, with more 

 or less consequent breakage of roots. The 8 plants remained 

 healthy, as did 16 others grown from the same tubers and by 

 themselves. This result was in marked contrast with a contem- 

 porary experiment, already described, wherein a certain method 

 of transferring plant lice resulted in ioo per cent infection. 



In the field, 18 healthy Green Mountain plants were grown 

 under cages together with mosaic plants. Six soon crowded out 

 their diseased companions by shading them. All remained 

 healthy except for the uppermost leaves of one stalk of one plant. 

 These were mottled on August 27, had aphid skins and aphids 

 clinging to them, and were near a small hole punched accidentally 

 in the cloth. 



In one of the tests with the seed-cutting knife, as described 

 above, a mixing of mosaic stock in the rows with rogued stock 

 did not result in any increase of mosaic over that seen in the 

 same rogued stock unmixed. 



In 191 7 five Green Mountain hills were selected as being 

 healthy and were harvested. In 1918 the hill lots from these 

 five hills were found to be partly mosaic, often with several 

 healthy . hills between successive mosaic ones in the row. The 

 healthy hills were harvested separately and classified according 

 to the proximity to mosaic hills. Class 1 consisted of hills each 

 between two mosaic hills, class 2 of hills each between a mosaic 

 and a healthy hill, and classes 3, 4, 5, 6, and 7 respectively of 

 hills each with 1, 2, 3, 4 and 5 healthy hills between it and the 

 nearest diseased hill. The next season the tubers, 337 in all, 

 were planted uncut. For class 1 the mosaic percentage was 54 

 per cent, for class 2, 63 per cent, and for the other 5 together, 

 40 per cent. For classes 3 to 7 the percentage was respectively 

 56, 24, 54, 24, and 17 per cent. Being next to a mosaic plant in 

 the same row thus seemed to increase the chance of infection 

 as much as 54 or 63 per cent is greater than 40 per cent. It 

 probably is a contributing factor in mosaic transmission only or 

 chiefly by aiding aphid transmission. 



Similar evidence was secured by comparing two treatments 

 of Green Mountain lots. Removal of both the mosaic hills and 



