330 Journal of Agricultural Research voi. xix, no. 7 



mosaic. None of the 38 tubers from caged untreated plants or of the 

 15 from plants fed upon by aphids from a healthy potato plant were 

 mosaic. Of the 34 tubers from uncaged and untreated plants i was 

 mosaic; it came from a half -tuber hill that early showed ruffling and 

 chlorosis along the veins but no typical mosaic mottling such as was 

 shown, in addition to these incomplete symptoms, by the correspond- 

 ing half-tuber hill after treatment with virulent aphids. Of the 2>7 

 tubers from plants fed upon by aphids from radish plants 4, or 11 per 

 cent, were mosaic; these 4 came from 2 plants recorded as having been 

 fumigated to eliminate a few aphids which were found on them and 

 which were of unknown origin, possibly from neighboring diseased plants. 



These results agree essentially with those which were secured previously 

 with the first generation of the same stocks and which were described to 

 prove the possibility of transmission by aphids. They also indicate that 

 (i) mosaic mottling may be restricted to the parts of the leaf along the 

 veins, (2) a plant with three stalks healthy and four mosaic may produce 

 three mosaic tubers and two healthy ones, thus explaining the partial 

 infection of hill lots, (3) plants treated with virulent aphids may appear 

 healthy but produce progeny that are all mosaic, as shown previously 

 by the writers,^ and (4) apparently healthy plants inspected often for 

 aphids and fumigated to eliminate these insects as soon as they are dis- 

 covered may produce progeny of which a small percentage is mosaic. 



In connection with the experiment just considered it was necessary 

 to treat a number of control plants by laying a mosaic leaf upon each. 

 These were kept in a dififerent greenhouse room where aphids were more 

 abundant, and they were never caged. Of 45 tubers from these, and 

 also of 25 tubers from similar plants with no leaf laid on, 20 per cent were 

 mosaic, all coming from plants recorded as being fumigated to eliminate 

 uncontrolled aphids found upon them. 



PROXIMITY STUDIES WITH PLOTS 



In 1 91 8, plots I, 2, and 3 were each rogued of mosaic hills three times. 

 Stocks from the first two, Green Mountain and Bliss Triumph, respec- 

 tively, each showed mosaic in 20 per cent of the hills in 191 9, while that 

 from No. 3, Green Mountain, next to No. 4, a Green Mountain plot with 

 45 per cent of the hills diseased, showed mosaic in 30 per cent of the 

 hills. In 1 91 9, each of the stocks was rogued several times and grew 

 between similar stocks. All these plots in both years were each 

 X acre in area. The greater percentage of mosaic in 191 9 in stock 

 from plot 3 can be explained best by the greater proximity in 191 8 to a 

 half-mosaic plot and by consideration of the apparently greater ease of 

 dispersal of aphids, which were numerous in 191 8, from the half -mosaic 

 plot to No. 3. Plots I, 2, and 3 were planted with stocks A, B, and C, 

 respectively, described in Table VII. 



1 ScHUiTZ, E. S., Foi,soM, Donald, Hildebrandt, F. M., and Hawkins, Lon A. op. ai. 



