26o 



Journal of Agricultural Research voi.xvn,No. 6 



Table V. — Swmmary of Table IV 



Total number of plants grown from above progeny in second generation is 44. 

 Number of second generation plants showing mottling is 44. 



Number of plants without mottling in first generation but mottled in second is 21. 

 Percentage of plants mottled in second generation but not in first is 48. 



Similar experiments were performed at Washington in the winter of 

 1 91 8-1 9. Bliss Triumph potatoes, from stock that had been rogued 

 during the preceding season in northern Maine, were planted in two 

 lots. One lot was kept free from aphids by fumigation while the other 

 was subject to a heavy infestation. In the former, ii per cent — the 

 progeny of 2 out of 18 halved tubers — became mottled as soon as the 

 first leaves appeared, evidently as a result of field infection. In the 

 latter, 67 per cent, or 31 out of 46 plants — progeny of 23 halved tubers — 

 developed mottling. The difference between 11 per cent and 67 per 

 cent evidently was the result of aphid dispersal from neighboring mosaic 

 plants of the same variety. The aphid-free lot was planted December 

 17, 1918, and was fully matured by March 22, 1919. The infested lot 

 was planted in 8-inch pots on February i, 191 9, in a separate greenhouse 

 but with growing conditions practically the same as those of the other. 

 Hundreds of aphids were present upon the plants by the time they had 

 developed to a height of 6 to 8 inches. The plants were arranged in 

 five rows, the plants in row i being in contact with the originally aphid- 

 infested plants and the other rows following in numerical order at re- 

 spectively greater distances from them and therefore being less infested 

 by the dispersing aphids. As shown in Table VI, all the plants in rows 

 I and 2 showed mottling by April 4, while at that time some but not all 

 of the plants in the other three rows were mottled.- 



