Sept. 15, 1919 Investigations on Mosaic Disease of the Irish Potato 257 



GREENHOUSE EXPERIMENTS WITHOUT INSECT CAGES 



Since greenhouse conditions are more favorable to the control of 

 aphids, experiments with the pink and green potato aphis were conducted 

 in the greenhouse at Washington, D. C, during the winter of 1917-18. 

 The insects were allowed to develop on stock of the Bliss Triumph 

 variety which during the preceding summer had been rogued in the field 

 in northern Maine — that is, had the plants showing mottling eliminated 

 from the stock. However, as Tables IV and V show, about 22 per cent 

 of the plants developed mottling on January 28, 191 8, when they were 

 from 2 to 6 inches tall. From these afifected plants the aphids were 

 permitted to disperse to the neighboring, apparently healthy plants; 

 and in addition on March 5 artificial transfers of aphids from diseased 

 to nonmottled plants were made on fully a dozen different plants. By 

 March 19, 191 8, it was noted that many of the plants infested with aphids 

 had developed a crinkling and mottling, on the newly formed leaves 

 only, very similar to mosaic mottling (Pi. 29, A). The number of such 

 mottled plants increased so that by April 6, 191 8, 50 per cent of the plants 

 showed mottling. On the other hand, only 15 per cent of the remainder 

 of this 1917-grown stock were diseased when grown at Presque Isle in 

 the season of 191 8. This 15 per cent, as well as the 22 per cent which 

 first showed mosaic in the greenhouse experiment described above, 

 undoubtedly were progeny of hills that had become diseased in 191 7 

 in spite of the roguing. The increase to 50 per cent seems to be explained 

 best by the dispersal of the aphids from the diseased plants. Moreover, 

 the percentage of plants to which the aphids transmitted the disease in 

 this experiment was really 100, inasmuch as all plants, whether or not 

 eventually becoming mottled in 191 7-1 8, produced progeny which was 

 decidedly mottled in the winter of 191 8-1 9. (See Table IV, "Perform- 

 ance of second generation.-') That is, all the tubers which were saved 

 from nonmottled plants, as well as all tubers from the mottled plants, 

 produced mosaic vines in the following winter when planted in the same 

 greenhouse with no aphids present. 



