Sept. IS. 1919 Investigations on Mosaic Disease of the Irish Potato 261 



Table; VI. — Relation of aphids to mosaic of potato: Continuation of experiments at 

 greenhouse, Washington, D. C, iQl8-ig 



Total number of plants is 46. 



Number of plants showing mottling Apr. 4, 1919, is 31. 



Percentage of plants showing mottling Apr. 4, 1919, is 67. 



Somewhat similar evidence was secured during the same winter at 

 Orono, Me. Some Green Mountain potatoes were used that had been 

 grown in a rogued plot in northern Maine during the season of 191 7 and 

 had been kept for about a year in cold storage. One lot of 10 tubers was 

 planted immediately and 2 of them, or 20 per cent, produced plants that 

 were mottled when very young, evidently through field infection. An- 

 other lot of 30 tubers was stored in a cellar for a few weeks and then was 

 found to have produced sprouts that had become lightly infested with 

 green peach or spinach aphids. These aphids apparently had dispersed 

 from a neighboring heavily infested lot of sprouted tubers that had 

 come from a purely mosaic stock and that later produced mosaic plants. 

 The number of insects on a tuber varied from o to 30, and there were 

 few skins and but little honey-dew deposit present. The infested lot 

 was fumigated and planted. Five tubers, or 17 per cent, produced 

 plants that became mottled when very small, in 25 to 30 days after 

 planting, evidently the result of field infection. In addition to these, 

 6 other tubers, or 20 per cent, produced both mottled and healthy shoots. 

 This increase can be explained only by the infestation of the sprouts by 

 the aphids from the diseased tubers. This explanation receives support 

 from the observation that the mottled shoots of the 6 partly diseased 



