Potato Mosaic. 169 



from the caged hills were found in 1919 to be healthy, while 

 nncaged stock of the same kind, grown near by, was mosaic in 

 1919 in 35 per cent of the hills for Bliss Triumphs and in 49 

 per cent for Green Mountains. 



These results, wherein the normal spread of mosaic in the 

 field was greatly reduced or eliminated by protecting healthy 

 plants with insect cages, merely support other evidence that in- 

 sects transmit the disease. Such evidence has required the trans- 

 fer of insects from mosaic to healthy plants, with check plants 

 both untreated and treated with insects from healthy plants. 



PLANT LICE IN THE GREENHOUSE. 



Since greenhouse conditions are more favorable to the con- 

 trol of insects, and since Allard 12 had demonstrated the trans- 

 mission of tobacco mosaic by aphids, experiments with one 

 species of these insects — the pink and green potato aphid, Mac- 

 rosiphum solanifolii Ashmead — were begun in the greenhouse 

 in the winter of 191 7-18. Bliss Triumph plants were grown, of 

 which about a fifth appeared mosaic when a few inches tall, as 

 the result of field infection during the preceding summer. Aphids 

 were permitted to disperse from these affected plants and also 

 artificial transfers were made. Within a few weeks half of the 

 plants were mosaic, the additional ones only in the youngest 

 leaves instead of in all the leaves as when following tuber trans- 

 mission. Moreover, all the progeny of these plants, even of 

 those apparently healthy, were mosaic when grown in the winter 

 of 1918-19. This and other cases of aphid transmission without 

 the appearance of mosaic until the second generation is grown, 

 affords the best explanation for the frequent appearance of 

 mosaic early in the growth of the progeny of plants selected as 

 healthy in a field containing aphids and mosaic plants. The same 

 stock grown in part in the greenhouse in 1917-18 was used for 

 field planting in 191 8 when less than a fifth of the hills were 

 mosaic. 



A similar experiment with the same variety was performed 

 in the greenhouse in 1918-19. Part of a lot of tubers was kept 

 free from aphids by fumigation and only 11 per cent of the 



12 Allard, H. A. Op. cit. 



