Potato Mosaic. 173 



PLAXT LICE IN THE FIELD. 



The positive results of the greenhouse experiments were 

 confirmed in the field in 1919. Eight Green Mountain tubers 

 from a mosaic- free stock were each divided into three parts, a 

 tuber furnishing the plants for each cage. In four cages the 

 plants were fed upon by spinach aphids which were introduced 

 from radish plants and they all remained healthy. In the other 

 four cages the plants were fed upon by aphids introduced from 

 mosaic potato plants and mosaic symptoms appeared in three of 

 the cages before the plants were harvested. Nine similar tubers 

 were split, a half from each being planted in the open where it 

 produced a healthy plant. The other halves were planted in 

 three cages into which spinach aphids from mosaic potatoes 

 were introduced. Three of these nine plants became mosaic. 

 The smaller percentage of infected plants in these field-cage 

 experiments, as compared with those in some of the greenhouse 

 experiments, is probably due to the much greater size of the 

 plants in relation to each hundred aphids used, the 19 19 season 

 being very unfavorable to the development of aphids and fav- 

 orable to the rapid, early growth of potato plants. 



The various proofs of aphid transmission furnish the best 

 explanation for certain field observations that have been made. 

 In 1918 aphids were unusually numerous in northern Maine, 

 much more so than in 1917. If they are an important cause of 

 the spreading of mosaic, such spread should be greater in 1918, 

 other conditions being equal, than in 191 7. In fact it was great- 

 er, judging from certain stocks, grown in fifth-acre plots, that 

 had all mosaic plants rogued out both in 1917 and 1918. These 

 showed mosaic in 1918 in from 11 to 16 per cent of the hills as 

 the result of infection from near-by diseased plots in 1917, while 

 they showed mosaic in 1919 in from 20 to 30 per cent of the 

 hills as the result of such infection in 1918. This difference is 

 made more striking by the fact that these stocks were grown 

 each next to all-mosaic plots in 1917, but in 1918 were grown 

 each next mosaic-free or half-mosaic stock. In 1918 one rogued 

 stock was grown next a half-mosaic stock while two rogued 

 stocks were grown nine and eighteen rows away, respectively. 

 The former showed mosaic in 30 per cent of the hills in 1919 

 and each of the latter in only 20 per cent. It is thus seen that 



