Sept. IS, 1919 Investigations on Mosaic Disease of the Irish Potato 265 



the 3 plants which had developed from the other half tubers and had 

 been kept free from aphids were free from mottling. On examination 

 of the control plants 4707 and 4787 as well as 470X and 478X on April 4, 

 no mottling whatever was found (PI. 30, B). 



In January, 191 9, some Green Mountain potatoes were secured at 

 Orono, Me., supposedly from a field that had been found free from mosaic 

 the previous season. Seventeen tubers were divided each into 6 seed 

 pieces. Eight tuber groups of 6 plants each developed mottling when 

 very small, and the other 9 did not. The 6 plants from each tuber 

 were subjected to 6 different treatments: One plant was kept as an 

 uncaged control ; another was a control , caged until the plant was over 2 

 feet tall; the third was grown intertwined with a mosaic potato plant 

 from a separate pot; the fourth was fed upon for a week by wingless 

 green peach aphids from a mosaic potato plant, an average number of 

 about 130 being introduced on a piece of gauze when the plant was 3 

 inches high; the fifth received the same treatment as the fourth except 

 that the average number of insects was about 170 and that they were 

 introduced on leaves which were impaled upon a sterile stick thrust 

 into the soil in such a way that there was no contact between the intro- 

 duced leaves and the plant or soil (see PI. 26, B) ; on the sixth plant when 

 I inch high there were placed 20 winged aphids secured from a mosaic 

 plant with a camel's-hair brush and introduced within a small open 

 bottle. 



All of the 18 controls remained healthy. Of the 9 plants with aphids 

 introduced on leaves, as described above, 8, or 89 per cent, became mot- 

 tled in 20 to 31 days — averaging 25 days — or when the plants had be- 

 come 14 to 29 inches high — averaging 25 inches. One of these plants, 

 together with an untreated plant from the same tuber, is shown in Plate 

 29, B, and corresponding leaves from these two plants are shown in 

 Plate 27, C. Of the 9 plants with aphids introduced on gauze, 2, or 22 

 per cent, became mottled in 20 to 26 days, when the plants were 22 to 29 

 inches high. Of the 9 plants with winged aphids introduced, i, or 11 

 per cent, showed signs of mosaic in 27 days, when 17 inches high. Of 

 the 9 plants kept in contact with mosaic plants, all remained healthy 

 but I. This I was brought into contact with the diseased plant on 

 March 7, was found to be free from aphids on March 31, was fumigated 

 on April 7 because of the presence of several aphids, and showed signs of 

 mosaic on April 17, when 35 inches high. This plant became diseased 

 apparently either because of transmission by very few aphids after 

 March 31 or because of contact. The latter cause seems more prob- 

 able, but would make this the only case of contact transmission known 

 at present to the writers. 



This experiment seems to have demonstrated that aphids can transmit 

 mosaic, even better than the first experiment conducted in this green- 

 house (p. 262-264). The same precautions were used in this experiment, 



