Potato Mosaic. 171 



130 in number when transferred and were allowed to remain for 

 a week before being killed by tobacco fumigation. The mottling 

 appeared after an average interval of 26 days, upon only the top- 

 most leaves of any shoot. Thirteen or 72 per cent of the 18 

 plants became mosaic, due to transmission by the aphids. Many 

 precautions were taken to eliminate the possibility of accidental 

 infection, the soil being steam-sterilized, the seed-cutting knife 

 flamed, contact avoided, white flies (Aleyrodes vaporariorum 

 Westw.) and any species of aphids other than the one in use 

 entirely eliminated, and conditions kept similar for controls and 

 inoculated plants regarding light, temperature, humidity, loca- 

 tion of seed piece in the tuber, soil fertilization and watering. 

 Moreover, with controls other than the untreated ones, aphid- 

 free mosaic leaves were laid upon the plants when young, or 

 aphids from healthy potato foliage and from radish plants were 

 introduced, all without any effect respecting mosaic. 



Since potato mosaic may be acquired by one generation 

 without the symptoms being shown until the tubers produce the 

 second generation of plants, the preceding groups of plants were 

 dug and the tubers planted in the field in 1919. The tubers from 

 mosaic shoots produced diseased plants. Even those from ap- 

 parently healthy shoots fed upon by aphids from diseased plants 

 also produced mosaic plants. Those from the 35 other healthy 

 plants produced healthy plants except the ones from two plants. 

 Upon these two a few aphids were found which were of un- 

 known origin, possibly from diseased plants. 



In a third experiment, 9 Green Mountain tubers produced 

 healthy plants, six seed pieces being cut from each tuber. The 

 six plants from each tuber were put into as many separate 

 groups having different treatments with results as follows. Two 

 groups of untreated plants, one caged and the other uncaged, 

 remained healthy. In the third group, caged, wingless spinach 

 aphids were introduced on leaves which were impaled upon a 

 stick thrust into the soil so that the aphids, about 170 in num- 

 ber, had to traverse the stick and soil to reach each caged plant, 

 and 89 per cent became mosaic. In the fourth group, caged, 

 the aphids, about 130 in number, were introduced to each plant 

 on a piece of gauze, and 22 per cent became mosaic. In the 

 fifth group, caged, 20 winged aphids were introduced into each 

 cage in a small bottle and 11 per cent became mosaic. In the 



