326 Journal of Agricultural Research voi.xix, no. 7 



In no case did any healthy units become infected even though they 

 happened to be treated immediately after a diseased plant had been 

 operated upon. This indicates that infection does not carry very readily 

 from one plant to another by merely rubbing the leaves of one plant and 

 subsequently practicing the same operation upon a neighboring plant. 



Late Application 



On July 12, 1 91 9, six healthy Green Mountain hills representing three 

 dififerent tuber units were inoculated with juice from mosaic Irish Cob- 

 bler vines. A second application was made upon these same plants a 

 week later, when the vines were in blossom. 



On August 15 distinct mottling was in evidence on the upper leaves 

 of the vines in each of the six treated hills, and by August 22 some of 

 the leaves were dying in spots and streaks as in the bad stage of mosaic. 



Inoculations similar to the foregoing were made July 20 upon the 

 vines of four hills in as many separate tuber units of the Irish Cobbler 

 variety with juices from mosaic-dwarf Green Mountain vines. The 

 plants at the time of the first inoculation had just finished blossoming. 

 By August 20 slight mottling was noted upon the upper leaves of the 

 inoculated vines and also slight streaking of the leaves as in bad mosaic 

 stages. The results in these experiments indicate that plants can be 

 inoculated successfully at the time of blossoming and later, as well as 

 earlier in their development. Also, as stated previously in connection 

 with insect transmission, even though mottling may not be in evidence 

 in the season when infection occurs, nevertheless such plants will not 

 fail to show distinct mottling under favorable environmental conditions 

 during the following season. 



INSECT TRANSMISSION 



greenhouse; experiment with aphids 



Green Mountain tubers furnished by C. I, Gilbert were used at Orono 

 with aphids in a greenhouse experiment because they were expected to 

 be disease-free. This stock was used later in two plots. One consisted 

 of 70 tuber units, of which only i was diseased early, evidently as the 

 result of infection in 191 8. The other, grown and observed in southern 

 Maine by Dr. W. J. Morse, consisted of 1,357 hiUs, of which less than 

 three-fourths of i per cent were mosaic. In the greenhouse experiment 

 10 tubers were each cut lengthwise with a flamed knife into four sets 

 and planted on March 17. Half the plants from each tuber were inclosed 

 with insect cages, into each of which about 150 individuals of the common 

 green peach aphis, or spinach aphis {Myzus persicae Sulz.) from mosaic 

 potato plants were introduced on April 13 to 16, when the plants were 

 from 2 to 9 inches high. To 15 plants aphids were introduced on leaves 

 on a stick thrust into the soil so that they dispersed without contact 

 between the diseased leaves and the treated plant. To 5 plants they 



