juiyi, I920 Transmission of Mosaic Disease of Irish Potatoes 327 



were introduced on terminal-shoot buds in a flask laid upon the soil. 

 The aphids were killed by nicotine fumigation on April 21. All plants 

 appeared healthy when observed by one of the writers on April 2 1 , when 

 from 10 to 25 inches high. Between April 21 and June 2 ^ mosaic 

 symptoms appeared on all of the 15 plants to which the aphids were 

 introduced on sticks. Of the 5 plants to which the aphids were intro- 

 duced in the flask only i became mottled, on July 8. When intro- 

 duced in the flask many aphids had been injured or killed by water 

 condensing on the interior of the flask following transpiration by the 

 bud. Nineteen of the 20 untreated plants remained healthy; i showed 

 slight symptoms on July 9. This plant was the only one found on or 

 before April 28 with uncontrolled aphids upon it — possibly from a 

 mosaic plant or a plant treated with virulent aphids. It was again 

 found to be infested on May 19 and 26. In the case of the 15 plants 

 treated with the stick method of introducing aphids, the percentage 

 showing infection and the average length of the period between treat- 

 ment and the appearance of the symptoms were greater than in 

 the case of plants treated similarly in a previous greenhouse experiment,^ 

 probably because in the later trial the plants elongated to heights of 

 from 44 to 72 inches and thus offered for a longer period a chance for 

 the initial display of mottling in the young leaves. 



FIELD EXPERIMENTS WITH CAGES 

 EFFECT OF THE USE OF CAGES IN 1918 



Although the cages for the control of insects in 1918 did not inhibit 

 completely the dispersal of aphids, nevertheless their use materially 

 checked transmission of mosaic. The effect of these cages upon 

 transmission of mosaic is indicated in Table VI. 



Table VI. — Effect of cages on transmission of mosaic 



Variety. 



Green Motmtain 



Do 



Do 



Bliss Triumph. . 

 Do 



Treatment in 1918. 



Uncaged 



Caged with mosaic hill 



Caged 



Uncaged ". . . . 



Caged 



Percent- 

 age of 

 mosaic 



49 



100 



o 



35 

 o 



The number of hills reported in Table VI includes only a small per- 

 centage, a representative lot, of the total number planted in 1918. How- 

 ever, each hill indicated was grown under a separate cage. While these 



• Observations after May i were made weekly by Viola L. Morris, laboratory assistant, and finally by 

 Dr. W. J. Morse, neither having any information regarding the previous treatment of any plant. 

 'SCHtn[,Tz, E. S., FoLsoM, Donald, Hildebrandt, F. M., and Hawkins, Lon A. op. err. 



177285°— 20 4 



