July 1, 1920 Transmission of Mosaic Disease of Irish Potatoes 333 



out and discarded and only the others were planted. The latter occupied 

 the third row, and the fourth row was used for a control lot prepared 

 similarly except that no all-mosaic stock was used. Upon examination 

 of the four rows on July 23 the control row was found to contain 85 mosaic 

 hills, the third row 72, and the first two 475 — that is, 75 excluding the 

 400 from all-mosaic stock. No change in the number of mosaic hills was 

 found on August 18. A X-acre plot of the rogued stock was planted 

 elsewhere and contained 80 mosaic hills in each 400. Evidently the fur- 

 nishing of conditions apparently optimum for knife transmission had no 

 effect upon the mosaic percentage. 



It was thought in 191 8 that the partial infection of tuber units might 

 be due to knife traosmission. As stated before (p. 318), in 191 9 when 

 tuber units were planted three knives were used in rotation, each one 

 being immersed in a 4 per cent formaldehyde solution when not in use. 

 However, the partial infection of tuber units and hill lots was as common 



as before. 



TESTS OF EFFECTS OF CONTACT 



GREENHOUSE EXPERIMENTS 



As has been reported,^ out of nine healthy plants kept in contact with 

 mosaic plants in a greenhouse one showed mosaic, but not until after a 

 few uncontrolled aphids, possibly from mosaic plants, were discovered 

 upon it. At about the same time, March 13, 191 9, each of 12 tubers was 

 split into three sets and planted in small pots. The plants from 4 

 tubers became mottled by April i when from 3 to 13 inches tall. The 

 other 24 were transplanted about April i into large pots, 2 from each 

 tuber into steam-sterilized soil and the third into soil containing a mosaic 

 plant. The transfer was made by knocking off the bottom of the small 

 pot and setting it into a hole formed by a small empty pot put in when 

 the mosaic set was planted. The method used permitted the mingling of 

 the roots of the two plants while it kept the two sets of tubers mostly 

 apart and facilitated harvesting them separately. The vines of the two 

 plants were twisted and tied together. All of the 24 plants remained 

 healthy until July 9. ^ They had ceased to elongate by this time and 

 soon afterwards were dug. The tubers were not planted, because of 

 the abundance of aphids on the plants in July. 



FIELD EXPERIMENTS WITH INSECT CAGES 



Nine tubers of the Gilbert stock were planted halved in 191 9, each 

 two sets being separated by a mosaic set and all three caged. On 

 July 30 three of the mosaic hills were dead or nearly so. The Gilbert 

 hills all remained healthy until August 9 and when dug on August 27 



' ScHULTz, E. S., FoisoM, Donald, Hildebr,\ndt, F. M., and Hawkins, Lon A. op. ot. 

 2 Observations after May i were made weekly by Viola L. Morris, Laboratory Assistant, and finally 

 by Dr. W. J. Morse. 



