3i6 



Journal of Agricultural Research 



Vol. XIX, No. 7 



shown by the diseased hills of any lot averaged either "slight plus," 

 "medium," or "medium plus," although it was usually "slight" for 

 many hills and often "bad" for some. "Slight" indicates characteristic 

 mottling sufficiently rare to require careful search; "slight plus" means 

 that mottling is' readily apparent but is unaccompanied by wrinkling, 

 "medium" represents both conspicuous mottling and some wrinkling, 

 becoming "medium plus " with marked ruffling and more or less dwarfing; 

 "bad" stands for extreme ruffling and dwarfing which may sometimes 

 cause the mottling to be obscured. 



Another similar series of small lots was grown in a second plot. In 

 these the percentage of mosaic hills varied from 4 to 63, being 40 per cent 

 for the 800 hills altogether. The severity of the symptoms shown by the 

 diseased hills was about the same as for the lots in the first plot. 



In addition to the healthy hill selections described above, stocks were 

 grown from hills that showed mosaic in 191 8. These contained 1,100 

 hills, of which only 5 had not yet shown mottling by July 30. These 5, 

 of which 4 came from one tuber, were not observed later. It is possible 

 that this healthy tuber, supposedly from an Irish Cobbler hill with bad 

 mosaic, was formed by a long rhizome of a neighboring healthy hill, such 

 as is seen occasionally, and was included with the tubers of the mosaic 

 hill in spite of the precautions usually taken. The severity of the 

 .symptoms in the mosaic stocks is indicated in Table I. 



Table I. — Comparison of mosaic stocks in igi8 and IQIQ 



Evidently there was, in the stocks described in Table I, a tendency for 

 the disease to change very little in severity as a result of transmission 

 through the tubers from 1918 to 191 9. 



Two larger plots, one Green Mountain and one Bliss Triumph, were 

 planted with stock from plots entirely mosaic in 1918. While the per- 

 centages of mottled plants on July 9 were, respectively, 67 and 89, all 

 plants were mottled by the last of July. Although in magnitude the 

 plants and yield were inferior to those of comparatively healthy lots, the 

 appearance of the plants and of the plot as a whole was no worse than for 

 the same stock during the three previous seasons. 



