Potato Mosaic. 165 



thereafter from year to year have a larger percentage of hills 

 affected, thus approaching the all-mosaic condition with which 

 the most of the yield comparisons are concerned. 



Transmission by the Tubers. 



Three questions are involved with tuber transmission — 

 whether diseased stock recovers to any extent, whether such 

 stock becomes worse from season to season, and whether tubers, 

 from apparently healthy plants may transmit mosaic through the 

 dormant season. 



It appears that when a plant once becomes diseased, there is 

 no recovery by it or by any of its progeny. In 191 7 over a hun- 

 dred mosaic hills were selected. Progeny of theirs, several hun- 

 dred hills, were grown in 19 18 and showed mosaic in all cases^ 

 All healthy plants were removed from two one-fifth-acre plots 

 grown in 1917, and in 1918 these two lots, grown again on fifth- 

 acre plots, contained no healthy plants. A number of small lots 

 all-mosaic in 19 17 were again entirely diseased in 1918. 



These results were duplicated with stocks grown in 191S 

 and tested in 1919. Over 150 hills were selected, furnishing 1100 

 hills of progeny, all mosaic. Stocks used previously in the two 

 larger plots and in smaller lots were continued in use but re- 

 mained diseased. 



It is commonly considered that mosaic increases in severity 

 from year to year and certain field observations seem to confirm 

 this idea. In these cases, however, there was a tendency for the 

 disease to change very little in severity as a result of transmis- 

 sion through the tubers from one season to the next. For ex- 

 ample, stock selected as showing a medium stage of mosaic 

 might show a slight stage in some hills and a bad stage in others 

 the next season, but the average would be medium. The same 

 was true of stock selected as showing a slight stage or a bad. 

 stage. 



In this connection it should be pointed out that treatment 

 of seed tubers with formaldehyde, corrosive sublimate, and heat 

 have not appreciably affected the percentage of mosaic 9 . This 

 is not surprising in view of the evidence, presented in this bul- 



9 Orton, W. A. Op. cit. 



