Potato Mosaic. 179 



the latter, 28 per cent of the hills upon each site showed mosaic, 

 mostly early, from infection in 1918. 



Similar negative results were given by a more extensive 

 test wherein 19 rows of rogued stock were planted across the 

 sites of fourteen of the 1918 plots. Practically all the mosaic 

 which occurred was shown by July 30 and so was judged to be 

 due to infection occurring in 1918. Mosaic hills were dug and, 

 disregarding volunteers, formed 23 per cent of the total, 4,466. 

 A record was kept for each of the 14 parts. Among these there 

 were only four marked variations from the average. These con- 

 sisted of a deviation upward and one downward for the sites of 

 two half-mosaic plots and similar deviations for the sites of two 

 comparatively mosaic-free plots. 



It therefore appears that the cause of potato mosaic is not 

 transmitted in soil, except in discarded or ungathered tubers. 

 These, however, may constitute an important means for the har- 

 boring of mosaic from season to season in a piece of land, since 

 they produce volunteer plants, which, if mosaic, furnish a num- 

 ber of well-distributed sources of infection through aphid trans- 

 mission. 



LOCALITY. 



The symptoms of mosaic may vary according to the region. 

 The same stock of mosaic potatoes has been divided, part being 

 sent to Colorado and part kept in northern Maine. The usual 

 mottling of mosaic stocks was seen in Maine but did not appear 

 in Colorado. With part of a stock sent to Washington, D. C, 

 some mottling was noted but there were a number of doubtful 

 cases, while the part kept in Maine was distinctly mottled. 



When a number of lots were divided and planted at the two 

 farms of the Maine Agricultural Experiment Station, the part 

 of a lot grown at Highmoor Farm in the southwestern portion 

 of the state often showed much less mottling than the part grown 

 at Aroostook Farm in the northeastern portion. In two out of 

 three seasons the difference has been very marked and the re- 

 verse was never seen. 



Such a difference is not due to recovery in one place but 

 rather to an obscuring of the mottling, as indicated by the reap- 

 pearance of distinct mottling in affected stock returned to north- 



