Effects of potato virus S and mild potato virus X on early and late harvest yields of the Kennebec cultivar in Maine

Yields of the Kennebec cultivar as affected by mild potato virus X (PVX) and/or potato virus S (PVS) were compared with those of a virus-free (VF) stock in both early and late harvest tests for the years 1972, 1973 and 1974. Results showed that VF stocks gave significantly higher yield weights than those with PVS plus mild PVX in late-harvested plots but in early-harvested plots results were reversed, though not significantly so at P=.05. Thus, the significant interaction of virus content with harvest date indicated that length of growing season can influence the outcome of tests designed to measure the effects of PVS and PVX on yield.