Field Resistance to Potato Virus Y in Potato Assessed by Cluster Analysis. R. H. Bagnall, Research Scientist, Agriculture Canada Research Station, Fredericton, NB, Canada E3B 4Z7. G. C. C. Tai, Research Scientist, Agriculture Canada Research Station, Fredericton, NB, Canada E3B 4Z7. Plant Dis. 70:301-304. Accepted for publication 11 October 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-301.

Forty potato cultivars tested for their responses to potato virus Y (PVY) at Fredericton, NB (1972–1983), were separated into five resistance groups by the Scott-Knott cluster analysis procedure. Susceptible groups A (52.1–60.4% infection) and B (35.4–47.5%) differed significantly from the moderately to highly resistant groups C (12.9–14.6%), D (4.6–9.6%), and E (0.0–2.5%) in a supplementary Duncan’s multiple range test, which otherwise gave much overlapping. Mean disease readings for the different categories of cultivars retained the same relative order (A, B, C, D, E) during each year of the trial. By comparison with these five groups, 42 additional commercial cultivars and many seedlings present in the trials for 4–8 of the 12 yr could be classified. The “old” strain of PVY was used in these trials, whereas the official Dutch variety ratings are based on the “new” strain of this virus. Nevertheless, 11 cultivars common to the two lists are rated quite similarly. In the New Brunswick seed potato crop, incidence of mosaic (caused largely by PVY) increased as cultivars progressed during 5 yr from “Elite” status through Foundation seed to Certified seed. This increase was most pronounced in groups A and B, moderate in groups C and D, and virtually absent in the most resistant group, E. To reduce the reservoir of PVY in potato crops, susceptible cultivars, now widely grown, need to be replaced with PVY-resistant cultivars.