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Susceptibility to Potato Leafroll Virus in Potato: Effects of Cultivar, Plant Age at Inoculation, and Inoculation Pressure on Tuber Infection. C. D. DiFONZO, Department of Entomology, University of Minnesota, St. Paul 55108. D. W. RAGSDALE, and E. B. RADCLIFFE, Department of Entomology, and E. E. BANTTARI, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 78:1173-1177. Accepted for publication 19 September 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1173.

Three potato (Solatium tuberosum) cultivars differing in susceptibility to potato Leafroll virus (PLRV)—Russet Burbank (highly susceptible), Kennebec (moderately resistant), and Cascade (resistant)—showed increasing resistance to PLRV in the field as plants aged. As inoculation age (measured in days after plant emergence) increased, percent tuber infection decreased significantly (P < 0.05) in each cultivar. As inoculation pressure (measured by viruliferous aphid-days) increased, percent tuber infection per plant increased significantly for Russet Burbank and Cascade. The percentage of PLRV-infected tubers for any inoculation age and aphid-day treatment combination was always less for Kennebec and Cascade than for Russet Burbank. In each cultivar, there was no significant difference in percent PLRV infection among four size categories of tubers. A greenhouse bioassay classified potato cultivars into PLRV resistance categories corresponding to previously reported field resistance. However, Cascade was not as resistant in the greenhouse bioassay as it was in field trials.