Verticillium Wilt Resistance Varies within Ploidy Levels in Strawberry (Fragaria spp.)

Verticillium wilt is a major obstacle to U.S. strawberry production. Our current and ongoing investigations include defining new sources of resistance in cultivated and wild strawberry germplasm, and advancing genetic studies on the basis of resistance/susceptibility. We screened 26 octoploid, 1 decaploid, and 23 diploid strawberry genotypes for response to root-dip inoculation with Verticillium dahliae isolate V1.  Inoculated plants were individually rated at eight weeks post-inoculation using a categorical scale: 1=healthy; 1.5=slightly symptomatic; 2=moderately symptomatic; 2.5=very symptomatic; 3=dead. Qualitative classifications were assigned to genotypes on the basis of their respective mean disease resistance ratings.  The rating ranges and corresponding classifications (in parentheses) were: 1.0-1.3 (very resistant = VR), 1.4-1.7 (moderately resistant = MR), 1.8-2.2 (intermediate = I), 2.3-2.6 (moderately susceptible = MS), and 2.7-3.0 (very susceptible = VS). Considerable variability in inoculation response existed within and between species at both the diploid and octoploid levels. VR or MR genotypes were found within each of the following species: diploids F. vesca, F. iinumae, and F. nipponica); and octoploids F. chiloensis, F. virginiana, and F. ×ananassa. MS and VS genotypes were documented within F. vesca, within each octoploid species, and in a genotype of decaploid F. cascadensis. We compared our screening results to those of previous studies and constructed a pedigree of the evaluated and related octoploid cultigens for visualization.  We also made resistant × susceptible crosses at both the diploid and octoploid levels as a step toward genetic analysis of wilt resistance/susceptibility and resistance gene identification in strawberry.