Oligogenic inheritance for resistance to Zucchini yellow mosaic virus in Cucurbita pepo

'True French' is an open-pollinated cultivar of the Zucchini (Courgette) Group of Cucurbita pepo and is susceptible to Zucchini yellow mosaic virus (ZYMV). Using C. moschata 'Menina' as the source of ZYMV resistance and following six generations of backcrossing, a true-breeding line nearly isogenic to 'True French', designated 381e, was recovered that carried ZYMV resistance, albeit not at as high a level as in 'Menina'. 'True French' and accession 381e were crossed, and their reciprocal F-1, F-2, and backcross progenies were grown in a chamber and inoculated with a highly virulent, non-aphid-transmissible strain of ZYMV. Nearly all F-1 plants and all plants of the backcross to 381e were classified as resistant. Segregation to resistant and susceptible individuals occurred in the backcross to the susceptible parent, in accordance with a 3:5 three-gene ratio of resistant: susceptible. The F2 Segregated in accordance with a ratio of 45 resistant : 19 susceptible, which would be obtained if there was one major gene for resistance, Zym-1 (Zym), and two other genes, herein designated Zym-2 and Zym-3, both of which are complementary to Zym-1. The presence of Zym-1 and either Zym-2 or Zym-3 is necessary for resistance to be expressed in young plants, but the presence of all three might be necessary for resistance to continue to be expressed during subsequent development of the plants. Evidently, Zym-2 and Zym-3 are ubiquitous in C. moschata but their susceptible alleles are much more common in C. pepo. As the level of resistance of 381e to ZYMV is not as high as that of C. moschata 'Menina', additional, as yet unidentified, genes must be involved in conferring high resistance to this virus.