An allotriploid derived from a amphidiploid × diploid mating in cucumis

Cucurbita pepo (pumpkin, squash, gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). `True Frenchʹ, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straightneck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita, and resistant plants were selected in the F2 generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to `True Frenchʹ. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlledenvironment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0.