Genetics and Nature of Resistance to Powdery Mildew in Crosses of Butternut with Calabaza Squash and ‘Seminole Pumpkin’

Abstract

Abstract The inheritance of resistance in butternut squash (Cucurbita moschata Poir) to powdery mildew (Erysiphe cichoracearum DC) was studied under greenhouse and field conditions using F1 F2, and BC1 generations derived from crosses of ‘La Primera’ (resistant) (calabaza type) with 2 butternut cultivars ‘Ponca’ (susceptible), and ‘Waltham’ (susceptible), and ‘Ponca’ x ‘Seminole Pumpkin’ (intermediate resistance). The breath-blowing method of inoculation was effective in greenhouse tests. Petiole and stem reactions of plants in both field and greenhouse trials and leafblade reactions of plants in the field tests indicated that 3 alleles at a single locus determined resistance reactions in crosses involving ‘Ponca’, ‘Waltham’, and ‘La Primera’. Proposed genotypes for the susceptible ‘Ponca’ and ‘Waltham’ butternut squash are pm – lp pm – lp and pm – lw pm – lw, respectively, and for the resistant ‘La Primera’, pm – lL pm – lL. The partial and complete dominance relations of these alleles for the leaf-blade (field) and petiole/stem reactions (field/greenhouse), respectively, are pm – lp > pm – lL > pm – lw. A different gene, pm – 2s, controlled the intermediate resistance of ‘Seminole Pumpkin’. However, a quantitative inheritance pattern of disease reactions of leaf blades was observed in the greenhouse trials in all crosses, presumably because of the higher inoculum load and the increased intermediate ratings of the heterozygotes under these conditions. Leaf-blade and stem/petiole resistance was completely associated in segregating progenies in the field but not in the greenhouse because of the intermediate susceptibility of the leaf blades of heterozygotes under those conditions. Scanning electron micrographs of powdery mildew on compatible and incompatible hosts showed that differential compatibility occurred in conidial germination. Resistance in ‘La Primera’ involved delayed conidia germination, retarded hyphal growth, shorter conidiophores, and weak sporulation.