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ADDITIONAL INDEX WORDS. Cucurbitaceae, Didymella bryoniae, disease resistance, vegetable breeding ABSTRACT. Heritability of resistance to gummy stem blight ( Didymella bryoniae (Auersw.) Rehm.) was measured in two diverse cucumber (Cucumis sativus L.) populations (North Carolina elite slicer 1 (NCES1) and North Carolina wide base pickle (NCWBP)). Heritability was estimated using parent-offspring regression and half-sib family analysis in North Carolina field tests during 1991 and 1992. NCES1 is a slicing cucumber population with a narrow genetic base, and NCWBP is a pickling cucumber population with a wide genetic base. Heritability estimates were low to moderate ranging from 0.12 to 0.49 for the gummy stem blight leaf rating and from -0.03 to 0.12 for stem rating. Estimates of gain from selection were at least two times larger for selection based on half-sib families than for mass selection for all traits in bot h populations. Approximately three to five cycles of selection would be required to improve the NCES1 population mean for gummy stem blight leaf resistance by one rating scale unit, and three to four cycles of selection would be required to improve the NCWBP population mean for gummy stem blight leaf resistance by one rating scale unit, based on half-sib family selection. One rating scale unit decrease is equivalent to an 11% reduction in susceptibility. Gain would be slower if selecting for stem, or leaf and stem resistance. A moderate amount of additive genetic variation exists in both populations for gummy stem blight leaf resistance, but estimates for additive genetic variation for stem resistance indicate little to no additive genetic variation. Development of populations specifically for greater initial resistance and greater additive variance than found in these populations should aid in selection for resistance. one cycle of mass selection within the open pollinated slicing cucumber 'Homegreen #2' was 0.14 to 0.35. However, we know of no other estimates of heritability or genetic variance of resis- tance to gummy stem blight in cucumber populations. The objective of this research was to estimate the heritability of leaf and stem resistance to gummy stem blight in two diverse cucumber populations. Other objectives were to estimate additive and environmental variation, to determine the suitability of the examined populations for improvement of resistance to gummy stem blight, and to predict genetic gain from selection in these populations.

Heritability and Genetic Variance Estimates for Leaf and Stem Resistance to Gummy Stem Blight in Two Cucumber Populations