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

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 both 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 resistance 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. Materials and Methods POPULATIONS TESTED. Two cucumber populations were used in this study: the North Carolina elite slicer 1 (NCES1) and the North Carolina wide base pickle (NCWBP). The NCES1 population was formed by intercrossing eight elite slicing type cultigens (Wehner, 1998). Because of the small number of parents used, that population was expected to contain less genetic variability than NCWBP, which was formed by intercrossing 1063 cultigens and selecting for pickle types (Wehner, 1997). Both populations were improved through recurrent selection cycles (seven for NCES1 and eight for NCWBP) for fruit yield (total, marketable, and early) and fruit quality (type, color, and shape) prior to this study. The parent populations used in this study were each formed by sampling five seeds from approximately 400 halfsib families and bulking the seeds. INOCULUM PREPARATION AND APPLICATION. Conidia were produced as reported previously (St. Amand and Wehner, 1995b, 2000). Only one locally collected, single spore isolate of D. bryoniae (DB-H-23) was used for inoculation because cultigen by isolate interactions are not known (St. Amand and Wehner, 1995a). Plants were sprayed with conidia nearly to runoff at the vine Gummy stem blight of cucumber (Cucumis sativus) is caused by the fungus Didymella bryoniae (Auersw.) Rehm., which causes severe defoliation and stem necrosis in the late stages of cucumber production. The incidence and severity of damage due to gummy stem blight on cucumber is second only to root knot nematode (Meloidogyne incognita, Kofoid and White) in North Carolina (St. Amand and Wehner, 1991), an important cucumber producing state. Control of gummy stem blight on watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] currently requires three to six applications of fungicide during the first half of the growing season or continuing all season (Keinath, 1995). Crop rotation is also useful for control on watermelon (Keinath, 1996). Similar practices are necessary for optimum control on cucumber. Although fungicides are available, genetic resistance is usually more effective, less costly, less subject to environmental conditions, and easier for growers to implement. Resistant cultigens (cultivars, plant introductions, and breeding lines) were used by Wehner and St. Amand (1993) to show that resistance was quantitatively inherited. Additional research showed that resistance was due to more than five genes (St. Amand and Wehner, 2000). Genetic improvement of plants for quantitative traits requires reliable estimates of heritability in order to plan an efficient breeding program (Dudley and Moll, 1969). Wyszogrodzka et al. (1986) reported the realized heritability for foliar resistance in Received for publication 26 May 1998. Accepted for publication 23 Aug. 2000. Research was supported in part by a grant from the North Carolina Pickle Producers Assoc., Mount Olive, NC 28365. We gratefully acknowledge technical assistance of R. R. Horton, Jr. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Former graduate research assistant. Currently assistant professor, Department of Agronomy, Kansas State University, Manhattan KS 66506. Professor; to whom reprint requests should be addressed.