Generation Means Analysis of Leaf and Stem Resistance to Gummy Stem Blight in Cucumber

Leaf and stem resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm.] in five resistant by susceptible crosses of cucumber (Cucumis sativus L.) was investigated using generation means analysis. No single gene of major effect controls either leaf or stem resistance to gummy stem blight in these five crosses. The mean number of effective factors controlling leaf resistance in the cross ‘Slice’ x ‘Wis. SMR 18’ was estimated to be at least five. Estimates of broadand narrow-sense heritabilities indicated that environmental effects were larger than genetic effects. In general, additive variance was the larger component of genetic variance. Epistasis was significant in most crosses, and dominance was present in several crosses. Additive gene effects contributed more to resistance than to susceptibility in contrast with dominance gene effects. Reciprocal differences for leaf rating were detected in the crosses M 17 x ‘Wis. SMR 18’ and ‘Slice’ x ‘Wis. SMR 18’. Phenotypic correlations between leaf and stem ratings were moderate (r = 0.52 to 0.72). Estimates of genetic gain for resistance to gummy stem blight ranged from low to moderate. Breeding methods that make best use of additive variance should be used because much of the variance for resistance is additive, and dominance effects, at least in these crosses, tended to contribute to susceptibility. of effective factors controlling resistance, and investigate the effects of cytoplasmic inheritance on resistance. Materials and Methods PLANT MATERIAL. Parents were selected based on their diversity and previously reported levels of resistance. Resistant cucumber parents used were: PI 200818, ‘Homegreen #2’, NCSU M 17, and ‘Slice’. Susceptible parents used were: ‘Wisconsin SMR 18’ and ‘Marketmore 76’. NCSU M 17, ‘Slice’, ‘Wis. SMR 18’, and ‘Marketmore 76’ are highly inbred lines. Seeds of PI 200818 and the open-pollinated ‘Homegreen #2’ used in this study were self-pollinated and selected for resistance for five generations before use. M 17 and ‘Wis. SMR 18’ are pickling types; ‘Homegreen #2’, ‘Slice’ and ‘Marketmore 76’ are slicing types; and PI 200818 is an accession from Burma. CROSSES AND GENERATIONS. Crosses were made between resistant and susceptible parents as follows: PI 200818 x ‘Wis. SMR 18’, ‘Homegreen #2’ x ‘Wis. SMR 18’, ‘Homegreen #2’ x ‘Marketmore 76’, M 17 x ‘Wis. SMR 18’, and ‘Slice’ x ‘Wis. SMR 18’. Generation means analysis was performed using each resistant (P1) and susceptible parent (P2), F1 and F2 generations including reciprocals (F1' and F2'), and backcrosses of the F1 to each parent (B1 and B2). All crosses were controlled pollinations in a greenhouse. INOCULUM PREPARATION AND APPLICATION. One virulent, single spore isolate of D. bryoniae (DB-H-23) collected from a diseased cucumber in North Carolina, was increased in petri plates containing 10 mL of cucumber malt extract agar (CMEA), using mycelial plug inoculation. The CMEA was prepared by tindalizing a purée of whole freshly harvested cucumber fruits and using the purée in place of one third of the water needed for standard malt extract agar (Difco, Detroit, Michigan, USA). Inoculated plates were then incubated for 10 to 14 d at 24 °C ± 2 °C, with a 12 h photoperiod. The florescent lamps provided ≈40 to 90 μmol·m·s PPFD at the upper surface of the plates. These culture conditions promoted formation of spore-producing pycnidia. Inoculum was prepared by flooding plates with 5 to 10 mL of acidified sterile distilled water and scraping the surface of the agar with a rubber spatula. The solution was Received for publication 18 May 1998. Accepted for publication 22 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. Gummy stem blight causes severe defoliation and stem necrosis in the late stages of cucumber production. It is the second most important cucumber (Cucumis sativus) disease in North Carolina (St. Amand and Wehner, 1991), and is a serious disease of greenhouse cucumbers in The Netherlands, where it causes fruit rot (Van Steekelenburg, 1982). Gummy stem blight of cucumber is caused by Didymella bryoniae (Auersw.) Rehm (synonyms: Mycosphaerella citrullina (C. O. Sm.) Gross., and Mycosphaerella melonis (Pass.) Chiu and Walker) and its anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc. (Farr et al., 1989) (synonyms: Ascochyta cucumis Fautr. and Roum., and Phyllosticta cucurbitacearum Sacc.). Several cultigens (breeding lines, cultivars, and plant introductions) resistant to gummy stem blight have been reported. Using field screening methods in Wisconsin, ‘Homegreen #2’ and PI 200818 were reported to be resistant (Wyszogrodzka et al., 1986). In The Netherlands, greenhouse screening methods were used to identify several plant introduction accessions as resistant, including PI 200818 (Van Der Meer et al., 1978). In North Carolina, PI 164433, ‘Slice’, PI 390264, M 17 and M 12 were reported resistant using field screening methods (Wehner and St. Amand, 1993). Wyszogrodzka et al. (1986) reported that the realized heritability for foliar resistance in one cycle of mass selection within ‘Homegreen #2’ was 0.14 to 0.35. However, a comprehensive assessment of the inheritance of resistance in cucumber has not been reported. Knowledge of the genetic basis and heritability of resistance to Didymella bryoniae is essential for efficient development of resistant cultivars. Thus, a study was designed to determine the types of gene action controlling foliar and stem resistance, estimate the genetic and environmental components of variance, estimate heritability and gain from selection, estimate the minimum number