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  • Author or Editor: Todd C. Wehner x
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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.

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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' × `Wis. SMR 18' was estimated to be at least five. Estimates of broad- and 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 × `Wis. SMR 18' and `Slice' × `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.

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Gummy stem blight [Didymella bryoniae (Auersw.) Rehm] is the second most important pathogen of field-grown cucumbers (Cucumis sativus L.) in North Carolina and a severe problem for greenhouse-grown cucumbers worldwide. To determine whether resistance exists under North Carolina field conditions, 83 cultigens [cultivars, breeding lines, and plant introduction (PI) accessions] were evaluated in the field for 4 years for their resistance to a mixture of D. bryoniae isolates. Plants were inoculated at the vine tip-over stage and rated for foliar lesion size and number. Cultigens identified as resistant in Wisconsin and The Netherlands were not resistant in North Carolina. When averaged over years and locations, the most resistant C. sativus cultigens were PI 164433, `Slice', PI 390264, M 17, and M 12. Several accessions of related Cucumis species were highly resistant: PI 299568 (C. myriocarpus Naud.), PI 282450 (C. zeyheri Sond.), PI 299572 (C. myriocarpus), and PI 233646 (C. anguria L.). The most susceptible cultivars were `Colet', `Meresto', `Supergreen', `Dura', `Pioneer', `Marketmore 76', `Pickmore', and `Addis'. `Calypso' and `Dasher II', popular cultivars in North Carolina, were moderately susceptible.

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Gummy stem blight (Didymella bryoniae (Auersw.) Rehm) is one of the major cucumber diseases, causing the second highest loss of any disease in North Carolina. Published methods of screening for resistance to this fungus are poorly correlated with field resistance. The objective of this study was to develop seedling or detached-leaf screening methods that are correlated with field resistance. Seedling tests examined the effects of: seedling age (1, 2 or 3 true leaves), days in humidity chamber, inoculum concentration (1×105, 1×106 or 1×107 spores per ml), time of inoculation (am vs. pm), fungal isolates, and cultigens. Detached leaf tests examined the effects of leaf age (1st, 2nd or 3rd true leaf), inoculum concentration (1×104, 1×105 or 1×106 spores per ml), and light levels during incubation (dark vs. 12h light/12h dark). Correlations between seedling tests and field data were moderate to high (r = 0.5 to 0.7). However, the coefficients of variation were also high. Correlations between detached leaf tests and field data were very low or negative.

Free access

Abstract

Growth patterns of cucumbers (Cucumis sativus L.) were studied by photographing developing fruit at 2- or 3-day intervals over a 30-day period beginning with pollination (day 0). Nine cultivars were studied: ‘Chinese Long Green’, ‘Sprint 440’, ‘Marketmore 76’, and ‘Minisol’ (all fresh-market types); ‘Riesenschal’ (a schälgurken type); and ‘Marbel’, ‘Kobus’, ‘Calypso’, and ‘Wisconsin SMR-18’ (all pickling types). Analysis of the photographs showed that all sections of the fruit grew in length at a constant rate during the 30-day period. The pattern of growth was fairly uniform, except that there was slightly more growth in the center section than at the ends, and slightly more growth at the blossom end than at the peduncle end of the fruit. Fresh-market and schälgurken types had the longest fruit over the 30-day period of growth, but pickling types had the highest percent change in length. Pickling types produced shorter fruit because they stopped growing earlier than the two other types (12 vs. 14 days, respectively).

Open Access

Abstract

Three rapid methods of measuring yield (small plots harvested at the green stage, and single plants harvested at the green or at the mature stage) were tested for correlation with yield from a replicated multiple-harvest yield trial to determine how well they predicted performance of 10 hybrids in 1981, and 18 lines and cultivars tested in 1982. Only 2 methods of measuring yield were evaluated in 1981, small plots harvested once-over at the green stage and the standard replicated multiple-harvest trial. In that year, the most efficient method for measuring yield, based on capacity of the method to handle lines and to predict multiple-harvest trial performance, was 2 or 3 replications of 3 m plots harvested once-over. Those methods had a calculated advantage (relative capacity × correlation coefficient) of 43% to 80% over the replicated multiple-harvest trial. The most efficient method for measuring yield, of the 4 tested in 1982, was single plots with 1, 2, or 3 replications harvested once-over at the green stage. The tests had a calculated advantage of 102 to 107% over the replicated multiple-harvest trial, respectively. It was concluded that 2 or 3 replications in the test would provide the best results by controlling environmental variability without using an excessive number of seeds per family. In addition, the best correlation with yield in the multiple-harvest test was obtained when all fruit from the plot were counted, rather than just those of marketable size (38 to 60 mm diameter).

Open Access

Abstract

Three versions of the hybrid ‘Meridian 76’ (‘Marketmore 76’ × ‘Tablegreen 72’) differing in sex expression were used to evaluate the effect of gynoecious expression on fruit yield and earliness. The versions tested were gynoecious × gynoecious (G×G), gynoecious × monoecious (G×M), and monoecious × monoecious (M×M). In 2 years of testing, the gynoecious (G×G and G×M) hybrids had 99% to 100% pistillate flowers on the first 10 nodes of the main stem of the plant, whereas the monoecious (M×M) hybrid had 3% to 5% pistillate flowers. The monoecious hybrid had a higher percentage of U.S. Fancy and No. 1 fruits and a lower percentage of culls than the gynoecious hybrids. There were few significant differences in yield among the 3 hybrids. The gynoecious hybrids were earlier than the monoecious hybrid, but there were no important differences in the yield of marketable fruit after the first harvest in either year. The G×M hybrid had a significantly higher yield than the M×M hybrid in the first harvest for both years. The G×M hybrid tended to outyield the G×G hybrid as well, but the differences were not always significant. The gynoecious hybrids (especially the G×M hybrid) of ‘Meridian 76’ provided an advantage in early yield but not in yield summed over all harvests (6 to 8 depending on year), compared to the monoecious hybrid.

Open Access

Abstract

The feasibility of testing the resistance of pickling cucumber (Cucumis sativus L.) cultivars to bloater damage was determined using a procedure of artificial carbonation at several concentrations of dissolved CO2 during brine storage of unfermented cucumbers. The use of grade 3-size fruit carbonated to saturation with CO2 during brine storage in 18-liter (5-gal) plastic pails appeared to be a simple, rapid (6 days), inexpensive, and reproducible method to test for cultivar differences in susceptibility to bloater damage. The method involves the use of a prescribed brine composition and sequential introduction of N2 and CO2 into the brined cucumbers. Thirty-five cucumber lines and cultivars were statistically separable using the method. Based on the data collected, it is recommended that 6 replications of 15 grade 3 fruit per cultivar or line be used, although good data can be obtained with as few as 2 replications of 5 fruit.

Open Access

Abstract

Six selection indices (Smith-Hazel, desired gain, simple-weighted, rank summation, Elston's weight-free, and Baker's standard deviation) were compared to determine the effectiveness of each in identifying superior families for improving 8 fruit yield and quality traits in 3 fresh-market cucumber populations differing in genetic diversity (elite, medium-base, and wide-base). The rank summation, Elston's weight-free, and Baker's standard deviation indices were constructed with 5 traits as well as with the full 8 traits to determine whether measurement of fewer traits would suffice. The Smith-Hazel and desired gain indices were constructed using 5 traits only, since the 8-trait indices had problems with trait colinearity. The effectiveness of the indices was measured by calculating selection differentials for each index. In the elite population, the Smith-Hazel index produced negative selection differentials for all 8 traits studied. In the medium-base and wide-base populations, the Smith-Hazel index had positive differentials, but the desired gain index had negative differentials for the 8 traits studied. The simple-weighted, rank-summation, Elston's weight-free, and Baker's standard deviation indices all had positive selection differentials for the traits of interest in all 3 populations. The best index was the rank summation for 5 traits, since it had the highest overall selection differential of those measured and was easiest to calculate.

Open Access

Abstract

Variance components for 3 fruit yield and 5 fruit quality traits in 3 cucumber (Cucumis sativus L.) populations were estimated using a North Carolina Design I analysis. Three populations were studied (elite, medium base, and wide base), having been formed by intercrossing lines to produce 3 levels of genetic diversity. Families were evaluated in 1.5 × 1.5 m plots using once-over harvest at the stage of 15% oversized fruits. Heritabilities based on full-sib family selection for fruit yield ranged from 0.03 to 0.25, and for the fruit quality traits 0.00 to 0.30. The wide base population would be best for long-term selection for the traits studied, because it had the highest predicted means for all traits except percentage of culls, fruit shape, and overall performance after 15 cycles of full-sib family selection.

Open Access