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- Author or Editor: Todd C. Wehner x
Genetic diversity and relatedness were assessed among 46 American cultivars of watermelon (Citrullus lanatus var. lanatus), and 12 U.S. Plant Introduction accessions (PIs) of Citrullus sp. using 25 randomly amplified polymorphic DNA (RAPD) primers. These primers produced 288 distinct reproducible bands that could be scored with high confidence among cultivars and PIs. Based on the RAPD data, genetic similarity coefficients were calculated and a dendrogram was constructed using the unweighted pair-group method with arithmetic average (UPGMA). The cultivars and C. lanatus var. lanatus PIs differentiated at the level of 92% to 99.6% and 88% to 95% genetic similarity, respectively. In contrast, the C. lanatus var. citroides, and C. colocynthis PIs were more divergent and differentiated at the level of 65% to 82.5% and 70.5% genetic similarity, respectively. The low genetic diversity among watermelon cultivars in this study emphasizes the need to expand the genetic base of cultivated watermelon.
Gummy stem blight (GSB), caused by three related species of Stagonosporopsis [Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), Stagonosporopsis citrulli, and Stagonosporopsis caricae], is a major disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] in most production areas of the United States. We studied the inheritance of resistance to GSB using three PI accessions of watermelon. Four families of six progenies (Pr, Ps, F1, F2, BC1Pr, and BC1Ps) were developed from four crosses of resistant PI accessions by susceptible cultivars. Each family was tested in 2002 and 2003 in North Carolina under field and greenhouse conditions for resistance to GSB. Artificial inoculation was used to induce uniform and strong epidemics. The effect of the Mendelian gene for resistance, db, was tested. Partial failure of the data to fit the single-gene inheritance suggested that resistance to GSB of PI 482283 and PI 526233 may be under the control of a more complex genetic system.
Cucumber (Cucumis sativus L.) and horned cucumber (C. metuliferus Naud.) germplasm were evaluated for their resistance to root-knot nematodes (Meloidogyne spp.). All 24 C. metuliferus cultigens evaluated were resistant to all root-knot nematodes tested-M. incognita (Kofoid and White) Chitwood race 3, M. arenaria (Neal) Chitwood race 2, and M. hapla Chitwood. All 884 C. sativus cultigens (cultivars, breeding lines, and plant introduction accessions) tested were resistant to M. hapla and few to M. incognita race 3. Only 50 of 884 C. sativus cultigens evaluated were somewhat resistant to M. arenaria race 2 and M. incognita race 3. A retest of the most resistant C. sativus cultigens revealed that LJ 90430 [an accession of C. sativus var. hardwickii (R.) Alef.] and `Mincu' were the only cultigens that were moderately resistant to M. arenaria race 2. LJ 90430 was the only cultigen, besides the two retested C. metuliferus cultigens, that was resistant to M. javanica (Treub) Chitwood. All C. sativus cultigens retested, including LJ 90430, were highly susceptible to M. incognita races 1 and 3. The two C. metuliferus cultigens retested were highly resistant to all root-knot nematodes tested-M. arenaria race 2, M. incognita races 1 and 3, and M. javanica.
Cucumber (Cucumis sativus L.) is one of the most popular vegetable crops grown in U.S. home and urban gardens. The objectives of this study were to identify cultivars and planting densities for high yield of container-grown cucumbers. Additional objectives were to determine the value of field trials for predicting cucumber performance in containers and to evaluate different plant types (dwarf-determinate vs. tall-indeterminate, gynoecious vs. monoecious, pickling vs. slicing) for container use and disease severity across cultivars. Fourteen cultivars and breeding lines were tested at three planting densities in two seasons for yield, quality, and disease resistance in field and patio trials. Significant differences were detected for seasons, cultivars, and densities. Yields were highest in the spring season compared with the summer season, and the best performance was obtained using three plants per 12 L container. There was a high correlation between patio and field trials, allowing extension specialists to recommend cucumber cultivars with high yield, high quality, and disease resistance based on field trial data. Home gardeners who want space-saving, high-yielding cucumbers with tender skin should consider a dwarf-determinate, pickling type that is monoecious. With monoecious type, no pollenizer is needed, and the harvest will be spread over more weeks than would be for gynoecious types.
The rate of natural outcrossing in cucumber (Cucumis sativus L.) was measured at 3 locations in North Carolina, using the dominant allele for scab resistance as the marker gene. Outcrossing was measured within and between 1.5 m plots in isolation blocks. Between-row outcrossing averaged 36% over the 3 locations, and within-row outcrossing averaged 17%. Self-pollination accounted for 47% of the pollinations. Results suggest that populations should be intercrossed either as single plants, or in family rows treated with growth regulators to control pollen flow and prevent sib- and self-pollination.
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.
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.
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.
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).