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  • Author or Editor: Todd C. Wehner x
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Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is a major crop in the southern U.S., where the most important virus diseases are papaya ringspot virus (PRSV), watermelon mosaic virus-2, and zucchini yellow mosaic. The most economical control of virus diseases of watermelon is probably through genetic resistance. Watermelon has not been screened extensively for resistance to PRSV. The objective of this research was to develop a suitable method for screening watermelons for resistance to PRSV and then to screen the USDA germplasm collection. To date, we have developed an effective method and have nearly completed the screening. Several of the 1283 accessions have shown resistance to the virus. Methods tests involved 10 isolates of PRSV, several watermelon accessions and multiple inoculation procedures. Seedlings were screened in greenhouse flats with six replications per test. Tests were rated visually on a 0 to 9 scale (0 = no damage, 9 = plant dead), as well as with ELISA to detect the presence of virus. The watermelon germplasm collection was screened in four separate runs of 1283 accessions with `Charleston Gray' as the susceptible check. This research will be useful for those interested in effective screening methods, and sources of resistance for development of improved watermelon cultivars.

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Seeds of cucumber (Cucumis sativus L.) cultigens were mixed and compared for fruit yield (Mg·ha-1) and crop value ($/ha). Three cultigen pairs (Gy 14A + M 21, Gy 4 + WI 2757, and `Regal' + `Carolina') and five component ratios of each cultigen pair (0:100, 25:75, 50:50,75:25, and 100:0) were evaluated in five plantings over a 3-year period. Fruits were harvested six to eight times for each planting date. Early and total fruit yields and crop values were greatest when either Gy 14A or Gy 4 were planted as pure stands. As the ratio of Gy 14A or Gy 4 increased in the seed mixture, the yield and crop value increased. However, when the two predominantly gynoecious hybrids, `Regal' and `Carolina', were mixed, the yield and crop value were greater than those of pure stands of either hybrid. The combination of 75% `Regal' and 25% `Carolina' resulted in higher early and total fruit yields and crop value than the other ratios of the same cultigen pair. The `Regal' + `Carolina' pair produced significantly higher early yield and early crop value (first two harvests) than did the other cultigen pairs tested. However, total yield and total value for `Regal' + `Carolina' did not differ from the Gy 14A + M 21 pair. The practice of mixing two cultivars in arbitrary combinations does not offer an advantage over single cultivars in a multiple-harvest system. In some cases, predominantly gynoecious-predominantly gynoecious mixtures may have advantages over monoecious-predominantly gynoecious cultigen mixtures if pollination is not limiting. Seed mixtures need to be evaluated to determine whether specific combinations offer advantages, as this study indicated that superior mixtures may exist.

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A new trait, twin fused fruit, was discovered in gynoecious cucumber (Cucumis sativus L.) line B 5263. Plants with the twin fused fruit trait had two fruit fused into a single unit. In addition to having the twin fused fruit trait, line B 5263 had fruit with necks, large tubercles (warts), and dark green skin. The inheritance of twin fused fruit was studied in populations resulting from crosses between gynoecious line B 5263 (twin fused fruit) and monoecious line B 5404 (single fruit). Research was done in 1999 to 2001 in the greenhouses of the Research Institute of Vegetable Crops, Skierniewice, Poland. The F1 progeny developed single fruit in all cases. The observed distribution of plant phenotypes in the F2 fitted the expected ratio of 3 with single fruit: 1 with twin fused fruit. The observed distribution of plant phenotypes in the BC1A fitted the expected ratio of 1 with single fruit: 1 with twin fused fruit. Twin fused fruit occurred only in gynoecious plants, and never in monoecious plants of the cross. In the F2 progeny, the ratio of twin fused fruit within gynoecious plants fitted the expected ratio but the gene was not expressed in monoecious plants. In the F2 generation, the observed distribution of plant phenotypes fitted the expected ratio of 9 gynoecious single: 4 monoecious single: 3 gynoecious twin fused: 0 monoecious twin fused, indicating that there was epistasis, with twin fused fruit hypostatic to monoecious. The new gene will be named tf (twin fused fruit).

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Gene linkage was investigated in 11 families using 18 genes in cucumber (Cucumis sativus L.). The genes studied were B (black spine), B-3 (Black spine-3), B-4 (Black spine-4), bi (bitterfree cotyledons), Bt (bitter fruit), Bt-2 (bitter fruit-2), D (dull fruit skin), df (delayed flowering), de (determinate habit), F (female sex expression), gl (glabrous foliage), lh (long hypocotyl), ns (numerous spines), pm-h [powdery mildew (Sphaerotheca fuliginea Schlecht.:Fr.) resistance expressed on the hypocotyl], ss (small spines), Tu (tuberculate fruit), u (uniform immature fruit color), and w (white immature fruit color). A major objective of this study was to measure linkages of genes for fruit bitterness (Bt and Bt-2), and spine color (B-3 and B-4) relative to previously studied loci: B, bi, D, de, df, F, gl, lh, ns, pm-h, ss, Tu, u, and w. The F2 progeny of LJ 90430 × PI 173889 segregated 13 bitter fruit: 3 nonbitter fruit, indicating that different genes are controlling fruit bitterness in these lines. Bt-2 is proposed as the gene controlling bitterness of fruit in LJ 90430. It is a separate locus from Bt, that causes bitter fruit in PI 173889. Several new gene linkages were found: biBt, (Bt-2)—de, D—(Bt-2), Dns, glF, ss—(Bt-2), Tu—(Bt-2), and u—(Bt-2). The Bt gene appears to be linked to bi and may be located on linkage group I. Bt-2 appears to be linked with several genes that could connect linkage groups I and IV. Bt-2 was linked to u, Tu, D, and ss, that are all on linkage group IV. Bt-2 was also found to be linked loosely to de, that is on linkage group I. No linkages were found between B-3 and B-4 and the genes evaluated in this study. Weak linkages (>25 cM) between several gene combinations [(Bt-2)-de, dens, dess, deTu, deu, nsF, and ssF] provided more evidence that linkage group I and IV may be linked. Due to the weak linkages, more information needs to be obtained using larger populations and more markers to confirm these findings.

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Cucumber (Cucumis sativus L.) plant introduction (PI) accessions from the regional PI station at Ames, Iowa were evaluated in open-field production for single-harvest yield at Clinton, N.C. and Ames, Iowa. Check cultivars and experimental inbreds were also tested for comparison with the PI accessions (the three groups hereafter collectively referred to as cultigens). In order to make the evaluation more uniform for all cultigens regardless of sex expression and fruit size, all were crossed with Gy 14, a gynoecious pickling cucumber inbred used commonly in the production of commercial hybrids. The resulting 761 gynoecious hybrids were tested for early, total, and marketable yield using recommended cultural practices. Results were obtained for 725 cultigens at both locations. Significant differences were observed among cultigens for all traits evaluated. Differences between the two locations were also significant for total yield, corrected total yield, and percentage of early fruit. The interaction of cultigen and location was significant for standardized total yield and standardized corrected total yield. The highest yielding hybrids at both locations were produced using the following cultigens as male (paternal) parents: PI 422185, PI 390253, PI 175120, PI 173889, PI 267087, PI 175686, PI 178888, PI 385967, PI 458851, and PI 171601. The highest and lowest yielding paternal parents from the germplasm screening study were retested, along with check cultigens in a multiple-harvest trial at Clinton, N.C. Cultigens were evaluated directly, rather than as hybrids with Gy 14, and fruit number, fruit weight, and sex expression were recorded. Most cultigens performed as expected for the yield traits in the retest study. The exceptions were `Wautoma' and PI 339250, which were among the low and high yielders in the first test, but were ranked as medium and low, respectively, in the retest study.

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Sources of resistance to the Zucchini yellow mosaic virus-Florida strain (ZYMV-FL) have been identified within the Citrullus genus. Inheritance of resistance to ZYMV-FL was studied in PI 595203 (Citrullus mucosospermus), a resistant watermelon accession. The F1, F2, and BC1 progenies derived from the cross ‘Calhoun Gray’ (CHG) × PI 595203 and ‘New Hampshire Midget’ (NHM) × PI 595203 were used to study the inheritance of resistance to ZYMV-FL. Seedlings were inoculated with a severe isolate of ZYMV-FL at the first true leaf stage and rated weekly for at least 6 weeks on a scale of 1 to 9 on the basis of severity of viral symptoms. A single recessive gene (zym-FL) was found to control the high level of resistance to ZYMV-FL in PI 595203.

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Cucurbit plants usually are sensitive to chilling and easily damaged. Although bottle gourds, which are members of the Cucurbitaceae family, are considered as fresh vegetables in some Asian countries, their main use in recent years is to be used as rootstocks in grafted watermelon cultivation. We tested 163 bottle gourd accessions of the U.S. Department of Agriculture (USDA) genebank for cold tolerance in the early seedling stage. The experiment was conducted using controlled environment chambers with 3 chilling durations (36, 48, and 60 hours) at 4 °C. Chilling damage was rated 0 to 9 (0 = no damage, 1 to 2 = trace of damage, 3 to 4 = slight damage, 5 to 6 = moderate damage, 7 to 8 = advanced damage, 9 = plant totally dead). We rated damage separately for the cotyledons, true leaf, and growing point. Cold damage was higher at a chilling duration of 60 hours, and decreased at 48 and 36 hours. Most tolerant cultigens were PI 491272, PI 491280, PI 491281, PI 491286, and PI 491326. Most susceptible were PI 381845, PI 381846, PI 534556, PI 636137, and PI 668365.

Open Access

Salted and sweet watermelon rind pickles are commonly produced in North America, Europe, and Asia using traditional recipes. Homeowners and small industries use the leftover watermelon crop, especially from cultivars having thick and crisp rind, to produce pickles. Recently, we classified rind thickness for a set of obsolete and heirloom cultivars used by home gardeners and heirloom collectors in the United States. In this study, we used elite cultivars for growers interested in high yield, fruit quality, adaptability, and disease resistance. The objective of this study was to classify modern cultivars (nine inbreds and 103 F1 hybrids) of watermelons available to growers for use in production of watermelon rind pickles. Based on the data, cultivars were divided into three groups of rind thickness and categorized according to pedigree (inbred or F1 hybrid), fruit type (seeded or seedless), and flesh color (red, orange, or yellow). Most of the cultivars tested (109 of 112) had rind thicker than 10 mm and could be used for pickle production.

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A second gene for bitterfree foliage in cucumber (Cucumis sativus L.) was discovered. In a cross between two inbred lines having bitterfree foliage (NCG-093 and WI2757), the F1 progeny were bitter, the F2 progeny segregation frequency fit a ratio of 9 bitter : 7 bitterfree, and the BC1 segregation frequencies fit a ratio of 1 bitter : 1 bitterfree. Thus, a second factor nonallelic to the previous bitterfree gene, bi, controls the bitterfree trait. When F2 and BC1 progeny resulting from crosses of bitterfree NCG-093 with other bitter lines were studied, the second factor for bitterfree in NCG-093 fit a recessive, single-gene model. The existence of a second, recessive bitterfree gene was confirmed in additional crosses, and the gene was designated bi-2. Further analysis of two crosses indicated that bi-2 was linked with the short petiole (sp) gene (map distance = 11 cM).

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