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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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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.

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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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All available cucumber (Cucumis sativus L.) cultigens were tested for combining ability for fruit storage ability by crossing them with the gynoecious inbred Gy 14. Fruit weight and firmness were measured before and after storage, and fruits were rated for water loss after storage. The cultigens with the lowest percentage of fruit weight loss during storage were PI 172839, PI 344067, PI 264667, PI 171612, PI 339245, PI 220171, PI 279469, and PI 368550; those with the lowest percentage of loss in fruit firmness were PI 379284, PI 339241, PI 414159, PI 422177, `Regal', PI 109483, `Addis', PI 285603, PI 257486, and `Calypso'. The cultigens demonstrating the least fruit shriveling were `Dasher II', `Sprint 440', `Texas Long', PI 390255, PI 432870, `Pacer', PI 419078, PI 390247, PI 321011, and PI 414158. The 10 best cultigens from the initial screening study, along with the four worst cultigens and six checks, were retested directly (not as F1 progeny) for fruit keeping ability in two storage conditions and at two harvest dates. No significant differences were detected between the two harvest dates and storage conditions.

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Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] flesh color is controlled by several genes to produce red, canary yellow, salmon yellow, and orange. Our objective was to study the interaction of three gene loci with two or three alleles at each C (canary yellow vs. red), y (salmon yellow vs. red), yo (orange), and i (inhibitory to C permitting Y to produce red flesh color). Five crosses were used to study gene action: `Yellow Baby' × `Sweet Princess', `Yellow Baby' × `Tendersweet Orange Flesh', `Yellow Baby' × `Golden Honey', `Yellow Doll' × `Tendersweet Orange Flesh', and `Yellow Doll' × `Golden Honey'. Based on the performance of six generations (PA, PB, F1, F2, BC1A, and BC1B), the parents had the following genotypes: `Yellow Baby' = CCYYII, `Yellow Doll' = CCYYII, `Sweet Princess' = ccYY ii, `Tendersweet Orange Flesh' = ccyoyoII, and `Golden Honey' = ccyyII. Segregation of flesh colors in the progeny of the five families demonstrated that there was a multiple allelic series at the y locus, where YY (red) was dominant to yo yo (orange) and yy (yellow). Also, yoyo was dominant to yy. In conclusion, epistasis is involved in genes for the major flesh colors in watermelon, with ii inhibitory to CC (Canary), resulting in red flesh, and CC in the absence of ii epistatic to YY, producing canary flesh.

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All available luffa (Luffa aegyptiaca Mill.) cultivars, breeding lines, and plant introduction accessions (collectively referred to as cultigens hereafter) were evaluated at Clinton, N.C., over 3 years. Plants were grown in plots 1.5-m-long on a 1.8-m-high trellis. Border rows and tiers on the sides and ends of the trial were used to reduce the edge effect. Plots were planted in May and evaluated for vine height and sex expression. Fruit were harvested in October to determine fruit number and length after frost killed the vines. Sponges were processed from the fruit and evaluated for seed cell number, wall thickness, sponge strength, fiber denseness, and other quality traits. The tallest vined cultigens were PI 286425 and Fletcher, and the shortest vined were PI 381869 and PI 540921. The highest yielding (sponge number per hectare) cultigens were PI 540921 and PI 391603, and the lowest yielding cultigens were Luffa 30310 and Luffa 97321. Of the cultigens tested, PI 391603 had the longest fruit overall, whereas PI 540921 had the shortest.

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