Browsing by Author "Todd C. Wehner, Committee Chair"

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Breeding Watermelon (Citrullus lanatus) for Resistance to Gummy Stem Blight (Didymella bryoniae)
(2003-03-31) Gusmini, Gabriele; Gerald J. Holmes, Committee Member; Edd S. Buckler, Committee Member; Todd C. Wehner, Committee Chair
Gummy stem blight, caused by Didymella bryoniae (Auersw.) Rehm is a major disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] in the U.S. Plant breeders need sources of resistance that can be incorporated into adapted breeding lines to keep epidemics of this disease under acceptable control. We tested all the available accessions from the United States Department of Agriculture, Agricultural Research Service (USDA-ARS) watermelon germplasm collection, including C. lanatus var. citroides, for resistance to gummy stem blight. To perform the screen, we adopted the following protocol: 1) we used spores from a virulent isolate (or mixture of virulent isolates) grown for 2 to 3 weeks on Potato Dextrose Agar under artificial white light (12 hour photoperiod); 2) we used as inoculum a water suspension of spores in deionized water (5*105 spores/ml); 3) we ensured high relative humidity immediately after inoculation (with the presence of free-water on the leaves of test plants), by irrigating the field and using a clear plastic disease chamber in the greenhouse with artificial mist; 4) prior to inoculation we injured the trichomes of the leaves by brushing the plants with a wood stake. Our experiment was a randomized complete block with 1,332 cultigens (elite cultivars, obsolete cultivars, breeding lines, PI accessions, and cucumber checks), two assays (field and greenhouse), two or four replications, and two to six plants per plot. Cultigens were significantly more or less resistant than the resistant check PI 189225 and the susceptible check 'Charleston Gray'. PI 279461, PI 482379, PI 254744, PI 526233, PI 482276, PI 271771, PI 164248, PI 244019, PI 296332, and PI 490383 were selected as the most resistant cultigens, based on low mean rating for gummy stem blight, similar reaction in field and greenhouse, low standard deviation, and high number of replicates. The most susceptible cultigens, based on high mean rating for gummy stem blight, similar reaction in field and greenhouse, low standard deviation, and high number of replicates, were: PI 183398, PI 169286, PI 223764, PI 226445, PI 525084, and PI 534597. Further analysis of the group of the most resistant PI accessions suggested that resistance to gummy stem blight in watermelon might be determined by a single gene (or a unique set of genes) with different pleiotropic effects at the juvenile stage in the greenhouse and at the adult stage in the field. For marketing of cultivars in drier climates (i.e. southwestern U.S.), where gummy stem blight typically appears at the seedling stage during transplant production, due to high relative humidity and warm temperatures in greenhouses, it should be sufficient to develop cultivars highly resistant in the greenhouse and moderately resistant in the field. In years of extraordinary humid weather, gummy stem blight in the field would be controlled with an integrated pest management program, while in regular year of dry weather it would not affect the crop.
Growth regulator Effects on Watermelon Chilling Resistance, Flowering, and Fruiting
(2008-08-22) Oh, Jiyoung; Jonathan R. Schultheis, Committee Member; Todd C. Wehner, Committee Chair; James D. Burton, Committee Member
Inheritance of Fruit Characteristics and Disease Resistance in Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]
(2005-03-24) Gusmini, Gabriele; Todd C. Wehner, Committee Chair; Gerald J. Holmes, Committee Member; Janet F. Spears, Committee Member; Ralph A. Dean, Committee Member
The watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] is a major vegetable crop in the United States. The genetics of this crop have been widely studied and several genes reported. Nevertheless, further investigation was needed for genes determining the appearance of the fruit (rind and flesh colors), the weight of single fruit, and resistance to gummy stem blight, a severe disease of watermelon caused by Didymella bryoniae (Auersw.) Rehm. In this work the inheritance of novel rind phenotypes was measured and the genetics of flesh color verified. Three new genes were identified: Dr for the deep-red flesh color of 'Dixielee' and 'Red-N-Sweet', Yb for the yellow belly of 'Black Diamond Yellow Belly', and is for the intermittent stripes of 'Navajo Sweet'. The spotted phenotype from 'Moon and Stars' was transferred to light green and gray cultivar for the development of novel varieties with distinctive rind patterns. Yield of 80 diverse cultivars was evaluated in replicated experiments. Some of the new, elite hybrid cultivars were in the top yielding group, however old, inbred cultivars appeared in the top group as well. Consistent and significant differences among the 80 cultivars tested suggests genetic variation for yield. Since most watermelon consumers are interested in smaller fruit, six adapted cultivars bearing the largest and smallest fruit were crossed in a half-diallel, producing F1, F2, and backcross generations. Genotypic variances, heritability, and gain from selection were estimated. High environmental variance and low narrow- and broad-sense heritability were recorded. Finally, the inheritance of resistance to gummy stem blight, previously attributed to the db gene, was verified. A genetic system more complex than a single gene seems to regulate the transmission of resistance from resistant to susceptible germplasm. Due to the complexity of phenotypic testing for this disease in watermelon, a new project for the development of molecular markers linked to resistance was initiated. Nevertheless, the use of F3 phenotypic data and F2 genotypic markers and the apparent complexity of the trait did not allow the identification of a tightly linked marker.
Inheritance of Fruit Characteristics in Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]
(2009-07-22) Lou, Lingli; Todd C. Wehner, Committee Chair; G. Craig Yencho, Committee Member; Gina Brown-Guedira, Committee Member
Watermelon fruit characters may affect customer acceptance of the watermelon fruit. The qualitative fruit traits, such as flesh color, seed size, seed coat color, rind pattern, fruit shape, exhibit wide ranges of phenotypes. The flesh color can be red, orange, yellow, or white. The seed length of watermelon also varies from 4.4mm to 16.5mm. The seeds can have various coat colors or other decorations. The rind of watermelon fruits can be striped or solid colored, which are further characterized by different stripe widths, stripe colors, backgrounds colors, and additional modifications. The fruit shape can be elongate, oblong, and round. Other fruit traits include shape of fruit blossom end, fruit surface characters, and hollow-hearted flesh. By crossing watermelon cultivars with different phenotypes, we studied the inheritance of the various phenotypes and identified and verified genes responsible for the flesh color, seed size, rind pattern, and fruit shape. In addition, we studied the quantitative traits of the fruit weight and total soluble solids content. The calculated broad-sense and narrow-sense heritability for fruit weight is low to medium, indicating large environmental effect on fruit weight. Medium to high heritability is found for the total soluble solid content, suggesting possible gains from selection.
Papaya Ringspot Virus Watermelon Strain and Zucchini Yellow Mosaic Virus Resistance in Watermelon
(2005-03-01) Guner, Nihat; Todd C. Wehner, Committee Chair
Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) is a major crop in the southern United States. Papaya ringspot virus-watermelon strain (PRSV-W, formerly Watermelon mosaic virus-1) and Zucchini yellow mosaic virus (ZYMV) affect all agricultural species of the Cucurbitaceae and are of great economic importance because of their destructiveness. Watermelon has not been screened extensively for resistance to PRSV-W. Although there is a resistance source for ZYMV resistance, the identification of additional sources of resistance to ZYMV would be highly desirable since the initial sources of resistance are temperature dependent, or not resistant to some of the more severe strains of the virus. The objectives of this study were 1) to screen the USDA watermelon germplasm collection along with available watermelon cultivars for PRSV-W resistance; 2) to verify the disease rating for the most resistant and most susceptible accessions; 3) to determine the genetic control of PRSV-W resistance; and 4) to screen the U.S.D.A. watermelon germplasm collection along with available watermelon cultivars to identify additional sources of resistance to ZYMV. A total of 1275 plant introduction accessions and 44 watermelon cultivars were screened for PRSV-W resistance. The experiment was a randomized complete block with five replications. Enzyme-linked immunosorbent assay (ELISA) was performed after the last rating to determine whether the virus was in the plant tissue. After the germplasm screening, the most resistant and most susceptible cultigens were retested to verify their reactions. Of the 60 resistant PI accessions in the final retest, eight had resistance with a rating of 3.6 or less for the best, average and maximum ratings: PI 244017, PI 244019, PI 482342, PI 482318, PI 485583, PI 482379, PI 595203, and PI 244018. None of the watermelon cultivars tested exhibited resistance to PRSV-W. Inheritance of PRSV-W resistance was studied in three C. lanatus var. citroides accessions: PI 244017, PI 244019, and PI 485583. Three susceptible parent lines, 'Allsweet', 'Calhoun Gray', and 'New Hampshire Midget', were crossed with resistant accessions to develop F1, F2, and BC1 generations for six families. A single recessive gene was found to control resistance to PRSV-W in all three resistant PI accessions. A test of allelism indicated that resistance to PRSV-W in the three PI accessions was due to the same gene. Therefore, the gene symbol 'prv' is proposed for PRSV-W resistance in PI 244017, PI 244019, and PI 485583 in watermelon. The USDA germplasm collection of 1613 introduction accessions, as well as 41 watermelon cultivars, was screened for resistance to the Florida strain of Zucchini yellow mosaic virus (ZYMV-FL). The experiment was a randomized complete block with four replications. A retest study was conducted after the germplasm screening to verify the reaction of the most resistant and susceptible accessions. The PI accessions with the highest resistance to ZYMV were PI 595203, PI 537277, PI 560016, PI 386016, PI 386019, PI 485580, PI 494529, and PI 595200. PI 595203 was the most resistant accession based on both the germplasm screening and the retest study. PI accessions with the highest resistance to ZYMV-FL that also had resistance to other watermelon viruses PRSV-W and WMV (Watermelon mosaic virus, formerly Watermelon mosaic virus-2) were PI 595203, PI 386015, PI 386016, PI 386024, PI 386025, PI 386026, PI 244018, PI 244019, PI 485583, and PI 494528, PI 494529.