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Nihat Guner and Todd C. Wehner

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is a major vegetable crop in the world, accounting for 6.8% of the world area devoted to vegetable crops. Watermelon is a useful vegetable crop for genetic research because of its small genome size, and the many available gene mutants. The watermelon genes were originally organized and summarized in 1944, and have been expanded and updated periodically. However, the action of some watermelon genes has not been described clearly in some cases. Also, the interaction of multiple gene loci that control similar traits needs to be described more clearly. Finally, it is necessary to identify the inbred lines having each published gene mutant, for use as type lines in studies of gene action, allelism, and linkage. The objective of this work was to update the gene list, identify the cultivar or line having each gene mutant, and collect seeds of the lines for use by interested researchers. In addition, the gene descriptions were expanded and clarified, information on gene interactions was added, and errors in naming or citing previously described genes were corrected. New genes that have not previously been described (cr, Ctr, dw-3, ms-2, Ti, ts and zym-FL) were added to the list, for a total of 163 watermelon gene mutants.

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Nihat Guner and Todd C. Wehner

Inheritance of resistance to zucchini yellow mosaic virus-Florida strain (ZYMV-FL) was studied in the resistant watermelon accession of PI 595203 (Citrulluslanatus var. lanatus), an egusi type originally collected in Nigeria. The F1, F2, and BC1 generations derived from the cross `Calhoun Gray' × PI 595203 and `New Hampshire Midget' × 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 on a 0 to 9 scale, based on the severity of virus symptoms. A single recessive gene was found to control resistance, for which we propose the symbol zym-FL2. The gene probably was not allelic to the previously published gene, zym-FL, for resistance to the Florida strain of ZYMV in the accession PI 482261, since PI 482261 was not resistant to the ZYMV-FL isolate used in our tests.

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Nihat Guner*, Zvezdana Pesic-VanEsbroeck and Todd Wehner

Sources of resistance to the watermelon strain of papaya ringspot virus (PRSV-W) have been identified within the watermelon (Citrullus lanatus) germplasm collection. Inheritance of resistance to papaya ringspot virus-watermelon strain was studied in three C. lanatus var. citroides accessions: PI 244017, PI 244019, and PI 485583. The 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. The gene symbol `prv' is proposed for PRSV-W resistance in watermelon. Additional work is needed to determine whether the genes in PI 244017, PI 244019, and PI 485583 are allelic for resistance to PRSV-W.

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Nihat Guner, Luis A. Rivera-Burgos and Todd C. Wehner

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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Nihat Guner, Zvezdana Pesic-VanEsbroeck, Luis A. Rivera-Burgos and Todd C. Wehner

Sources of resistance to the watermelon strain of papaya ringspot virus-watermelon strain (PRSV-W) have been identified within the watermelon (Citrullus lanatus) germplasm collection. Inheritance of the resistance to PRSV-W was studied in three Citrullus amarus (formerly C. lanatus var. citroides) PI accessions: PI 244017, PI 244019, and PI 485583. Three susceptible parent lines, ‘Allsweet’, ‘Calhoun Gray’, and ‘New Hampshire Midget’, were crossed with resistant PI accessions to develop F1, F2, and BC1 progenies in six families. A single recessive gene was found to control the resistance to PRSV-W in all three resistant PI accessions. Allelism tests indicated that the three PI accessions carry the same resistance allele to PRSV-W. The gene symbol ‘prv’ is proposed for PRSV-W resistance in PI 244017, PI 244019, and PI 485583 in watermelon.

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Amnon Levi, John Coffey, Laura Massey, Nihat Guner, Elad Oren, Yaakov Tadmor and Kai-shu Ling

The bitter desert watermelon, Citrullus colocynthis (L.) Schrad is a wild species valuable for biotic and abiotic stress resistance that could be exploited for improving watermelon cultivars [Citrullus lanatus (Thunberg) Matsum & Nakai var. lanatus]. The objective of this study was to survey and identify C. colocynthis accessions displaying resistance to the Papaya ringspot virus-watermelon strain (PRSV-W). Thirty-one accessions of C. colocynthis, collected in Africa, the Middle East, southwest Asia, and India were evaluated for PRSV-W resistance. Of these 31 accessions, 4 U.S. Plant Introduction (PI) accessions, including 525080 (collected in Qena, Egypt) and PI 537277, PI 652554, and Griffin 14201 (collected at the northern Indian desert of Rajasthan and the neighboring region of Punjab, Pakistan) showed high resistance to PRSV-W. Plants of these four resistant PIs were self-pollinated to produce S1 and S2 seeds that continued to maintain the high levels of PRSV resistance. Since there is a wide genetic distance between watermelon cultivars and C. colocynthis, we performed crosses and backcrosses with watermelon cultivars, including ‘Charleston Gray’ and ‘Sugar Baby’ to produce viable seed that would be useful in the development of genetic populations and in introducing the resistance into watermelon cultivars.

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Nihat Guner, Zvezdana Pesic-VanEsbroeck, Luis A. Rivera-Burgos and Todd C. Wehner

The Florida strain of Zucchini yellow mosaic virus (ZYMV-FL) is one of the major viral diseases of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. Although some screening has been performed for ZYMV resistance in watermelon, the germplasm collection has not been screened extensively for resistance. The objectives of this study were to screen the U.S. Department of Agriculture (USDA) watermelon germplasm collection for ZYMV resistance and to verify the disease rating for the most resistant and most susceptible accessions. We used a randomized complete block with four replications, 1613 PI accessions, and 41 cultivars. ‘Charleston Gray’ and ‘Crimson Sweet’ susceptible controls were used to verify that the ZYMV inoculum was virulent. After the last rating, an enzyme-linked immunosorbent assay (ELISA) was performed to determine the presence of a virus in the plant tissue. The PI accessions with high resistance to ZYMV-FL that also exhibited resistance to other watermelon viruses were PI 595203, PI 386015, PI 386016, PI 386024, PI 386025, PI 386026, PI 244018, PI 244019, PI 485583, PI 494528, and PI 494529. The ZYMV-FL retest of the most resistant 46 PI accessions showed that there were some escapes. Sixteen resistant PI accessions had a rating of 3.0 or less for the average and maximum ratings: PI 595203, PI 537277, PI 560016, PI 386016, PI 386019, PI 485580, PI 494529, PI 595200, PI 494528, PI 595201, PI 386025, PI 494530, PI 386015, PI 386021, PI 386026, and PI 596662. Overall, PI 595203 had the highest resistance according to both the germplasm screening and the retest studies.