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 (P_{r}, P_{s}, F_{1}, F_{2}, BC_{1}P_{r}, and BC_{1}P_{s}) 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.

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- Author or Editor: Luis A. Rivera-Burgos x

### Gabriele Gusmini, Luis A. Rivera-Burgos and Todd C. Wehner

### 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 F_{1}, F_{2}, and BC_{1} 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.

### 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 F_{1}, F_{2}, and BC_{1} 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.

### 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.