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Raphael Z. Gilbert, Molly M. Kyle, Henry M. Munger, and Stewart M. Gray

Resistance to watermelon mosaic virus (WMV) was transferred by successive backcrossing with selection from Cucumis melo PI 414723 to three melon varieties. Levels of resistance to virus accumulation in leaf tissue were evaluated using enzyme-linked immunosorbent assay, and procedures are described to select resistant individuals efficiently and accurately in segregating populations. Resistance is controlled by a single dominant. gene designated Wmr. Plants that carry this gene initially develop mosaic symptoms on inoculated leaves, but eventually recover from symptoms, and low or no virus can be detected in the youngest leaves. In contrast, susceptible plants show similar symptoms initially, but remain stunted and symptomatic with reduced fruit yield and fruit quality. Co-infection with other cucurbit viruses, specifically cucumber mosaic virus, papaya ringspot virus, and zucchini yellow mosaic virus, did not overcome resistance to WMV conferred by Wmr.

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S. Alan Walters

Plastic mulches and rowcovers were evaluated in southern Illinois to determine their influence on watermelon mosaic virus (WMV) disease incidence and symptom severity in susceptible and tolerant summer squash (Cucurbita pepo). The use of either black or white mulch produced greater early and total marketable yields than no mulch (bare soil) on `Dividend' and `Multipik'. More fruit had WMV symptoms with no mulch than with mulch, regardless of cultivar. However, more severe WMV symptoms developed on the fruit of susceptible `Multipik' compared to tolerant `Dividend'. The use of plastic mulches provided greater and longer protection to `Dividend' compared to `Multipik'. However, `Dividend' fruit did eventually develop virus symptoms as disease incidence in production fields increased. Rowcovers reduced the number of alate aphids landing on plants which resulted in fewer plants with WMV symptoms and suppression of symptoms on squash plants regardless of mulch type. Rowcovers had a greater influence on reducing the incidence of WMV and the severity of symptoms on `Dividend' compared to `Elite'. Rowcovers did not reduce WMV on `Elite' by the end of the season and were more effective when used with white mulch compared to black mulch. Rowcovers suppressed the incidence and severity of WMV symptoms that developed on a virus tolerant squash cultivar for a greater length of time compared to a susceptible cultivar, which related to increased yields and fewer culls with virus symptoms on the tolerant cultivar.

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S. Alan Walters

Watermelon mosaic virus (WMV) is often the most limiting factor to cucumber (Cucumis sativus) production in the midwestern U.S. The influence of WMV on farm-gate revenues for nine slicing cucumber (or fresh market cucumber) cultivars was determined under high WMV disease incidence during 2000 and 2001. Over the two growing seasons, most cucumber cultivars produced excessive amounts of unmarketable WMV symptomatic fruit; however, no WMV symptoms were observed on any fruit produced by `Daytona' or `Indy'. `Thunder' produced some WMV symptomatic fruit but was significantly (P ≤ 0.05) less than that produced by all other cucumber cultivars, except for `Daytona' and `Indy.' Consistent high total farm gate-revenues over both years were produced by `Daytona' and `Indy' compared to other cucumber cultivars evaluated with the exception of `Thunder'. `Daytona,' `Indy,' and `Thunder' tended to produce greater early-season farm-gate revenues. However, late-season revenues of `Thunder' were reduced compared to `Daytona' and `Indy'. `Dasher II,' `General Lee,' `Greensleeves,' `Marketmore 76,' `Speedway,' and `Turbo' produced excessive amounts of unmarketable WMV symptomatic fruit which led to reduced farm-gate revenues. Cucumber cultivars without some level of resistance to WMV produced substantially less cumulative farm-gate revenues than those that had some level of resistance. `Daytona,' `Indy,' and `Thunder' were not the highest yielding cucumber cultivars evaluated in this study, but produced the highest farm-gate revenues due to higher levels of genetic resistance to WMV.

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Richard G. Snyder, Frank Killebrew, and Joseph A. Fox

Yellow squash (Cucurbita pepo L.) planted after early spring in Mississippi have a strong likelihood of developing green streaks and blotches on the fruit-symptoms of watermelon mosaic virus strain 2. Cultivars with the relatively new precocious yellow gene (PYG) tend to show such symptoms less prominently, and in some cases not at all, when infected. Field trials were conducted at two locations to evaluate several PYG cultivars and compare their WMV-2 symptoms to those of standard, non-PYG types. In both cases, the PYG cultivars had fewer unmarketable fruit due to WMV-2 symptoms, although they were not entirely immune to the virus.

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Eileen Kabelka and Rebecca Grumet

Potyviruses cause severe loss in cucurbit crops. Inbred lines derived from the Chinese cucumber cultivar, Taichung Mau Gua (TMG), have been identified to be resistant to several potyviruses including zucchini yellow mosaic virus (ZYMV), zucchini yellow fleck virus, watermelon mosaic virus, and the watermelon strain of papaya ringspot virus. Recently, an additional virus that infects cucurbits, the Moroccan watermelon mosaic virus (MWMV), has been identified to be a distinct member of the potyvirus group. In this study, we sought to determine if TMG-1 is resistant to MWMV and, if so, examine whether a relationship exists between resistance to MWMV and resistance to ZYMV. Progeny analyses show that TMG-1 is resistant to MWMV and, like resistance to ZYMV, MWMV resistance is conferred by a single recessive gene. Sequential inoculation of progeny possessing resistance to ZYMV followed by MWMV (or MWMV followed by ZYMV) suggests that both resistances are conferred by the same gene, or two tightly linked genes. Additionally, all F3 families derived from F2 individuals selected for resistance to ZYMV, were resistant to MWMV. A second source of resistance to ZYMV, allelic to the TMG-1 source, has been incorporated into the Dutch hybrid Dina. Progeny analyses show Dina to posses a single recessive gene for MWMV resistance. As with TMG-1, no segregation of resistances was observed when ZYMV resistant progeny were inoculated with MWMV (or MWMV followed by ZYMV). Collectively, these results suggest that a single gene, or two tightly linked genes, control resistance to the potyviruses ZYMV and MWMV.

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Konstantinos Anagnostou, Molly Kyle, and Rafael Perl-Treves

We have studied the relationship of resistance to watermelon mosaic virus (WMV), zucchini yellow mosaic virus (ZYMV), papaya ringspot virus (PRSV), and powdery mildew (PM) in melon (Cucumis melo). We have confirmed monogenic dominant inheritance of these four resistances and report that PI414723-4S3, which was initially selected as a source of ZYMR, is also a source of dominant monogenic resistance to PRSV. Further, we observed departure from independent assortment for resistance to WMV and ZYMV in a study of 73 (UC Top Mark × PI414723-4S3) F3 families (χ2 = 39.87 significant at both 0.01 and 0.05 levels), indicating linkage between Wmv and Zym. The map distance between these resistance genes calculated from the number of recombinant families (RF% = 9.58) was 10.5 cM. Compari-sons among WMV, PM, ZYMV-PM, PRSV-PM, ZYMV-PRSV, and WMV-PRSV of 48 (TM × PI414723-4S3) F3 families, which were screened with all four pathogens, showed no consistent cosegregation.

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Albert N. Kishaba, Steven J. Castle, Donald L. Coudriet, James D. McCreight, and G. Weston Bohn

Abbreviations: ANOVA, analysis of variance; AR HBJ, AR Hale's Best Jumbo; AR TM, AR Topmark; AR 45, aphid-resistant `PMR 45'; CMV, cucumber mosaic virus; HBJ, `Hale's Best Jumbo'; EN 50 , 50% infection; WMV, watermelon mosaic virus; ZYMV, zucchini

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A. Graves Gillaspie Jr. and Robert L. Jarret

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T. Wai and R. Grumet

The inbred cucumber (Cucumis sativus L.) line TMG-1 is resistant to three potyviruses: zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus (WMV), and the watermelon strain of papaya ringspot virus (PRSV-W). In this study we sought to determine the genetics of resistance to PRSV-W. TMG-1 was crossed with WI-2757, an inbred line susceptible to all three viruses. Segregation data indicated that resistance to PRSV-W was due to a single dominant gene (proposed designation, Prsv-2). Enzyme-linked immunosorbent assay (ELISA) data suggested that the mechanism of resistance to PRSV-W differs from that for ZYMV and WMV, and may be better described as tolerance. Although the plants were free of symptoms, high PRSV-W titers existed in young expanding leaves of the TMG-1 plants and the WI-2757 × TMG-1 F1 progeny.

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S. Alan Walters, Jeffrey D. Kindhart, Houston A. Hobbs, and Darin M. Eastburn

Viruses are a serious threat to cucurbit production in southern Illinois. The most prevalent viruses infecting cucurbit crops in the region were determined during the 1998, 1999, and 2000 growing seasons to enable growers to make better decisions on viral disease management. Watermelon mosaic virus (WMV) was the most prevalent virus as it was found in ≈84% of samples over the three years. Cucumber mosaic virus (CMV), papaya ringspot virus (PRSV), squash mosaic virus (SqMV), and zucchini yellow mosaic virus (ZYMV) were detected in ≈8%, 6%, 9%, and 1% of samples, respectively, over the 3-year period. WMV was generally the only virus isolated from samples collected before mid-September. Other viruses, including CMV, PRSV, SqMV, and ZYMV, were generally first detected after mid-September and were usually found as mixed infections with WMV.