Grafting watermelon (Citrullus lanatus) to control Fusarium wilt has been practiced in Europe, the Middle East, and the Far East for decades. Until recently, grafting watermelon has not been practiced in the United States due to labor costs and land availability. There is some disagreement in the literature as to the effects that grafting has on watermelon fruit quality. This study was designed to determine the effects of grafted watermelon on fruit firmness, lycopene content, and total soluble solids (TSS) using five different rootstocks. When using Cucurbita ficifolia or Cucurbita maxima x Cucurbita moschata hybrid as the rootstock, watermelon fruit consistently had higher fruit firmness values. Other C. maxima x C. moschata hybrids or Lagenaria siceraria rootstocks generally produced lower or more varied fruit firmness values. Grafting increased fruit firmness by as much as 25% in some cases, but field and year effects were observed. In addition, grafting had no effect on lycopene content or TSS.Furthermore, no off-flavors were detected in fruit from grafted plants, but there was a 5-to 7 day delay in fruit maturity compared to their non-grafted counterpart. Although environment can have a major influence on fruit quality attributes, rootstock selection may be equally important in achieving the desired outcome.Keywords: Citrullus lanatus, Cucurbita spp., Lagenaria sp., grafting, lycopene, fruit firmness, total soluble solids.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products of vendors that may also be suitable.

Three races (0, 1, and 2) of Fusarium oxysporum f. sp. niveum have been previously described in watermelon (Citrullus lanatus) based on their ability to cause disease on differential watermelon genotypes. Four isolates of F. oxysporum f. sp. niveum collected from wilted watermelon plants or infested soil in Maryland, along with reference isolates of races 0, 1, and 2, were compared for virulence, host range, and vegetative compatibility. Race identification was made on the watermelon differentials Sugar Baby, Charleston Gray, Dixielee, Calhoun Gray, and PI-296341-FR using a root-dip, tray-dip, or pipette inoculation method. All four Maryland isolates were highly virulent, causing 78 to 100% wilt on all differentials, one of which was PI-296341-FR, considered highly resistant to race 2. The isolates also produced significantly greater colonization in the lower stems of PI-296341-FR than a standard race 2 reference isolate. In field microplots, two of the isolates caused over 90% wilt on PI-296341-FR, whereas no disease was caused by a race 2 isolate. All four isolates were nonpathogenic on muskmelon, cucumber, pumpkin, and squash, confirming their host specific pathogenicity to watermelon. The Maryland isolates were vegetatively compatible to each other but not compatible with the race 2 isolates evaluated, indicating their genetic difference from race 2. This study proposes that the Maryland isolates belong to a new race, race 3, the most virulent race of F. oxysporum f. sp. niveum described to date.

Cucurbit yellow vine disease (CYVD), which can inflict heavy losses to watermelon, pumpkin, cantaloupe, and squash in U.S. production areas from the midwest to northeastern states, causes phloem discoloration, foliar yellowing, wilting, and plant decline. Bacteria were cultured from the phloem of crown sections of symptomatic plants of Citrullus lanatas and Cucurbita pepo. Those bacteria testing positive in CYVD-specific polymerase chain reaction (PCR) were all gram negative and appeared morphologically identical, producing creamy white, smooth, entire, convex colonies on Luria-Bertani or nutrient agar. Characterized cucurbit-derived strains of Serratia marcescens were introduced into greenhouse-grown squash plants by puncture inoculation and into field-grown squash plants by enclosure with S. marcescens-fed squash bugs, Anasa tristis. Up to 60% of the bacteria-inoculated plants in the greenhouse and up to 17% of field plants caged with inoculative squash bugs developed phloem discoloration and tested positive for S. marcescens by CYVD-specific PCR. None of the controls developed phloem discoloration or tested positive by PCR. Of the diseased field plants, 12% (2 of 35) also yellowed, wilted, and collapsed, exhibiting full symptom development of CYVD. However, neither plant collapse nor decline was observed in the greenhouse-grown, puncture-inoculated plants. The morphology, growth habit, and PCR reaction of bacteria cultured from crown tissue of a subset of plants in each experimental group were indistinguishable from those of the inoculum bacteria. Evidence presented from our studies confirms that the squash bug can transmit S. marcescens, the CYVD causal bacterium. The S. marcescens-A. tristis relationship described here is the first instance in which the squash bug has been identified as a vector of a plant pathogen. Our experiments represent a completion of the steps of Koch's postulates, demonstrating that S. marcescens is the causal agent of CYVD and that the squash bug, A. tristis, is a vector of the pathogen.

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The pathogenicity of Acremonium cucurbitacearum to 31 cucurbits, 18 crop plant species, and 15 weed species was evaluated under greenhouse conditions. This study demonstrates that many other cucurbits in addition to muskmelon and watermelon can potentially serve as hosts of A. cucurbitacearum. Using a disease severity index (DSI), all muskmelon and watermelon cultivars were ranked as susceptible or highly susceptible, with the exception of honeydew cultivar TAM Dew Improved, which ranked resistant in one study. Highly resistant cucurbits included Cucurbita maxima (autumn squash), Luffa acutangula, L. aegyptiaca (sponge gourd) and Benincasa hispida (ash gourd). Cyclanthera pedata and Cucumis miriocarpus were rated as resistant. The percentage recovery of A. cucurbitacearum ranged from 8% to 100% and tended to be higher in plants with a higher DSI. Other crops and weed species common to muskmelon production fields are not likely to be involved in the perpetuation or propagation of the fungus.

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Field surveys were conducted to determine the distribution and frequency of viruses infecting watermelon and other cucurbits in the southern US in 2010 and 2011. Leaf samples were collected from 715 symptomatic plants from 10 states and were tested by dot-immunobinding assays or reverse transcriptionpolymerase chain reaction for 17 viruses that included Alfalfa mosaic virus (AMV), Bean pod mottle virus (BPMV), Cucurbit aphid born yellows virus (CABYV), Cucurbit yellow stunting disorder virus (CYSDV), Cucumber green mottle mosaic virus (CGMMV), Cucumber mosaic virus (CMV), Melon necrotic spot virus (MNSV), Papaya ringspot virus-W (PRSV-W), Squash leaf curl virus (SLCuV), Soybean mosaic virus (SMV), Squash mosaic virus (SqMV), Squash vein yellowing virus (SqVYV), Tobacco ringspot virus (TRSV), Watermelon mosaic virus (WMV), Watermelon silver mottle virus (WSMoV), Zucchini yellow mosaic virus (ZYMV), and Zucchini green mottle mosaic virus (ZGMMV). Thirteen out of 17 viruses were detected in this study. The distribution of detected viruses varied with the highest average frequency for WMV (30.6%), followed by PRSV-W (24.7%), ZYMV (13.9%), TRSV (5.7%), SqMV (3.5%), and MNSV (2.6%). The percent frequency of the remaining viruses was less than 2%. Seven viruses (AMV, BPMV, CMV, SqMV, TRSV, PRSV-W, and ZYMV) were also detected either from nearby agricultural crops or weeds species. Mixed infections were also recorded for some viruses with potyviruses being the most common. There is limited information on frequency and distribution of viruses that occur on watermelon and other cucurbits. These results indicate that potyviruses, particularly PRSV-W, WMV, and ZYMV, are frequently present in infected watermelon and other cucurbits in the southern US. Accepted for publication 30 July 2012. Published 24 August 2012.