Watermelon (Citrullus lanatus) production in the United States has almost doubled since the 1989 Watermelon Research and Promotion Act (United States Code, 2019), and in 2018, about two million metric tons of watermelon were produced, with Florida, Texas, and California being the top-producing states (Economic Research Service, 2013; National Agricultural Statistics Service, 2019). In Washington State, watermelon is primarily grown in the Columbia River Basin, where current production is estimated at 243 ha (Timothy Waters, personal communication). One of the barriers to increasing watermelon production in this region of southeastern Washington is verticillium wilt (caused by Verticillium dahliae), which can cause rapid vine decline and yield loss. Watermelon does not have resistance to V. dahliae (Pscheidt and Ocamb, 2019), and so management tactics are aimed at reducing V. dahliae microsclerotia, which serve as the primary inoculum, and include fumigation and crop rotation. However, these controls are not always effective.
Recent studies in Washington have shown that grafting watermelon onto resistant rootstocks is a suitable management option for verticillium wilt (Buller et al., 2013; Dabirian et al., 2017; Wimer et al., 2015a). On a commercial scale, growers in Asia have been using grafting for several decades as a management strategy for biotic stresses such as fusarium wilt and verticillium wilt, and abiotic stresses such as soil salinity and cold stress in watermelon. For example, Japan has used commercial watermelon grafting since 1927 (Tateishi, 1927), and cucurbit rootstocks in Japan have been well researched. In comparison, commercial grafting in the United States began only in the 1990s (Kubota, 2016), which could be the reason that few watermelon rootstocks are available in the domestic market. Most rootstock cultivars are imported into the United States from Asia by seed companies that are involved in breeding these rootstocks, and seeds generally are not sold directly to growers. Currently, the cost of a grafted watermelon seedling can be up to five times greater than that of a nongrafted seedling (Galinato et al., 2016) and can discourage growers from using grafted transplants as a disease management strategy. The cost of labor represents 48% to 60% of the total cost of each grafted watermelon seedling (Galinato and Gallardo, 2017; Lewis et al., 2014), and the cost of rootstock seeds can be twice that of watermelon seeds on average, depending on the cultivar and on the number of seeds purchased (Lewis et al., 2014).
The rootstock species that are most commonly used for watermelon grafting are bottle gourd [Lagenaria siceraria (Mol.) Standl.], interspecific hybrids between Cucurbita maxima Duch. and C. moschata Duch., and wild watermelon (Citrullus spp.) (Davis et al., 2008). These rootstocks are used because of their grafting compatibility with watermelon and their resistance to biotic and abiotic stresses. Grafting compatibility can be defined as the successful union of the rootstock and scion in a manner that does not compromise fruit yield and quality. Previous studies have found that yield of watermelon grafted onto interspecific Cucurbita hybrids and L. siceraria can be equivalent to or greater than that of nongrafted watermelon (Bekhradi et al., 2011; Paroussi et al., 2007; Wimer et al., 2015a). Yetışır et al. (2003) also reported that watermelon grafted onto L. siceraria produced higher yield than Cucurbita hybrids. Use of Benincasa hispida (Thunb.) Cogn. as a rootstock has been attempted in the past, but difficulties regarding seedling emergence have hampered further investigations (Yetışır et al., 2003). In some studies, the screening of cucurbit germplasm for resistances to diseases such as powdery mildew (Block and Reitsma, 2005), Zucchini yellow mosaic virus (Ling and Levi, 2007), and fusarium wilt (Huh et al., 2001) has led to the identification of resistant accessions for these diseases. Only a few studies have been carried out for verticillium wilt on cucurbits although identification of accessions with verticillium wilt resistance could be helpful for rootstock development. For example, Wimer et al. (2015b) demonstrated that several germplasm accessions are resistant to verticillium wilt, but their suitability for use as rootstocks was not examined.
The rootstock as well as the scion can affect fruit quality of grafted watermelon. For instance, fruit TSS of ‘Sugar Baby’ was highest when grafted onto C. maxima × C. moschata, whereas ‘Crimson Sweet’ fruit had the greatest TSS when grafted onto L. siceraria compared with nongrafted fruit (Petropoulosa et al., 2012). In the same study, fruit from ‘Crimson Sweet’ grafted onto C. maxima × C. moschata had greater firmness than those from nongrafted treatments, but total soluble solids (TSS) and lycopene were similar to nongrafted watermelon. In contrast, TSS was reported to be lower on ‘Crimson Tide’ grafted to C. maxima and C. moschata (Yetışır et al., 2003), and ‘Extazy’ grafted onto Cucurbita rootstocks also had reduced TSS (Edelstein et al., 2014). Lycopene in seedless watermelon fruit from grafted plants has been found to be greater than from nongrafted plants (Liu et al., 2017). Kyriacou et al. (2015) reported that when compared with nongrafted seeded watermelon, watermelon grafted onto interspecific Cucurbita rootstock had greater lycopene content, and the rootstock accounted for ≈45% of its total variance.
The objective of this study was to identify cucurbit germplasm entries from the USDA NPGS that are resistant to verticillium wilt, compatible for grafting with watermelon, and have a positive impact on fruit quality. The potential outcomes of this study would expand the genetic diversity of rootstocks, reduce seed costs for rootstock, and increase access to rootstock seeds, all of which would aid adoption of vegetable grafting as an integrated pest management strategy for verticillium wilt.
AgWeatherNet2019The Washington agricultural weather network: Monthly weather data. Washington State Univ. 4 Feb. 2019. <https://weather.wsu.edu/?p=93150>
BekhradiF.KashiA.DelshadM.2011Effect of three cucurbits rootstocks on vegetative and yield of ‘Charleston Gray’ watermelonIntl. J. Plant Prod.5218
BerbegalM.OrtegaA.García-JiménezJ.ArmengolJ.2007Inoculum density-disease development relationship in verticillium wilt of artichoke caused by Verticillium dahliaePlant Dis.9111311136
BullerS.InglisD.MilesC.2013Plant growth, fruit yield and quality, and tolerance to verticillium wilt of grafted watermelon and tomato in field production in the Pacific NorthwestHortScience4810031009
DabirianS.InglisD.MilesC.A.2017Grafting watermelon and using plastic mulch to control verticillium wilt caused by Verticillium dahliae in WashingtonHortScience52349356
DavisA.R.Perkins-VeazieP.SakataY.López-GalarzaS.MarotoJ.V.LeeS.G.HuhY.C.SunZ.MiguelA.KingS.R.CohenR.2008Cucurbit graftingCrit. Rev. Plant Sci.275074
Economic Research Service2013United States Department of Agriculture. U.S. Watermelon Ind. 21 Mar. 2019. <https://usda.library.cornell.edu/concern/publications/zw12z528p?locale=en>
EdelsteinM.TyutyunikJ.FallikE.MeirA.TadmorY.CohenR.2014Horticultural evaluation of exotic watermelon germplasm as potential rootstocksScientia Hort.165196202
GalinatoS.P.MilesC.A.WimerJ.A.2016Non-grafted and grafted seedless watermelon transplants: Comparative economic feasibility analysis. Washington State Univ. Ext. Pub. TB08
GalinatoS.P.GallardoR.K.2017Cost analysis for vegetable grafting. In: C. Kubota C. Miles and X. Zhao (eds.). Chapter 6 Grafting Manual: How to produce grafted vegetable plants. 10 Feb. 2019. <http://www.vegetablegrafting.org/wp/wpcontent/uploads/2018/05/CostAnalysis6-22-17.pdf>
GoudJ.C.TermorshuizenA.J.GamsW.2003Morphology of Verticillium dahliae and V. tricorpus on semi-selective media used for the detection of V. dahliae in soilMycol. Res.107822830
HuberD.M.1980The role of mineral nutrition in defense p. 381–406. In: J.G. Horsefall and E.B. Cowling (eds.). Plant disease: An advanced treatise: How plants defend themselves. Academic Press New York
HuhY.C.OmY.H.LeeJ.M.2001Utilization of Citrullus germplasm with resistance to fusarium wilt (Fusarium oxysporum f. sp. niveum) for watermelon rootstocks. Proc. II Intl. Symp. Cucurbits 588:127–132
JohnsonS.J.MilesC.A.2011Effect of healing chamber design on the survival of grafted eggplant, tomato, and watermelonHortTechnology21752758
KubotaC.2016History of grafting chap. 1. In: C. Kubota C. Miles and X. Zhao (eds.). Grafting manual: How to produce grafted vegetable plants. 6 Mar. 2019. <http://www.vegetablegrafting.org/resources/grafting-manual/>
KyriacouM.C.SoteriouG.A.RouphaelY.SiomosA.S.GerasopoulosD.2015Configuration of watermelon fruit quality in response to rootstock-mediated harvest maturity and postharvest storageJ. Sci. Food Agr.9624002409
LewisM.KubotaC.TronstadR.SonY.J.2014Scenario-based cost analysis for vegetable grafting nurseries of different technologies and sizesHortScience49917930
LiuQ.ZhaoX.BrechtJ.K.SimsC.A.SanchezT.DufaultN.S.2017Fruit quality of seedless watermelon grafted onto squash rootstocks under different production systemsJ. Sci. Food Agr.9747044711
Martínez-BallestaM.C.Alcaraz-LópezC.MuriesB.Mota-CadenasC.CarvajalM.2010Physiological aspects of rootstock–scion interactionsScientia Hort.127112118
MendiburuF.D.2015Agricolae: Statistical procedures for agricultural research. R Package Version 1.2-3
MilesC.HesnaultL.JohnsonS.KreiderP.DabirianS.2016Vegetable grafting: Watermelon. Washington State Univ. Ext. Pub. FS100E. 15 Sept. 2019. <http://cru.cahe.wsu.edu/CEPublications/FS100E/FS100E.pdf>
MillerG.KhalilianA.AdelbergJ.W.FarahaniH.J.HassellR.L.WellsC.E.2013Grafted watermelon root length density and distribution under different soil moisture treatmentsHortScience4810211026
National Agricultural Statistics Service2019United States Department of Agriculture. Vegetables Summary 2018. 21 Mar. 2019. <https://usda.library.cornell.edu/concern/publications/02870v86p?locale=en>
ParoussiG.BletsosF.BardasG.A.KouvelosJ.A.KlonariA.2007Control of fusarium and verticillium wilt of watermelon by grafting and its effect on fruit yield and quality. Proc. III Balkan Symp. Vegetables Potatoes 729:281–285
PetropoulosaS.A.KhahbE.M.PassamcH.C.2012Evaluation of rootstocks for watermelon grafting with reference to plant development, yield and fruit qualityIntl. J. Plant Prod.617356814
PohlertT.2018PMCMRplus: Calculate pairwise multiple comparisons of mean rank sums extended. R package version 1.4.1
PscheidtJ.W.OcambC.M.2019Watermelon (Citrullus spp.) verticillium wilt. PNW Dis. Mgt. Handbook. Oregon State Univ. 7 Aug. 2019. <https://pnwhandbooks.org/plantdisease/host-disease/watermelon-citrullus-sp-verticillium-wilt>
R Core Team2013R: A language and environment for statistical computing. R Foundation for Statistical Computing Vienna Austria. <http://www.R-project.org>
ShanerG.FinneyR.E.1977The effect of nitrogen fertilization on the expression of slow-mildewing resistance in Knox wheatPhytopathology6710511056
United States Code2019Office of Law Revision Counsel Title 7. Ch. 80: Watermelon Research and Promotion Act. 20 Mar. 2019. <http://uscode.house.gov/view.xhtml?path=/prelim@title7/chapter80&edition=prelim>
U.S. Department of Agriculture2019Web soil survey. 3 Oct. 2019. <https://websoilsurvey.sc.egov.usda.gov>
WheelerD.L.JohnsonD.A.2016Verticillium dahliae infects, alters plant biomass, and produces inoculum on rotation cropsPhytopathology106602613
WimerJ.InglisD.MilesC.2015aEvaluating grafted watermelon for verticillium wilt severity, yield, and fruit quality in Washington StateHortScience5013321337
WimerJ.InglisD.MilesC.2015bField and greenhouse evaluations of cucurbit rootstocks to improve verticillium resistance for grafted watermelonHortScience5016251630
YetışırH.SariN.YücelS.2003Rootstock resistance to fusarium wilt and effect on watermelon fruit yield and qualityPhytoparasitica31163169