Verticillium wilt (caused by V. dahliae) is an important soilborne disease that limits watermelon (C. lanatus) production in Washington State and worldwide (du Toit et al., 2005; Dung and Weiland, 2014; Johnson, 2012; Paplomatas et al., 2000; Paroussi et al., 2007; Sunseri and Johnson, 2001; Wimer et al., 2015). Once established in the field, V. dahliae is extremely difficult to manage because of its wide host range and long-lived survival structures (microsclerotia), which can persist in soil or plant debris for up to 14 years, and can remain viable up to 30 °C (Berlanger and Powelson, 2000; Klosterman et al., 2009; Tjamos, 1989). Losses from verticillium wilt have led at least one major Washington grower to discontinue watermelon production, and other growers have experienced yield reduction of 25% to 75% (J. Loos and M. Nelson, personal communication, 2015). Preplant soil fumigation with methyl bromide had been used for over 50 years to control V. dahliae (Carpenter et al., 2000; Klosterman et al., 2009); however, methyl bromide was eliminated from use in most countries under the Montreal Protocol and as part of the Clean Air Act in the United States (Carpenter et al., 2000; U.S. Environmental Protection Agency, 2015). Other available soil fumigants such as Telone (1,3-dichloropropene + chloropicrin) and Vapam HL (metam sodium) are not always reliable for controlling V. dahliae (Davis et al., 2008; Klosterman et al., 2009; Woodward et al., 2011). As there is no resistance in watermelon against V. dahliae, alternative strategies that are environmentally sustainable are critically needed to achieve successful management.
Commercial grafting of cucurbits originated in the 1920s in Japan with the primary intent of managing soilborne pathogens (Oda, 2007; Sakata et al., 2007). The practice of grafting was expanded as a means to improve management of abiotic stresses (e.g., high soil salinity, drought, high temperature) to reduce the reliance on chemical and fertilizer inputs, and enhance fruit quality (Colla et al., 2010; Proietti et al., 2008). Today, grafting cucurbitaceous crops is an important integrated pest management strategy used worldwide to control several soilborne pathogens, including V. dahliae (Buller et al., 2013; Cohen et al., 2007; Davis et al., 2008; Guan et al., 2012; Louws et al., 2010), but has not been widely adopted in the United States, primarily because it is perceived as expensive and growers have limited knowledge about grafting (Cushman, 2009; Davis et al., 2008; Leonardi and Romano, 2004). Yet, researchers have consistently demonstrated that grafting can reduce severity of V. dahliae in watermelon (Johnson, 2012; King et al., 2008; Louws et al., 2010; Wimer et al., 2015). Grafting watermelon onto disease-tolerant rootstocks can delay onset of verticillium wilt symptoms by up to 3 weeks, thus allowing sufficient time for crop maturation (Paplomatas et al., 2002; Wimer et al., 2015). Paroussi et al. (2007) observed that verticillium wilt incidence decreased on grafted watermelon plants grown in artificially inoculated soil, and Buller et al. (2013) and Wimer et al. (2015) both observed reduced verticillium wilt severity of grafted watermelon plants at a naturally infested field site in western Washington harboring a relatively high soil population of V. dahliae (18.0 cfu/g).
In addition to grafting, the use of plastic mulch is another alternative disease management practice. Although black polyethylene mulch is the standard mulch type used in vegetable production (Gordon et al., 2010), clear plastic mulch absorbs 5%, reflects 11%, and transmits 84% of radiation, thereby leading to higher soil temperature under the mulch (Gough, 2001; Tarara, 2000). Soil solarization can occur when temperature exceeds 30 °C and has been shown to be effective in controlling soilborne pathogens and nematodes, as well as weed seeds and seedlings (Elmore et al., 1997). Studies show that by using a clear plastic tarp on soil to achieve solarization, verticillium and fusarium wilts of several crops can be successfully controlled (Ashworth and Gaona 1982; Katan, 1984). Moreover, Rajablarijani and Aghaalikhani (2011) indicated that plants grown with clear plastic mulch had the highest early yields compared with plants grown with black and silver/black plastic mulch. In Washington, some watermelon growers sometimes use clear plastic mulch to increase plant growth rate, thereby decreasing days to maturity. The objective of this study was to investigate the efficacy of grafting onto disease-resistant rootstocks and using clear plastic mulch for controlling verticillium wilt in watermelon.
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