Throughout the 20th century, watermelon (C. lanatus Thunb.) grafting was adopted in many areas of the world to manage soilborne diseases, primarily fusarium wilt caused by Fusarium oxysporum Schlecht. (Davis et al., 2008; King et al., 2008; Miguel et al., 2004; Sakata et al., 2007). Other soilborne diseases of watermelon can also be controlled by grafting, such as monosporascus vine decline (caused by Monosporascus cannonballus Pollack & Uecker), phytophthora crown rot (caused by Phytophthora capsici Leonian), and verticillium wilt (caused by Verticillium dahliae Kleb.) (Louws et al., 2010). In Washington, verticillium wilt is problematic for watermelon growers due to the widespread distribution of the pathogen (Sunseri and Johnson, 2001). To complicate matters, V. dahliae has a wide host range and can persist in the soil for long periods (Berlanger and Powelson, 2000). Farmers have traditionally used soil fumigation to control this pathogen, but some products (e.g., methyl bromide) are no longer available, whereas others have questionable efficacy (Klosterman et al., 2009; U.S. Environmental Protection Agency, 2013; Woodward et al., 2011). Thus, grafting onto disease-resistant or -tolerant rootstocks may be the only feasible way to manage severe infestations.
The first step in developing a successful management strategy for verticillium wilt in watermelon via the use of grafted plants is to evaluate rootstocks for resistance to V. dahliae. Paplomatas et al. (2000) are the only researchers to date that have carried out such an experiment. These researchers screened 33 cucurbit rootstocks for resistance to V. dahliae in Greece and found some degree of tolerance in zucchini, pumpkin, and bottle gourd, but none of the tested rootstocks were resistant. Of the many commercial watermelon rootstocks on the market, none have been sufficiently tested for verticillium wilt resistance or tolerance in Washington. The USDA NPGS contains over 1000 PI accessions in the Cucurbit family, including several genera and species that are used for commercial rootstocks for watermelon. Among these are B. hispida Thunb., Cucurbita maxima Duchesne, C. moschata Duchesne ex Poir., and L. siceraria Molina Standl. There are additional genera and species that may be suitable for use as rootstocks, but are so far untested. An extensive search for PI accessions with resistance or tolerance to specific pathogens is much needed and could result in the development of affordable and regionally adapted rootstocks in the United States (Lee et al., 1998).
Although Paplomatas et al. (2002), Paroussi et al. (2007), Buller et al. (2013), and Wimer et al. (2015) have demonstrated that grafting can significantly reduce verticillium wilt of watermelon, no commercial growers in Washington are currently using grafted plants to manage this disease. Adoption of this practice is hindered by the increased costs of grafted seedling production attributed to the extra (and often expensive) seed needed to grow the rootstock, the labor needed to perform the grafting and care for the grafted seedlings, and the special facilities that are required for proper graft union formation (Boughalleb et al., 2007; King et al., 2010; Lee et al., 2010). In addition, there is currently a lack of published information regarding the performance of commercially available rootstocks in specific environments (Kubota et al., 2008). For large-scale growers in Washington (each of whom requires tens of thousands of plants), adoption of this practice is contingent on the resolution of these issues.
To identify verticillium wilt–resistant rootstocks, an efficient disease evaluation system is needed. Field assays can be used for this purpose, but they tend to be lengthy and confined to each growing season’s constraints (Bae et al., 2011). Greenhouse assays, however, are rapid and can be conducted most times of the year. Bae et al. (2011) developed a greenhouse assay to screen potato seedlings for verticillium wilt and demonstrated that resistance, tolerance, and susceptibility to this disease can be accurately assessed. The protocol used in the Bae et al., study involved submerging exposed roots into a V. dahliae suspension. Dervis et al. (2009), Boughalleb et al. (2007), Paplomatas et al. (2000), and others used a similar procedure to test the pathogenicity of various pathogens on multiple cucurbit crops in the greenhouse. Although such greenhouse studies can be useful in determining resistance, tolerance, or susceptibility, the correlation with field studies can vary considerably (Zhang et al., 1997). Thus, greenhouse assays that can be used to predict field performance need to be developed to ensure cost-effective and efficacious rootstock development.
One objective of this study was to identify commercially available rootstocks and PI accessions that could be used to manage verticillium wilt of watermelon in Washington. The second objective was to compare the verticillium wilt reactions of entries in field and greenhouse assays. To achieve these objectives, plants were first evaluated in a field naturally infested with V. dahliae, then in the greenhouse using artificial inoculation.
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