Grafting Watermelon to Manage Verticillium Wilt in Washington State

Grafting watermelon (Citrullus lanatus) for soilborne disease control is common in many areas of the world but has not yet been widely adopted in the United States. This 2014 study investigated the use of grafting to manage Verticillium wilt (caused by V. dahliae) at three diverse field locations: Othello and Eltopia in the Columbia Basin of eastern Washington, and Mount Vernon in northwestern Washington. The V. dahliae population density was estimated to be <1.0, 5.7, and 18.0 colony forming units per gram (cfu·g^-1) of soil at Othello, Eltopia, and Mount Vernon, respectively. At each site, the experiment was arranged as a randomized complete block with three replications, and the treatments consisted of Verticillium wilt-susceptible ‘Sugar Baby’ non-grafted and grafted onto commercial rootstocks ‘Marvel’, ‘Rampart’, ‘Tetsukabuto’, and ‘Titan’. Plots were rated visually for Verticillium wilt 4–5 times beginning at symptom onset, and fruit yield and quality were assessed. Area under disease progress curve (AUDPC) values differed significantly among treatments at Eltopia and Mount Vernon, with non-grafted ‘Sugar Baby’ having the highest AUDPC value (most severe disease) and ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ having the lowest. At Othello, where disease pressure was lowest, AUDPC values did not differ significantly among treatments. Non-grafted ‘Sugar Baby’ had significantly lower marketable fruit weight per plant than all other treatments at Eltopia, while at Othello marketable fruit weight per plant did not differ significantly among treatments. Total soluble solids (TSS) differed significantly among treatments at Eltopia and Othello, although none were significantly different than non-grafted ‘Sugar Baby’. Flesh firmness was significantly increased at Eltopia by grafting ‘Sugar Baby’ onto ‘Titan’ and ‘Marvel’, but there was no difference at Othello. Lycopene content did not differ significantly among treatments at Eltopia or Othello. At Mount Vernon, fruit did not reach maturity and only total fruit weight was measured; ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had significantly greater total fruit weight per plant than all other treatments. All treatments at Eltopia and Mount Vernon were assayed for Verticillium spp. (at Othello, plants were removed by the grower prior to assay), and microsclerotia characteristic of V. dahliae were observed in all treatment samples. Results indicate that watermelon grafting can be used effectively to manage Verticillium wilt of watermelon in areas of Washington where the V. dahliae soil density exceeds 5.0 cfu·g^-1. Furthermore, grafting does not lead to reduced fruit quality and certain rootstock-scion combinations can actually improve flesh firmness.