Grafting watermelon to manage verticillium wilt in Washington state

Jesse Aaron Wimer

Verticillium wilt caused by Verticillium dahliae is a soilborne disease affecting watermelon production in Washington State. Although grafting has been shown to reduce soilborne disease severity of watermelon, not much is known about the efficacy of grafting for managing Verticillium wilt. In Year 1, potential watermelon rootstocks were screened for Verticillium wilt resistance in a naturally infested field at WSU Mount Vernon. One grafted watermelon entry was also included, as were two Verticillium wilt-susceptible non-grafted watermelon controls (‘Sugar Baby’ and ‘Crimson Sweet’). Typical Verticillium wilt symptoms were observed on all entries, and 21 had significantly lower disease severity than the controls. A follow-up greenhouse experiment tested 11 of the rootstocks and ‘Sugar Baby’ against V. dahliae. At 22 days after inoculation, ‘Sugar Baby’ had a significantly higher disease rating than most entries. Results from Year 1 suggest that watermelon is susceptible to Verticillium wilt, while most of the potential rootstocks are tolerant. In Year 2, grafted watermelon plants were evaluated in fields near Othello, Eltopia, and Mount Vernon, WA, where V. dahliae soil densities were low, moderate, and high, respectively, and the climates/growing conditions ranged from dryland irrigated to cool maritime. The treatments were non-grafted ‘Sugar Baby’ and ‘Sugar Baby’ grafted onto the rootstocks ‘Marvel’, ‘Rampart’, ‘Tetsukabuto’, and ‘Titan’. At Othello, there were no significant differences in disease severity or marketable yield among treatments. At Eltopia, non-grafted ‘Sugar Baby’ had significantly higher Verticillium wilt severity than ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ and ‘Rampart’, and significantly lower marketable yield than all treatments. Fruit quality was unaffected by grafting other than increased flesh firmness observed for ‘Sugar Baby’ grafted onto ‘Marvel’ and ‘Titan’ at Eltopia. At Mount Vernon, all grafted treatments had significantly lower Verticillium wilt severity than non-grafted ‘Sugar Baby’. Fruit did not reach full maturity at Mount Vernon, but non-grafted ‘Sugar Baby’ had significantly lower immature fruit yield than all treatments except ‘Sugar Baby’ grafted onto ‘Rampart’. Results from Year 2 suggest that grafting can be used to effectively manage Verticillium wilt in Washington when the pathogen density is moderate or high, and can increase yield while maintaining fruit quality.

Grafting watermelon to manage verticillium wilt in Washington state

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