Optimizing watermelon grafting to control verticillium wilt in Washington

Sahar Dabirian

Verticillium wilt, caused by the soilborne fungus Verticillium dahliae, negatively impact watermelon (Citrullus lanatus) production in Washington State. Although grafting can reduce Verticillium wilt severity, the cost of grafted transplants limits adoption. In this thesis project, one field study and two greenhouse studies were carried out with grafted watermelon plants. The field study was conducted at three locations in 2015 to evaluate the reactions of grafted vs. nongrafted watermelon plants grown with clear or black plastic mulch to V. dahliae and impact on fruit yield and quality. Treatments in this study were non-grafted watermelon 'TriX Palomar' and 'TriX Palomar' grafted onto the commercial rootstocks 'Super Shintosa', 'Tetsukabuto', and 'Just'. Results showed that grafted watermelon had lower Verticillium wilt severity than nongrafted plants, and disease severity tended to be lower for plants grown with clear plastic mulch than with black plastic mulch. The two greenhouse studies were conducted in 2015 and 2016 to test whether antitranspirants or sucrose solution could increase the survival of grafted watermelon transplants. In the first greenhouse study, two commercial antitranspirant products, Moisturin and Root-Zone, were applied to 'TriX Palomar' watermelon grafted onto 'Tetsukabuto' in 2015 and 'Emphasis' in 2016 using the one-cotyledon method. Results showed that survival of watermelon transplants increased about 25% by application of Root-Zone to both the scion and rootstock seedlings before grafting. Additional studies are needed to evaluate other rootstock-scion combinations and if both the scion and rootstock seedlings need to be treated. The second greenhouse study was carried out in 2016 to test whether sucrose solution (0%, 1%, 2%, 3%) applied to rootstock seedlings (cv. Tetsukabuto) prior to grafting can increase the survival of grafted watermelon transplants when the splice grafting method is used (both cotyledons removed from the rootstock). Results showed that starch levels increased in rootstock seedlings with sucrose application, and survival of watermelon transplants increased 31% with 2% sucrose solution. Increasing the survival of grafted watermelon transplants along with increasing the ease of grafting and eliminating rootstock regrowth can reduce the cost of grafted watermelon transplant production, which may increase the use of grafted watermelon.

Optimizing watermelon grafting to control verticillium wilt in Washington

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