Grafting Watermelon to Control Verticillium Wilt Caused By Verticillium dahliae in Washington

Verticillium wilt, caused by the soilborne fungus Verticillium dahliae, is a significant disease affecting watermelon production in Washington State. In 2015, a field study was conducted at three locations in Washington to investigate the Verticillium wilt reactions, and fruit yield and quality of watermelon cv. TriX Palomar non-grafted (control) and grafted with rootstocks Super Shintosa, Tetsukabuto and Just grown with black and clear plastic mulch. At planting, soil in each main plot had less than 1 cfu^.g^-1 of soil V. dahliae at Eltopia and Othello and 28 cfu^.g^-1 of soil at Mount Vernon. After harvest V. dahliae soil density under black mulch increased to 4, 4.5 and 55 cfu^.g^-1 of soil at Eltopia, Othello, and Mount Vernon, respectively. In contrast, V. dahiae under clear mulch was similar to the level at planting at each location (<1, 2.6, and 27 cfu^.g^-1 of soil, respectively). Area under disease progress curve (AUDPC) values differed significantly among treatments at all locations. AUDPC was greater for plants grown with black mulch (385) than for plants grown with clear mulch (237). AUDPC was greatest at Mount Vernon (680), and lowest at Eltopia (71). And AUDPC was greater for non-grafted TriX Palomar (707) than for grafted TriX Palomar (average 201). There was a significant interaction between location and rootstock for fruit yield per plant. At Mount Vernon, where Verticillium wilt pressure was highest, yield of non-grafted TriX Palomar was lower (7.4 kg.plant^-1) than grafted treatments (average 13.0 kg.plant^-1). At Othello and Eltopia, there was no significant difference in yield due to grafting. At season’s end, V. dahliae presence in stems, was greatest for non-grafted TriX Palomar at Mount Vernon and Eltopia (87.50 and 89.06, repectively), and there was no difference at Othello (average 22.67). Fruit firmness was greater at Mount Vernon (3.00 N) than Othello and Eltopia (2.44 and 2.20 N, respectively) and was lower for non-grafted TriX Palomar (2.20 N) than for all 3 grafted treatments (average 2.67 N). Additionally, fruit firmness was greater for TriX Palomar grafted onto Tetsukabuto grown with black mulch; however there was no difference for fruit grown with clear mulch. At Mount Vernon TSS (%) was greater for non-grafted TriX Palomar (11.04%) and lowest for TriX Palomar grafted to Tetsukabuto and Super Shintosa (10.25% and 10.27%, respectively). There was no significant difference in TSS at Othello (average 10.58%) or Eltopia (average 10.28%).

Key words: Grafting, Verticillium wilt, Rootstock