2018 ASHS Annual Conference

Verticillium wilt, caused by Verticillium dahliae, is a destructive soilborne disease that limits watermelon production worldwide. Control of this disease is becoming challenging for watermelon producers in Washington state due to the widespread distribution of the pathogen, the persistence of its microsclerotia in soil for many years, and phase-out of soil fumigants such as methyl bromide. Grafting watermelon onto resistant rootstock represents a promising disease management strategy, but there is little information regarding rootstock resistance to verticillium wilt. Evaluating rootstocks for resistance to V. dahliae is the first step in developing a management strategy for controlling this pathogen via the use of grafted plants. This study investigated the reactions of seven commercially available rootstocks, ‘Tetsukabuto’, ‘Super Shintosa’, ‘Java’, ‘Flexifort’, ‘Shintosa Camelforce’, ‘Cobalt’ and ‘Carnivor’, and seedless watermelon ‘Fascination’ (verticillium wilt- susceptible control) to V. dahliae in a naturally infested field (5 cfu/g of soil) at Washington State University Northwestern Washington Research and Extension Center. To evaluate the disease severity in combination with crop management, we tested black and clear plastic mulch, and two irrigation treatments, management allowance depletion (MAD) water application vs. scheduled water applications through drip irrigation. The irrigation time difference between scheduled and MAD treatment was 30-40 minutes per irrigation application, and overall, 33% less water was applied in MAD treatment compared to scheduled treatment. Area under disease progress curve (AUDPC) values did not differ significantly due to irrigation treatment or mulch treatment. The overall AUDPC value for verticillium wilt was higher for non-grafted watermelon than for the seven rootstock treatments (P = 0.0006). Results indicate that verticillium wilt severity was greater for ‘Fascination’ than for the seven rootstock treatments at the end of the growing season, but irrigation treatment and type of plastic mulch did not significantly effect on the disease severity in this study.

Keywords. Verticillium dahliae, soilborne disease, grafting, AUDPC