Evaluation of Watermelon Rootstocks for Resistance to Verticillium Wilt in Northwestern Washington State

Watermelon (Citrullus lanatus) grafting is common in areas of the world where production is affected by soilborne diseases. One such disease is Verticillium wilt caused by V. dahliae. Watermelon does not have known resistance to this pathogen and chemical control options are limited. Verticillium wilt has the potential to become increasingly problematic for watermelon growers throughout Washington State due to the long-term persistence of the pathogen in the soil. This 2013 study investigated the reactions of 11 commercially available non-grafted rootstocks and 14 non-grafted cucurbit species which have the potential to be used as rootstocks. This study was carried out in a field naturally infested with Verticillium spp. Entries were obtained from various seed companies as well as the USDA National Plant Germplasm System (NPGS). One watermelon entry, ‘Crimson Sweet’ grafted onto ‘Shintoza’, and two non-grafted watermelon entries ‘Crimson Sweet’ and ‘Sugar Baby’ also were included. The experiment was arranged as a randomized complete block design with three replications; transplanting occurred from 12 June to 21 June.  Plots were rated for Verticillium wilt severity once per week from 2 Aug through 18 Sep. ‘Sugar Baby’ and ‘Crimson Sweet’ had the highest relative area under disease progress curve (RAUDPC) values (26.8 and 15.6, respectively), while PI 419060 and ‘Crimson Sweet’ grafted onto ‘Shintoza’ had the lowest RAUDPC values (1.5 and 2.0, respectively). Twenty-one of the entries—10 commercial varieties and 11 germplasm accessions—had RAUDPC values lower than the overall mean (6.6). In addition, 5 commercial varieties and 6 germplasm accessions had RAUDPC values lower than 5, which was identified as a threshold of acceptable sustained disease severity in our study. The mean RAUDPC value for commercial varieties (5.7) was not significantly different from the mean RAUDPC value for germplasm accessions (5.5). One plant per entry was assayed for V. dahliae using a stem incubation method. Microsclerotia were observed on all samples except those of PI 181913 ‘Hama No 9 Long’ and ‘Crimson Sweet’ grafted onto ‘Shintoza’. Eighteen Verticillium isolates were obtained and eight were sent to the WSU Molecular Laboratory for species identification. Isolates obtained from ‘Crimson Sweet’, ‘Sugar Baby’ and PI 419060 were identified as V. dahliae, while the remaining five were V. isaacii. A follow-up greenhouse inoculation study will be conducted using selected rootstock entries and results from both field and greenhouse will be used to designate rootstocks as resistant, tolerant or susceptible to Verticillium wilt.