Verticillium wilt caused by Verticillium dahliae is a serious disease for watermelon growers in Washington State. Grafting represents a possible alternative disease management strategy, but little is known about rootstock resistance to verticillium wilt or the performance of grafted watermelon in the different production regions of the state. In this study, verticillium wilt severity, yield, and fruit quality were evaluated at three contrasting field sites in Washington using verticillium wilt-susceptible ‘Sugar Baby’ (diploid) watermelon grafted onto four commercial rootstock cultivars (Marvel, Rampart, Tetsukabuto, and Titan); nongrafted ‘Sugar Baby’ was included as the control. Verticillium dahliae soil densities varied at each site (<1.0, 5.7, and 18.0 colony-forming units (cfu)/g soil at Othello, Eltopia, and Mount Vernon, respectively). Area under disease progress curve (AUDPC) values differed significantly among treatments at Eltopia and Mount Vernon. Nongrafted ‘Sugar Baby’ had the highest AUDPC value at all three sites, while ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had the lowest AUDPC value at Eltopia and Mount Vernon. Nongrafted ‘Sugar Baby’ also had the lowest fruit weight per plant at all sites, but ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had the highest fruit weight per plant at Eltopia and Mount Vernon. Marketable fruit weight per plant did not differ among treatments at Othello. Yield was negatively correlated with AUDPC values at both Eltopia and Mount Vernon. Fruit number per plant was only significantly impacted at Eltopia, where ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had more fruit per plant than all other treatments except ‘Sugar Baby’ grafted onto ‘Rampart’. Fruit quality (flesh firmness, total soluble solids, and lycopene content) was unaffected by grafting at either Othello or Eltopia, except for increased flesh firmness for ‘Sugar Baby’ grafted onto ‘Marvel’ and ‘Titan’ as compared with nongrafted ‘Sugar Baby’ at Eltopia. At season’s end, plants were sampled from all treatments at Eltopia and Mount Vernon and assayed for V. dahliae. Microsclerotia typical of this organism were observed in all samples. Results from this study indicate that verticillium wilt of watermelon can be successfully managed by grafting when the V. dahliae soil density exceeds 5.0 cfu/g in Washington. In addition, grafting does not reduce fruit quality and using certain rootstocks can improve the quality of flesh firmness at certain locations.

Grafting watermelon (Citrullus lanatus Thunb.) onto resistant rootstocks is used in many areas of the world to overcome soilborne disease losses including verticillium wilt caused by Verticillium dahliae Kleb. Currently, this disease poses a serious risk to watermelon growers in Washington State. To identify resistant rootstocks, the verticillium wilt reactions (chlorosis, necrosis, and wilting) of 14 nongrafted PI accessions including Benincasa hispida Thunb., Cucurbita moschata Duchesne ex Poir., and Lagenaria siceraria Molina Standl. from the U.S. Department of Agriculture National Plant Germplasm System (USDA NPGS); 11 nongrafted commercially available rootstocks; and, nongrafted ‘Sugar Baby’ watermelon (verticillium wilt–susceptible control) were visually assessed in a field naturally infested with V. dahliae at a rate of 17 colony-forming units (cfu) per gram of soil. Typical symptoms of verticillium wilt were observed on all entries. ‘Sugar Baby’ had the highest relative area under disease progress curve (RAUDPC) value (26.80), which was not significantly different from ‘64-19 RZ’, ‘Marvel’, PI 368638, PI 634982, and PI 642045 (average = 10.16). PI 419060 (1.46) had the lowest RAUDPC value, which was not significantly different from ‘Miniature Bottle Gourd’, PI 326320, PI 419016, PI 536494, PI 636137, ‘Strong Tosa’, ‘Strongtosa’, and ‘TZ 148’ (average = 3.36). The mean RAUDPC value of PI accessions (5.49) did not differ significantly from the mean value of the commercial rootstocks (5.68). Microsclerotia typical of Verticillium spp. were observed in the stems of all but one entry (PI 181913). In a greenhouse study, a subset of 12 entries were inoculated with V. dahliae, and by 22 days after inoculation (DAI), ‘Sugar Baby’ had a significantly higher disease rating than all entries except PI 419060, PI 438548, and ‘Titan’. A strong positive correlation was observed between the field and greenhouse studies. Results indicate that commercial rootstocks as well as PI accessions could be used to successfully manage verticillium wilt in Washington; however, grafting compatibility with watermelon must first be ascertained for the promising PI accessions. Although greenhouse-based verticillium wilt assays can be used to help predict rootstock performance in the field, accurate assessment may require manipulating environmental conditions (e.g., temperature and humidity) to approximate field conditions.

Verticillium dahliae affects many crops in Washington State, and in 2011 was present at 3-4 cfu. g-1 soil in two sampled eggplant (Solanum melongena) and tomato (Solanum lycopersicum) fields in the Columbia Basin of eastern Washington, and at 18 cfu. g-1 soil in one northwestern Washington vegetable field. Field studies showed that 'Epic' eggplant was susceptible to Verticillium wilt at both locations, but 'Cherokee Purple' heirloom tomato was not. Grafting 'Epic' eggplant onto 'Beaufort' rootstock decreased eggplant wilt severity by 25% (P≤0.05), even though microsclerotia of the pathogen were recovered from all stems, indicating this rootstock is tolerant but not resistant to the pathogen. Grafting eggplant onto Solanum aethiopicum rootstock did not decrease disease severity as compared to the susceptible non-grafted control. In a replicated study in 2013 at the 18 cfu. g-1 soil field, 'Millionaire' eggplant grafted onto 'Java', 'Red Scorpion', and 'Meet' rootstocks all had comparable Verticillium wilt incidence (98-100%; P=0.52) and severity (34-40%; P=0.83) relative to non-grafted 'Millionaire' (the susceptible control) at the end of the growing season. Area under disease progress curve (AUDPC) values based on seven weekly disease ratings ranged from 1480 to 1777, and did not differ significantly among treatments (P=0.84). For tomato at the same field site, 'Stupice' grafted onto 'Beaufort', 'Maxifort', 'Colossus', 'Supernatural', 'Dro138TX', and 'Estamino' rootstocks showed 5% disease severity on average throughout the growing season, and did not differ significantly from non-grafted 'Stupice' (the susceptible control) (P=0.60). AUDPC values ranged from 123 to 295, and again did not differ significantly among treatments (P=0.37). However, foliar symptoms on tomato in these studies were not typical of Verticillium wilt. In 2014, additional rootstocks will be evaluated to identify resistance/tolerance to Verticillium wilt for eggplant, and heirloom tomato cultivars will be further observed for Verticillium wilt symptom expression.

Figure 4. Mean RAUDPC values for each entry. Means separation letters are based on rank transformed data.

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