Grafting eggplant and tomato for Verticillium wilt resistance

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.