A strain of tomato spotted wilt virus that has the ability to overcome the resistance of the Sw5 gene is continuing to give growers issues. Beginning in 2016 significant levels of tomato spotted wilt was seen in production areas of Fresno County, with plants showing symptoms at levels between 30 and 50 percent.
“The genetics of the virus infecting those plants was consistent with a resistance-breaking strain reported in Spain several years earlier. This was the first report of a resistance-breaking strain of tomato spotted wilt virus in the continental U.S. and it was in Fresno County,” said Tom Turini, vegetable crops Farm Advisor for Fresno County. “The area where we’ve identified the resistance-breaking strain has expanded annually.”
After dealing with the virus for about 15 years, the industry developed best practices for integrated pest management, sanitation, and planting strategy. The Sw5 gene was a significant breakthrough in keeping plants safe from the virus. “We got to the point where the best varieties had resistance to spotted wilt and there was large scale adoption of those varieties to the point where actually the vast majority now is Sw5 in both fresh market and processing tomatoes,” said Turini.
Research will continue through this season, with testing alternative varieties for resistance capabilities that can be effective against the resistance-breaking strain of the virus. “What we’re doing at this point is both looking at short-term and long-term management strategies. We’re going back to some of what we did in terms of IPM strategies before we had the resistance gene and that’s still going to be our best tactic,” Turini noted. “We still have a lot of areas where we haven’t reported this resistance-breaking strain, so certainly the Sw5 still has value, but I would caution against total reliance on Sw5 at this point.”