Evidence for fungicide-resistant seed-borne inoculum for gummy stem blight of watermelon.
The fungus Didymella bryoniae, which causes gummy stem blight (GSB), a very destructive disease of watermelon, has a history of developing resistance to systemic fungicides used for disease management. Fungicide resistance management is based on the assumption that resistance develops in populations repeatedly exposed to high-risk fungicides. However, unexpected occurrences of resistance in fields where fungicides have never been used suggest the introduction of resistant isolates from an external source. This research was conducted to evaluate watermelon seed as a potential source of fungicide-resistant inoculum. Using direct plating, sweat box and blotter assays, 5800 seeds from commercial watermelon seed lots were examined for signs and symptoms of GSB. Three isolates of D. bryoniae were obtained from two seed lots and were tested in vitro for their sensitivity to boscalid, tebuconazole, azoxystrobin and thiophanate-methyl, four fungicides commonly used for GSB management. The isolates were found to be resistant to thiophanate-methyl. This is the first report of isolation of D. bryoniae from naturally infested commercial watermelon seeds and the first report documenting that seed can harbour fungicide-resistant D. bryoniae. These results warrant a new approach to fungicide resistance management in watermelon production.