Susceptibility of Cucurbit Rootstocks to Didymella bryoniae and Control of Gummy Stem Blight on Grafted Watermelon Seedlings with Fungicides.

Seedlings of watermelon are susceptible to Didymella bryoniae, the cucurbit pathogen that causes gummy stem blight, particularly when they are grown in the greenhouse for use as transplants. Seedlings of bottle gourd (Lagenaria siceraria) and interspecific hybrid squash (Cucurbita moschata × C. maxima) that are used as rootstocks for grafting watermelon are susceptible to gummy stem blight when wounded. Nonwounded rootstock seedlings of both genera were as susceptible to gummy stem blight as seedless watermelon. Because grafted plants must be misted or held at high relative humidity for 1 week so the graft union will heal, fungicides may be necessary to manage gummy stem blight under these disease-conducive environmental conditions. Nine fungicides were applied as foliar treatments at labeled rates per 467 liters/ha water to nongrafted seedlings of watermelon and five rootstock cultivars. Fluopyram + tebuconazole injured all five bottle gourd and hybrid squash cultivars and stunted watermelon and hybrid squash seedlings. Cyprodinil + difenoconazole injured all five rootstock cultivars and watermelon. Tebuconazole stunted bottle gourd and watermelon seedlings. Four of the five fungicides that were not phytotoxic reduced incidence and severity of gummy stem blight on seedless watermelon grafted onto bottle gourd Emphasis and hybrid squash Strong Tosa. Difenoconazole and cyprodinil were more effective than mancozeb or cyprodinil + fludioxonil, which were more effective than thiophanate-methyl, which was not significantly different from the water control (P = 0.01). Nongrafted watermelon seedlings and watermelon seedlings grafted onto watermelon as the rootstock were as susceptible to gummy stem blight as watermelon seedlings grafted onto cucurbits. Although difenoconazole and cyprodinil are not registered currently on cucurbits, transplant growers can apply mancozeb or cyprodinil + fludioxonil to manage gummy stem blight on watermelon and rootstock seedlings during greenhouse production.