Controlling powdery mildew on cucurbit rootstock seedlings in the greenhouse with fungicides and biofungicides

Abstract Powdery mildew ( Podosphaera xanthii ) affects seedlings of inter-specific hybrid squash ( Cucurbita moschata  ×  Cucurbita maxima ) and bottle gourd ( Lagenaria siceraria ) used as rootstocks to graft seedless watermelon ( Citrullus lanatus var. lanatus ). Because powdery mildew grows primarily on the leaf surface where contact fungicides are effective, biofungicides may be effective preventative treatments for powdery mildew. The objectives of this study were to determine which biofungicides, organic fungicides, and conventional synthetic fungicides provided the best control of powdery mildew and least phytotoxicity on cucurbit rootstock seedlings in the greenhouse. Sixteen treatments (six biopesticides, four additional organic-approved fungicides, and six conventional synthetic fungicides) were tested. Four experiments were conducted and all were repeated once. Hybrid squash ‘Strong Tosa’ seedlings were used in the first three experiments, and bottle gourd ‘Emphasis’ seedlings were used in experiment four. In experiments one, two, and four, seedlings were sprayed three times at 5-day intervals and exposed to powdery mildew continuously after the first application. In the third experiment, seedlings were exposed to inoculum for 7 days, sprayed once, and held in a humidity chamber for 7 days under conditions used for healing after grafting. The most effective organic-approved fungicides were sulfur and fish oil + sesame oil, and the most effective conventional fungicides were penthiopyrad, myclobutanil, and cyprodinil plus fludioxonil. Quinoxyfen was phytotoxic to cotyledons of both species, and tebuconazole stunted both species. To manage powdery mildew, one or two preventative applications of sulfur or fish oil + sesame oil and one application of myclobutanil or penthiopyrad, if needed, are recommended.