Fungicide Options for Powdery Mildew Management in Organic Cucurbits

Powdery mildew colonies on the underside of a pumpkin leaf. Fungicide applications should start when these colonies are first observed during scouting. It is important to check both surfaces of the leaves. Photo by Josh Amrhein.

Powdery mildew will soon be appearing on squash, pumpkins, and other cucurbits.  Organic growers should always start with varieties with some degree of resistance to powdery mildew – seed catalogues often call partial resistance “tolerance”.  Although resistance will generally not be complete, efforts to manage powdery mildew with organic-acceptable products will be more productive if growers start with a variety that can put up a fight on its own than one that is highly susceptible.

In 2021 we evaluated  OMRI-approved fungicides for efficacy against powdery  mildew  in our standard bioassay. Young pumpkin plants were produced in a greenhouse, one plant per pot, sprayed with a fungicide, and taken to a pumpkin field with active powdery mildew.  After about  24 hours of exposure, the plants were returned to the greenhouse and evaluated for disease development 7 and 10 days after exposure.

Seven days after exposure (DAE) to powdery mildew in the field, disease severity was moderate (24.2%) on non-treated control bioassay plants. All of the OMRI-listed products significantly reduced powdery mildew severity compared to the non-treated control. Regalia, Milstop, and Microthiol Disperss reduced disease severity to <1%, providing 96-100% control, significantly lower than severity on plants treated with Badge X2 (7.8%; 68% control) or Serifel (9.5%; 61% control). Powdery mildew severity on pumpkin plants treated with Sonata (4.2%; 83% control) was statistically similar to that of plants treated with Milstop (1%; 96% control), Badge X2 and Serifel. Ten DAE to powdery mildew, disease severity on non-treated control plants increased to 42.1%. Powdery mildew severity was lowest on plants treated with Regalia (0.6%) and Microthiol Disperss (0%), followed by Milstop (9.8%), and Sonata (23%), corresponding to 99, 100, 77 and 45% control, respectively. Treatment with Serifel or Badge X2 did not significantly reduce powdery mildew on pumpkin plants 10 DAE compared to the non-treated control.

Best results are obtained when these products are used preventatively or at the very first signs of powdery mildew (see photo above), usually in mid-July in Ohio.  If you wait until powdery mildew has progressed significantly, it will probably be too late to get it under control.

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