Alert: First Report of Pumpkin Downy Mildew in Ohio and the Great Lakes Region

CDM.ipmpipe map of pumpkin downy mildew reports, August 10, 2022

We have had a confirmed report today from Bill Holdsworth of Rupp Seeds of downy mildew on pumpkins in research plots near Wauseon, Ohio, in Fulton County. This is the first report of downy mildew on pumpkins, not only in Ohio but in the Great Lakes region and in fact a big swath of the Midwest and Northeast

CDM.ipmpipe map of cucumber downy mildew reports, August 10, 2022

(map on left). Compare this to the cucumber downy mildew map on the right, with widespread distribution. As a reminder, these maps are constructed from voluntary reports of downy mildew. The disease is likely more widespread than the maps show. Bill has also seen downy mildew on gourds in the area.

The downy mildew pathogen, Pseudoperonospora cubensis, exists as two clades. Clade 2 is quite specific to cucumbers and melons and tends to circulate early in the Great Lakes region, having possibly overwintered on cucumbers in greenhouses. It also makes its way up the Eastern US from southeastern states. Clade 1 has a broader host range, attacking all cucurbits, and also originates in the Southeast, moving up the through the eastern states. Due to typical wind currents from west to east, we don’t usually see clade 1 outbreaks in the Midwest until mid-August or September, when spores can be carried in storms that are remnants of hurricanes originating in the Southeast. We don’t know with certainty yet if the Fulton County outbreak was caused by clade 1 strains, but this is likely. We hope to conduct molecular tests to confirm the clade in the near future.

Downy mildew symptoms on a pumpkin leaf (top)

Downy mildew symptoms on the underside of a pumpkin leaf.

Downy mildew can be a bit harder to diagnose in pumpkins, squash and other Cucurbita species since the lesions may be smaller and not always crisply angular as in cucumbers. Downy mildew can also cause a lot of damage on pumpkins, squash, gourds and other Cucurbita spp.

Growers should be scouting all cucurbits for symptoms of downy mildew. The best time to do this is in the morning before 9 or 10 am before the lesions dry out and the sporangia disperse. If you find suspicious symptoms you may text me photos of the underside and top of symptomatic leaves (330-466-5249) and/or send us samples for confirmation.

Growers of any cucurbits throughout Ohio should apply appropriate protectant fungicides such as chlorothalanil (e.g. Bravo). Growers in Fulton County should begin a fungicide program with downy mildew-effective fungicides now. See my July 11 post for a list of recommended fungicides.

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