Downy Mildew Now Widespread in Ohio Cucurbits

Our first report in 2016 of downy mildew on pumpkins in Ohio came from Pike County, in the southern part of the state. Late last week we found downy mildew on pumpkins and squash in our sentinel plot in Wooster (Wayne County) for the first time this year. At the same time we found downy mildew in pumpkins in research plots at Waterman Farm on the OSU campus (Franklin County) and OSU-OARDC’s Western Agricultural Research Station in South Charleston (WARS; Clark County).  We also found downy mildew on cucumbers at the WARS and Waterman Farm locations for the first time this year.

Our monitoring of downy mildew outbreaks is not complete – while we frequently check our sentinel and research plots in several locations and diagnose samples from multiple Ohio locations, we haven’t confirmed the disease in most Ohio counties despite the fact that it is likely to be present.  Nonetheless, these observations confirm what we have been seeing the last few years – there are likely at least two populations of the downy mildew pathogen in Ohio.  These populations probably have different sources and different host ranges.

The northern population has a high affinity for cucumbers and melons and appears in northern Ohio by the 4th of July in most years (blue oval on map below). This population does not appear to cause disease on pumpkins, squash or watermelon.  In our sentinel plots in northern Ohio, we never see downy mildew early in the summer on the pumpkins, squash or watermelon, even

when the cucumbers in the same field are nearly dead and melons quite damaged due to downy mildew.   This population is likely surviving over the winter on cucumbers in greenhouses in the Great Lakes region.  The southern population has a high affinity for squash and pumpkins – it appears later in the season – often not until mid-to-late August or early September (pink oval on map). It is probably arriving from the southeastern US on late summer/early fall storms related to hurricane activity, although sources to the west are also possible.  This  population usually appears first in southern Ohio and works its way up to central and eventually northern Ohio.  Where the two ovals overlap in central Ohio, we tend to find downy mildew on all cucurbit types at about the same time.  We don’t know if this is due to movement of the northern type into central Ohio, or a broader host range of the southern populations.

This means that pumpkin and squash fields needing to maintain foliage for additional time this season should be scouted and fungicides effective against downy mildew should be applied if the disease is found.  It is also a good idea to apply fungicides preventatively if downy mildew has been confirmed nearby.  See the September 2 post for fungicide recommendations.


A very rough estimate of downy mildew populations in Ohio. Note overlap of the northern and southern populations in central OH.


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