As of Monday, most reports of downy mildew in our neighboring states – MI, KY, PA – were in cucumbers, as were our previous reports. However, we found downy mildew on acorn squash in our sentinel plot on the OSU North Central Agricultural Experiment Station in Sandusky County and on cantaloupe in our sentinel plot on the OSU Muck Crops Experiment Station in Huron County. While we have not had reports from most Ohio counties, it is likely that downy mildew is widespread in Ohio. Now we have evidence that the strain that infects most cucurbits, including squash and pumpkins, is present here. The cucurbit growing season is winding down, but if cucurbit crops are expected to be in the field in the next few weeks they should be protected with fungicides. See my blog post on August 22 below for fungicide recommendations.
The downy mildew pathogen does not infect cucurbit fruits but if plants are defoliated the fruits are at risk of sunburn. Sunburn may not be obvious immediately in the field but may develop in storage. In fields with significant defoliation, pumpkins and winter squash should be removed from the field and into shade for curing.
Finally, the downy mildew pathogen does not survive over the winter in the absence of living cucurbit plants. However once harvesting is completed, plants should be disked as soon as possible to kill remaining green tissues that may otherwise be infected and serve as sources of inoculum. This will reduce downy mildew inoculum that can spread to cucurbit crops near and far.