Recent foggy, humid weather favors downy mildew

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

The wet, foggy and humid weather that occurred over the weekend and extended through the early part of this week will cause downy mildew to increase rapidly and spread. This year’s weather is much more favorable for downy mildew than what we experienced last year. That means that if you were able to control downy mildew on cucumber last year by spraying every 10 days you should not assume that the same approach will work this year. Since the weather is cooler and wetter than last year, fungicide sprays will need to be applied to cucumber more frequently than every 10 days in order to adequately control downy mildew. It is currently recommended that all cucumber growers apply a fungicide at least as often as every seven days. For cucumber growers who find downy mildew in the field, the sprays will need to be tightened up to five-day application intervals.

At this point, there have not been additional fields found with downy mildew beyond those identified earlier in Monroe and Allegan counties. However, given the increasing spore counts as determined by the spore traps, cucumbers in other fields or in homeowner gardens must be infected with downy mildew, but are not being reported. Since even seedlings at the cotyledon stage can become infected, it is critical that all fields be scouted frequently so the spray interval can be tightened as needed if downy mildew is found.

We have received some cucumber samples with downy mildew-like symptoms, but found alternaria leaf blight only. We welcome all samples that appear to have downy mildew-like symptoms. We can look at the leaves under the microscope and determine within a few minutes whether there is downy mildew. In some cases, we can arrange to pick up the samples if the location is near the spore trap sites.

Given the extremely favorable weather for downy mildew and the increasing spore counts in some areas of the state, all melon growers should be certain to be using downy mildew fungicides at a spray interval of every 10 days. Melon growers in Monroe and Allegan counties should consider downy mildew fungicide sprays every seven days since the disease is present in those counties and the weather has been very favorable for disease. In previous years, melons have become infected with downy mildew, ranking second behind cucumber in susceptibility to the disease.

I’ve had a lot of questions about how to use the new fungicides Presidio and Revus. Presidio can be used for both downy mildew and Phytophthora crown and fruit rot. The label for Presidio specifies that another fungicide needs to be mixed with Presidio. When using Presidio for control of downy mildew either Bravo of Dithane can be used as a mix partner. When using Presidio to target Phytophthora crown and fruit rot, fungicides such as Gavel or Acrobat can be considered as tank-mix partners. Revus is best used for control of Phytophthora. While I do not think that Revus is one of the top downy mildew fungicides that we have available, it is a good choice for Phytophthora control.

Squash, pumpkin, and zucchini growers can continue to use downy mildew fungicides every 10 days, but should be scouting fields for any early downy mildew infections. In the last few years, downy mildew has not been an especially difficult problem in squash, pumpkin, or zucchini fields.

Melon, pumpkin and squash growers need to remember that powdery mildew has also been found in the state and will need to be scouted and treated. The products used to treat powdery mildew are different than those used to treat downy mildew. So, those growers who are on a downy mildew program will need to make adjustments in their spray program to be protected from both downy mildew and powdery mildew.

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