Downy mildew on cukes – now what?

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.

Downy mildew was confirmed on June 9 on slicing cucumbers in Monroe County. Currently, there are two 8- to 10-acre fields with advanced disease. These two fields were planted in late April by direct seeding and plugs grown in an on-site greenhouse. At this time, there is no visible disease on nearby cantaloupe plantings. Other cucumbers in the Southeast area are only recently emerged and are not diseased. At present, cucumbers and cantaloupe are most at risk. It is less likely that pumpkins and squash will become infected by downy mildew. At this time, it is not known whether this is an isolated incident or more widespread problem. Many cucumber fields in other regions of the state have been scouted and no disease has been found.

What to look for

Downy mildew causes symptoms on the leaves similar to a mosaic or angular leaf spot. The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf, giving a somewhat “dirty” or “velvety” appearance. This fuzz may be most evident in the morning. Leaves of all ages can become infected, including cotyledons.

What to do if you think you have it

Contact your local MSU Extension office immediately. All samples must be examined by someone in my laboratory or Dr. Jan Byrne in MSU Diagnostic Services for downy mildew to be confirmed.

Cucumbers and cantaloupes growing in southeast Michigan have highest risk of downy mildew

At this point in time, a 5-day spray interval is recommended for this region. This will provide the needed protection until the situation can be fully assessed. Based on MSU trials from last year, Previcur Flex and Tanos are the recommended anchor products in a downy mildew fungicide program. These products should be used in alternation so that the downy mildew does not develop resistance. To further delay the development of resistance and enhance the ability of the anchor products (Previcur Flex, Tanos) to control disease, a protectant fungicide (mancozeb or Bravo, for example) should be added to each spray.

Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)

Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

Growers of cucumbers and/or cantaloupes outside of the southeast Michigan region should also use the above program but can use a 7-day spray interval.

Many growers have reported that Previcur Flex may not be readily available. In that case, lead off with Tanos 50 DF. Two Tanos 50 DF applications can be made in a row prior to changing up the spray program with Previcur Flex. Overall, fungicides are more likely to be effective when applied prior to the appearance of downy mildew. Tanos 50 DF has a 3-day PHI and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to five days. Bravo has a zero-day PHI.

If you grow watermelons

An application of mancozeb is adequate at this time while the disease situation in the state is assessed.

If you grow squash and pumpkins

Fungicide sprays are not critical at this time. Stay tuned for a change in this recommendation.

Don’t cut rates!

In a situation like this year, it is tempting to cut fungicide rates. Let me say it again, don’t cut rates! Cutting labeled rates is never a good idea. We’ve shown in other trials that cutting rates reduces protection and doesn’t work. It is better to reduce costs by banding the application if plants are small or stretch the interval by a day or two.

Previcur Flex or Tanos 50 DF were outstanding products in our MSU trial last year. (Note: the Curzate fungicide has one of the active ingredients that is present in Tanos 50 DF and can be used in place of Tanos 50DF). If Curzate is used, it should be mixed with either Bravo or Mancozeb. Curzate should not be used in alternation with Tanos 50DF because they have an important active ingredient in common.

How many downy mildew sprays will need to be applied?

I hope that once we get past the next week or two, that we will learn that downy mildew is not widespread and is less of a threat than last year. If that is the case, then we may be able to increase the spray interval or go to a mancozeb only program to reduce costs, yet provide a measure of protection. However, we must be vigilant! My lab (517-355-4576) and Diagnostic Services (517-355-4536) on campus are available to look at any potential downy mildew samples. I can be reached via cell phone at 517-927-4532. See for results of last year’s downy mildew fungicide trial.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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