Downy mildew detected on Michigan cucumbers, melons

Michigan State University Extension is encouraging all Michigan cucumber and melon growers to employ downy mildew fungicides now.

The first symptoms of cucurbit downy mildew disease on pickling cucumber plants in commercial production fields has been found by the Hausbeck Lab in Muskegon and Allegan counties on the morning of July 25. The statewide spore trapping network to detect downy mildew spores in the air has positives for six counties from the sampling period that ended July 17.

These results just became available in the last 36 hours from the qPCR processing. The counties with downy mildew positives from the spore traps include Bay, Allegan, Monroe, Muskegon, Ingham and Berrien. In all of the years that our lab has run this spore trapping program, I’ve not seen so many counties have positives from the air samples all at once. This indicates an influx of downy mildew sporangia in the state at a statewide level.

MSU Extension’s Downy Mildew Spore Trap webpage is updated with the latest results of confirmed positives for field disease and spore trap air samples. The results from the spore traps are a few days behind due to processing of the spore tapes. We are able to distinguish between cucumber and hop downy mildew spores using molecular tools and both are reported. View spore trap results and current downy mildew news.

I believe that our delayed downy mildew development in the state is the result of the high temperatures and dry conditions that we have been experiencing. Since the cucurbit downy mildew pathogen (Pseudoperonospora cubensis) prefers cooler temperatures, the current temperatures which are now more moderate, along with the rain over the weekend, has set the stage for a rapid escalation of the disease in the state if fungicide sprays are delayed.

Suspect samples should be submitted for a diagnosis. See instructions on how to submit samples.

It is very important cucumber growers use proven downy mildew fungicides (shown below in alphabetical order). These fungicides were effective in our 2021 research field plots and include:

• Elumin + chlorothalonil or mancozeb
• Omega (Orbus) + chlorothalonil or mancozeb
• *Orondis Opti (chlorothalonil is part of the premix)
• Previcur Flex + chlorothalonil or mancozeb
• *Ranman + chlorothalonil or mancozeb
• Zampro + chlorothalonil or mancozeb

*Across many years of field tests, Orondis Opti and Ranman have been the leading fungicides in cucurbit downy mildew control in Michigan and would be especially recommended given the positive downy mildew air samples detected across the state and the positive disease samples from two counties.

Fungicides should be alternated so that resistance of the pathogen to the fungicide’s active ingredient does not develop. It is important that the fungicides be applied prior to plant infection. Waiting until downy mildew has developed in the field prior to applying fungicides can lead to control failure and pathogen resistance. In many parts of the state, the second pickling cucumber crop has now been planted. Downy mildew can infect the cotyledons of cucumbers and has been observed in previous years for late-season plantings.

– Mary Hausbeck, Annika Peterson and Carmen Medina-Mora, Michigan State University Extension; Photo at top: A close-up of the underside of the leaf with the dark “mold” of the downy mildew pathogen. Photo by David Perla, MSU.