First 2025 cucurbit downy mildew spores identified in three Michigan counties
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EmailGrowers are urged to scout cucumbers and melons and check the Downy Mildew News website for updates.
Cucurbit downy mildew spores have been verified in air samples from Allegan, Muskegon and Bay counties over the last two weeks. Allegan had one positive sample during the last week of May. Muskegon had multiple days with positive samples during the first week of June whereas Bay had one positive day for the same time period. The Hausbeck lab at Michigan State University uses a Burkard volumetric spore trap coupled with qPCR molecular analysis of the spore trap tape and the positive samples are subsequently verified with microscopy.
Allegan and Muskegon counties are located in the state’s west side that hosts significant cucumber acreage. Bay county is located on the east side of the state in the “Thumb” region and hosts vegetable production for the fresh and processing markets. These positive air samples confirm that airborne downy mildew sporangia have arrived in the state this year. We have detected Clade 2 of the cucurbit downy mildew pathogen spores in the air which most commonly infect cucumber and melon.
A cucurbit downy mildew disease outbreak on cucumber (or other cucurbit) plants has not been reported in Michigan. The spore trapping network to detect downy mildew spores in the air has been established by our lab in several Michigan counties and serves to alert our producers that action may be needed. Downy mildew spores have not been detected in air samples from other monitoring sites. We are watching all spore traps carefully for the detection of downy mildew spores in the air samples from Michigan’s cucumber production regions. Our spore trap webpage is updated with the latest results. Results from the spore traps are several days behind due to processing of the spore tapes.
We can distinguish between cucumber and hop downy mildew spores using molecular tools. Keeping an eye on the spore trap results across the state will be helpful in knowing when the cucumber downy mildew pathogen has been detected in the air samples from a particular production region. The cucurbit downy mildew pathogen does not overwinter in northern growing regions; the spores move from overwintering sites such as production greenhouses or the southern U.S. production regions via air currents. Visit the Downy Mildew News page for spore trap results and current downy mildew news.
This is about the time in Michigan’s growing season for the first occurrence of cucurbit downy mildew spores in air samples from the field. It is notable that we have detected multiple positive samples in one week from the Muskegon site. While in past years we have detected the first disease outbreak in a Michigan field around the first week of July, the first field occurrence may be earlier this year if the cooler temperatures and frequent rains continue. The detection of downy mildew spores in the air usually occurs a few days or a couple of weeks prior to a field outbreak. While it is concerning that an influx of sporangia has been detected, these sporangia must land on a suitable plant such as a cucumber or melon and then the weather conditions must allow the spores to germinate and infect.
The cucurbit downy mildew pathogen (Pseudoperonospora cubensis) prefers cooler temperatures and humid and wet conditions. The weather earlier this growing season had been favorable for downy mildew disease (cooler temperatures and rain). Hot temperatures and dry conditions normally stall the downy mildew pathogen. Michigan State University Extension recommends growers and scouts keep a close eye on cucumber and melon plantings that may have been established under low or high tunnels and scout frequently. A protective fungicide application may be considered now, especially for growers on the west side of the state.
The use of recently developed molecular diagnostics coupled with microscopy ensures the accurate confirmation of this pathogen from our spore trap air samples. The cucurbit downy mildew sporangia detected in the air may have originated from local/regional cucumber production greenhouses or from early field plantings of cucumbers and melons being grown in low/high tunnels. Elsewhere in the U.S., cucurbit downy mildew outbreaks in the field have been reported in Florida, Alabama, Georgia and South Carolina.
We urge growers, scouts, consultants and Extension educators to monitor greenhouse cucumber production and early cucumber and melon field plantings that may be in high or low tunnels. Suspect samples should be submitted for a diagnosis. See instructions on how to submit samples.
It is very important that cucumber growers use proven downy mildew fungicides (shown below in alphabetical order). These fungicides were effective in our yearly 2021-2024 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
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 the plants becoming infected. Waiting until the disease develops in the field before applying fungicides can lead to control failure and pathogen resistance. Currently, many of the state’s pickling cucumber crops are in early development. Downy mildew infection at this stage would likely hurt yields significantly. Downy mildew can infect the cotyledons of cucumbers and has been observed in previous years for the later season plantings.
