Mildew disease confirmed on cucumbers in Branch and St. Joseph counties
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Downy mildew disease on cucumber plants in a commercial planting that had received downy mildew fungicide sprays has been verified in Branch and St. Joseph counties on Aug. 12, 2026, which is seven weeks later than last year. These counties are in southern Michigan. Results were confirmed by qPCR and indicate that this is Clade 2, which prefers cucumber and cantaloupe. Clade 1 has not been found in Michigan this season and favors watermelon, pumpkin and squash. Only growers of cucumber and cantaloupe need to protect their crops from downy mildew at this time.
Our spore trapping early alert network indicates there were many days in later July and early August where the spore traps did not detect any downy mildew spores. Recently, there have been two days where the spore trap samples were positive for downy mildew in Allegan County (Aug. 3 and 9) and one day (Aug. 10) in Berrien County. On the east side of the state, the spore traps have not detected downy mildew spores for two weeks.
Why is downy mildew so late in developing in the state this year? The pathogen doesn’t overwinter in Michigan and other northern regions and must move via air currents from southern areas where it is more likely to overwinter. Downy mildew outbreaks in the southeast U.S. were much delayed this year, likely from the significant drought that occurred in that part of the country earlier this spring. Florida, Georgia, Tennessee and North Carolina experienced downy mildew outbreaks much later than usual. Most recently, field outbreaks were found in northern Indiana and Ohio.
The MSU Downy Mildew News website 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 the processing of the spore tapes. We can distinguish between cucumber and hop downy mildew spores using molecular tools. Visit the MSU Downy Mildew News website for spore trap results and current downy mildew news. Air samples are processed on Tuesday and Wednesday, with our online report updated each Wednesday evening and Thursday morning. Suspect plant samples should be submitted for diagnosis. The positive plant samples can be tracked on our map. See instructions on how to submit samples.
Downy mildew development is influenced by temperature and moisture (relative humidity and the duration of leaf wetness). The cucurbit downy mildew pathogen (Pseudoperonospora cubensis) prefers cooler temperatures, overcast conditions, high humidity, and long dew periods and can rapidly escalate in the state if fungicide sprays are delayed. The recent dry conditions in Michigan along with high temperatures earlier this month may have helped delay the initial onset of downy mildew. The state’s second pickling cucumber plantings should be protected with fungicides to preserve fruit quality and yield.
It is very important that cucumber growers use proven downy mildew fungicides (shown below in alphabetical order). These fungicides were effective in our 2025 research field plots and include:
- Elumin + chlorothalonil or mancozeb
- Omega (Orbus) + chlorothalonil or mancozeb
- *Orondis Opti (chlorothalonil is part of the premix, additional chlorothalonil is suggested [see label for maximum chlorothalonil rates])
- 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.
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. Downy mildew can infect the cotyledons of cucumbers and has been observed in previous years under high disease pressure.
This material is based on work is supported by MSU Project GREEEN and the Agricultural Research Fund and Pickle and Pepper Research Committee of Pickle Packers International.
Reference to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned.
