New downy mildew outbreaks have also been confirmed in Sandusky, Holmes, and Ashland counties, where diagnostic microscopy and spore‐trap monitoring detected abundant Pseudoperonospora cubensis sporangia in each area. In all three counties, plants showing the angular, vein‐limited chlorotic lesions typical of early infection, with sporulation readily visible on the underside of affected leaves during morning dew. Warm days followed by cool, humid nights over the past week have created ideal conditions for pathogen development and spread.
To limit further disease progression, initiate a protectant–curative fungicide rotation, for example alternating chlorothalonil (FRAC M05) or mancozeb (M03) with a systemic such as mandipropamid (FRAC 40) or cyazofamid (U8), and include oxathiapiprolin (FRAC 49) or a Zampro® (ametoctradin + dimethomorph, FRAC 40 + 45) treatment at least once per ten‐day cycle. Apply sprays preventively—before visible symptoms appear—and shorten intervals to seven days if nightly leaf wetness persists. Improve air movement by removing lower leaves and trellising vines to reduce canopy humidity, and humid-prone irrigation should be switched to drip systems or scheduled for early morning to allow foliage to dry quickly. Continue scouting every three to four days, removing and destroying any heavily diseased foliage, and disinfect tools and harvest equipment between fields to minimize local inoculum buildup and cross‐contamination.
Confirmed Downy Mildew (Pseudoperonospora cubensis) in Ohio Cucurbit Fields by County (July 2025)
First confirmation of Downy Mildew (Pseudoperonospora cubensis) in Sandusky County
Field view of a cucumber vine leaf exhibiting scattered, vein-confined chlorotic flecks characteristic of early downy mildew infection on the adaxial surface.
Free Sample Diagnosis
Please consider submitting fresh or well-preserved samples to the Plant and Pest Diagnostic Clinic.
This process ensures that your management decisions, fungicide choice, cultural controls, resistant varieties, are based on precise diagnosis rather than assumption.
How to Submit:
- Complete the online Plant Diagnostic Form:
https://ppdc.osu.edu/forms/plant-diagnostic-form - Ship or deliver your samples according to the instructions on the form. Samples may be mailed:
C. Wayne Ellett Plant and Pest Plant Diagnostic Clinic
Ohio State CFAES Wooster Campus
c/o Dr. Francesca Rotondo
234 Selby Hall, 1680 Madison Ave., Wooster, OH 44691 - Samples may be hand-delivered:
Requires coordination with Dr. Rotondo: (330-263-3721) | rotondo.11@osu.edu