Downy Mildew and Phytophthora in Ohio. Getting ready!

In Ohio, as of this writing, we have not seen downy mildew in our sentinel plots at Fremont, Huron, Wayne, or Piketon research stations; however, given the regional influx of spores and favorable weather forecasts, we expect to observe spore detections and symptoms in the coming weeks.

With Ohio’s warm nights, humid afternoons, and frequent rain showers mirroring those in neighboring states, our cucurbit crops face a heightened risk of both downy mildew (Pseudoperonospora cubensis) and Phytophthora blight (P. capsici). Michigan’s earliest downy mildew outbreak in years—now confirmed in pickling cucumber fields in Monroe, Lenawee, Cass, and Van Buren counties—provides a cautionary example of how rapidly these pathogens can establish and spread under overcast, wet conditions (https://www.canr.msu.edu/news/downy-mildew-confirmed-on-cucumbers-in-four-michigan-counties2025). Likewise, eastern North Carolina confirmed downy mildew on cucumber on June 10, 2025, at approximately ten percent disease severity in commercial plantings (https://plantpathology.ces.ncsu.edu/2025/06/downy-mildew-confirmed-on-cucumbers-in-eastern-north-carolina/).

Regional Outbreak Timeline

  • June 10, 2025: Downy mildew confirmed on cucumber in eastern North Carolina; severity ~ 10 % in observed fields, Clade 2 confirmed by microscopic diagnostics

  • June 26, 2025: Michigan reports earliest downy mildew in years on pickling cucumbers in southeast and southwest production regions, supported by positive spore‐trap readings and rapid progression under moderate temperatures.

  • Ongoing: Ohio sentinel plots at Fremont, Huron, Wayne, and Piketon research stations continue weekly spore‐trap monitoring and scouting for first symptoms.

  • Two separate migrations of CDM, caused by Pseudoperonospora cubensis: • Great Lakes Region - • Group II (Mating type A1) • Primarily on cucumber • cantaloupe • Southeastern US to MW and NE • Group I (Mating type A2) • Other cucurbits

    Two separate migrations of CDM,
    caused by Pseudoperonospora cubensis:
    • Great Lakes Region -Group II (Mating type A1) Primarily on cucumber cantaloupe
    • Southeastern US to
    MW and NE Group I (Mating type A2) Primarily on other cucurbits. Image credit: Francesca Rotondo

    Key Environmental Triggers

    • Downy Mildew thrives when nights stay above 50 °F, days range 60 °F–75 °F, and leaf wetness persists for six hours or more.

    • Phytophthora Blight intensifies when soil temperatures exceed 65 °F, air temperatures are 75 °F–85 °F, and soils remain saturated by rain or irrigation.

    With Ohio’s forecast calling for continued warm, humid nights and frequent showers, conditions remain ideal for both pathogens.

Monitoring and Early Detection

Weekly Scouting: Examine leaves for angular chlorotic lesions and gray–purple sporulation under leaf surfaces (downy mildew), and for water-soaked crowns or wilting (Phytophthora).

Diagnostic Confirmation: Submit suspect samples to the OSU Plant and Pest Diagnostic Clinic, directed by Dr. Francesca Rotondo. Early confirmation allows for timely interventions (https://ppdc.osu.edu/).

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

Fungicide Recommendations for Downy Mildew

Products proven especially effective in recent regional trials include Orondis Opti, Ranman, and Zampro. Avoid back-to-back applications of the same mode of action to slow resistance.

Integrated Phytophthora Management

Phytophthora symptoms: water-soaked crowns, black-brown slimy lesions at the soil line, and sudden wilting

Phytophthora symptoms: water-soaked crowns, black-brown slimy lesions at the soil line, and sudden wilting. Image credit: Andres Sanabria-Velazquez

Phytophthora blight has already been confirmed in Ohio cucurbit fields this season. Symptoms include water-soaked crowns, black-brown, slimy lesions at the soil line, and sudden wilting (caused by Phytophthora).

  • Site Selection: Use well-drained fields, raised beds, and plastic mulch to reduce soil saturation and splashing of inoculum.

  • Crop Rotation: Rotate away from cucurbits and peppers for at least four years after any Phytophthora outbreak to deplete soil oospores.

  • Irrigation Practices: Minimize overhead irrigation; allow soil to dry sufficiently between events.

  • Sanitation: Clean equipment, tools, and footwear between fields; incorporate and destroy crop residues promptly.

Llmon-shaped sporangia with apical papillae and coenocytic hyphae confirm the presence of Phytophthora capsici in Ohio cucurbit fields. Image credit: Francesca Rotondo

Lemon-shaped sporangia with apical papillae and coenocytic hyphae confirm the presence of Phytophthora capsici in Ohio cucurbit fields. Image credit: Francesca Rotondo