Detection of Downy Mildew in Sandusky, Holmes, Ashland County

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)

Confirmed Downy Mildew (Pseudoperonospora cubensis) in Ohio Cucurbit Fields by County (July 2025)

First Detection of Downy Mildew (Pseudoperonospora cubensis) in a Wayne County Cucumber Block

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.

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:

  1. Complete the online Plant Diagnostic Form:
    https://ppdc.osu.edu/forms/plant-diagnostic-form
  2. 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
  3. Samples may be hand-delivered:
    Requires coordination with Dr. Rotondo: (330-263-3721) | rotondo.11@osu.edu

 

 

 

First Report: Downy Mildew of Cucurbits in Ohio

Downy Mildew of Cucurbits in Ohio
Date: July 15, 2025
Location: Homerville, Medina County, Ohio

Ohio counties with Downy mildew confirmed

Samples provided by the OSU Extension were submitted to the OSU Diagnostic Clinic, where microscopic examination confirmed downy mildew caused by Pseudoperonospora cubensis in a 4-acre cucurbit planting near Homerville. Approximately 10 percent of plants exhibit angular chlorotic lesions with characteristic purple-gray sporulation on the undersides of leaves, and on symptomatic plants disease severity averages 2 percent of total leaf area.

Downy mildew in cucumber.

Downy mildew in cucumber. Small, pale yellow lesions, often confined by the leaf’s veins, so that each spot takes on an angular shape. Image credit Frank Becker

underside of a cucurbit leaf—most likely cucumber—covered in tiny beads of dew. Along the veins you can see angular, brownish to grayish patches where the tissue has died, and within those patches a fine, dark-gray to purplish fuzz is just beginning to develop. That fuzzy growth is the sporulation of the downy mildew pathogen, emerging from the necrotic spots. The overall scene—water droplets on the cell-textured surface, contrasting with the darkened, angular lesions—is textbook for early downy mildew infection on cucurbits.

underside of a cucurbit leaf—most likely cucumber—covered in tiny beads of dew. Along the veins you can see angular, brownish to grayish patches where the tissue has died, and within those patches a fine, dark-gray to purplish fuzz is just beginning to develop. That fuzzy growth is the sporulation of the downy mildew pathogen, emerging from the necrotic spots. The overall scene—water droplets on the cell-textured surface, contrasting with the darkened, angular lesions—is textbook for early downy mildew infection on cucurbits.

Downy mildew in cucumber. Underside of a cucumber leaf. Along the veins you can see angular, brownish to grayish patches where the tissue has died, and within those patches a fine, dark-gray to purplish fuzz is just beginning to develop. That fuzzy growth is the sporulation of the downy mildew pathogen, emerging from the necrotic spots. Image credit Frank Becker

Confirmation in the Diagnostic Clinic. Characteristic sporangia and sporangiophore structures of a downy-mildew pathogen (in this case, Pseudoperonospora cubensis on cucumber). Scattered across the field are dozens of ovoid to ellipsoid sporangia, each roughly 20–30 µm long, with smooth, thin walls and a pale greenish tint. Image credit Francesca Rotondo

Recent climatic conditions, including frequent overnight leaf wetness from dew, daytime temperatures in the mid-70s Fahrenheit, and intermittent rainfall, have created an ideal environment for pathogen development. Although current incidence and severity remain relatively low, the pathogen’s rapid disease progression under these conditions poses a significant threat to yield if left unmanaged.

Immediate management began on July 15 with a protectant fungicide spray program targeting oomycete pathogens and subsequent applications planned at weekly intervals. Canopy thinning was performed to improve air circulation and reduce leaf wetness. Fields will be scouted every three to five days to assess treatment efficacy and adjust application timing. Severely affected foliage will be removed and destroyed to limit inoculum, and equipment sanitation protocols have been reinforced to prevent spread to adjacent fields.

Recommended active ingredients include chlorothalonil, mancozeb, cyazofamid (Ranman), the ametoctradin + dimethomorph combination (Zampro), and oxathiapiprolin (Orondis Opti). To reduce the likelihood of resistance, these fungicides should be rotated based on their modes of action. Moreover, preventive applications made before pathogen establishment are essential for the most effective disease control.

Submit your sample

Field diagnosis alone can be misleading. Many foliar symptoms overlap, and mixed infections are common. Accurate identification requires observation of the pathogen’s microscopic structures (conidia, sporangia, bacterial streaming, or hyphal morphology) under a compound microscope.

Please consider submitting fresh or well-preserved samples to the Plant and Pest Diagnostic Clinic. Our team will:

  • Macerate tissue and plate it on selective media
  • Examine spores, conidia or bacterial cells under the microscope
  • Run confirmatory tests when necessary

This process ensures that your management decisions, fungicide choice, cultural controls, resistant varieties, are based on precise diagnosis rather than assumption.

How to Submit:

  1. Complete the online Plant Diagnostic Form:
    https://ppdc.osu.edu/forms/plant-diagnostic-form
  2. 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
  3. Samples may be hand-delivered:
    Requires coordination with Dr. Rotondo: (330-263-3721) | rotondo.11@osu.edu

Ohio State University Extension Specialty Crop Team Newsletter

Editors- Thomas Becker, Agriculture and Natural Resources Educator, Lorain County and Frank Becker, Agriculture and Natural Resources Educator, Wayne County. Contributions made from team members around the state of Ohio. 

Warm and wet conditions across much of the state have continued to drive pest and disease activity in both fruit and vegetable crops. As we hit peak summer production, timely scouting and integrated pest management remain your best tools for maintaining crop health and quality.

Fruit Crops:
Spotted Wing Drosophila (SWD) remains a top concern for small fruit growers. This invasive fruit fly lays eggs inside ripening berries, with larvae feeding internally—causing soft fruit, collapse, and post-harvest losses.

To manage SWD, growers should monitor fields using baited traps placed within the fruiting zone. Traps should be checked regularly, ideally weekly, to assess fly presence. It’s important to harvest fruit frequently and remove any overripe or dropped berries to reduce egg-laying sites. At the first signs of fruit color change, apply protectant insecticides, rotating among different chemical classes such as spinosyns, pyrethroids, and organophosphates to delay resistance development. Post-harvest, rapid cooling of fruit and maintaining cold chain storage can significantly slow larval development and help protect product quality.

Sun Scald was reported in brambles following the extreme heat conditions. White or pale drupelets may appear on raspberry fruit due to intense sun exposure, a condition known as sun scald. While this disorder affects visual quality and salability, it has limited impact on flavor or nutritional value. To minimize sun scald, growers should maintain a healthy canopy by using proper pruning techniques that allow leaves to shield developing fruit. In extreme heat, consider deploying shade cloth or temporary row covers. Consistent irrigation is also critical to reduce plant stress and support recovery from heat damage.

Other pests in small fruit & orchards being reported are leafhoppers, aphids—including woolly apple, green, and rosy aphids—and spider mites (such as two-spotted and European red). These pests are showing up in scouting reports across the state. These pests often thrive during hot, dry spells and can cause significant foliage and fruit damage if left unchecked.

Regular field scouting is essential, especially on the undersides of leaves where many of these pests feed and reproduce. Growers should look for signs like stippling, curling, or honeydew. Maintaining beneficial insect populations such as lady beetles, lacewings, and predatory mites can provide natural pest control. Cultural practices such as avoiding excessive nitrogen applications can reduce lush growth that attracts pests. If pest thresholds are exceeded, selective insecticides or miticides may be used, always rotating products to reduce the risk of resistance.

Vegetable Crops
Phytophthora Root Rot has already been reported in several areas of the state, following some extremely saturated field conditions and heavy rain fall events. Persistently saturated soils have created ideal conditions for root rot diseases, particularly Phytophthora, which affects crops like peppers and cucurbits. This disease thrives in poorly drained soils and can quickly devastate fields if unmanaged. To reduce risk, improve field drainage by planting on raised beds or using plasticulture systems that shed excess water. Where appropriate, choose resistant or tolerant cultivars. Preventive fungicide applications can help protect plants during periods of prolonged soil saturation.

Powdery mildew has been confirmed in several squash plantings. Early symptoms include white, powdery patches on upper leaf surfaces and stems, which can reduce photosynthetic capacity and fruit yield. Management begins with early detection and protectant fungicide sprays.  Ensuring adequate airflow by properly spacing plants and minimizing leaf wetness also helps suppress disease development. After harvest, remove infected plant debris to reduce inoculum for future crops.

Squash bugs, stink bugs, and tarnished plant bugs have been observed feeding on vegetable crops statewide. These insects damage plants by piercing and sucking, which can lead to fruit deformities, discoloration, or aborted development.

For effective management, monitor plants closely for egg masses and early nymph stages, particularly on the undersides of leaves. When practical, remove egg masses by hand and destroy them. Cultural practices like eliminating crop debris and rotating host crops each season will reduce overwintering populations. Insecticide applications are most effective against young nymphs, as adults are often more difficult to control.

Cucumber beetles and Colorado potato beetles remain highly active and continue to pose serious challenges in cucurbit and solanaceous crops. Regular scouting is critical. Consider using sticky traps or direct plant inspections to detect rising populations early. Chemical options should be applied according to threshold guidelines, rotating among different IRAC classes to delay resistance development—especially important in managing Colorado potato beetle, which has a history of rapid resistance buildup.

In sweet corn fields, damage from European corn borer (ECB) has been reported. Monitoring ECB with pheromone traps can help time insecticide applications precisely. Applications are most effective when timed just after egg hatch, before larvae have a chance to bore into stalks and ears where they are protected from contact sprays. For growers planting sweet corn regularly, Bt hybrids are an effective tool for reducing ECB damage.

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