Author: Andres Sanabria-Velazquez

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

 

 

 

New Report: Downy Mildew of Cucurbits in Wayne County

Date: July 18, 2025
Location:  Wayne County, Ohio

Samples provided by the OSU Extension were submitted to the campus diagnostic clinic, where microscopic examination verified downy mildew caused by Pseudoperonospora cubensis. In this 2-acre trial block, 8 percent of plants displayed the angular, vein-limited chlorotic lesions typical of early infection, and sporulation was evident on the underside of affected leaves. Concurrently, spore traps deployed within the field captured abundant P. cubensis sporangia, confirming active pathogen dispersal in the canopy. Immediate management measures—including a preventative fungicide application and removal of severely affected foliage—have been initiated to curtail further spread.

Ohio counties with Downy mildew confirmed

Ohio counties with Downy mildew confirmed. Image Credit: Andres Sanabria-Velazquez

 

Downy Mildew sporangia underneath the leaves

Downy Mildew sporangia underneath the leaves. Image Credit: Andres Sanabria-Velazquez

 

Cucumber sample from Wayne County, Ohio

Cucumber sample from Wayne County, Ohio. Image Credit: Francesca Rotondo

Downy mildew causal angent, Pseudoperonospora cubensis sporangia and sporangiophores captured by spore traps confirming active airborne dispersal.

Downy mildew causal agent, Pseudoperonospora cubensis sporangia and sporangiophores captured by spore traps, confirming active airborne dispersal. Image Credit: Andres Sanabria-Velazquez

Management 

Implement a weekly preventive fungicide program rotating FRAC groups (Oxathiapiprolin  and Cyazofamid  have show good control). Improve air flow and reduce leaf wetness, scout the block every three days to catch new lesions early, remove and destroy any heavily diseased leaves, and avoid overhead irrigation during evening hours to minimize canopy moisture.

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

Cucurbit Disease Update: No Downy Mildew Yet but Powdery Mildew, Bacterial Wilt, Angular Leaf Spot, and Alternaria Leaf Spot Present – July 11, 2025

As of July 11, 2025, our sentinel plots and grower reports have yielded no confirmed cases of Downy Mildew on cucurbits in Ohio. Instead, we are observing several other foliar and vascular diseases that can be easily confused in the field:

1. Powdery Mildew
Powdery Mildew typically appears as white, flour-like patches on leaves and stems. Under humid conditions, it can spread rapidly and reduce photosynthetic capacity. Although its symptoms are fairly distinctive, severe infections or mixed diseases can obscure diagnosis.

Figure 3. Advanced symptoms of powdery mildew on Zucchini leaf. Image Credit: Andres Sanabria-Velazquez

Figure 1. Advanced symptoms of powdery mildew on Zucchini leaf. Image Credit: Andres Sanabria-Velazquez

2. Bacterial Wilt

We have detected Bacterial Wilt caused by Erwinia tracheiphila in melon and squash. Infected vines wilt suddenly and irreversibly during the heat of the day. Because early symptoms—such as yellowing of lower leaves—can mimic other wilts, laboratory confirmation is essential. Cucumber beetles transmit the bacteria; therefore, as their populations increase, the incidence of the disease will be higher.

Bacterial Wilt caused by Erwinia tracheiphila

Figure 2. Bacterial Wilt caused by Erwinia tracheiphila. Image Credit: Andres Sanabria-Velazquez

3. Cucurbit Angular Leaf Spot
Identified on cucumber and watermelon, Angular Leaf Spot produces water-soaked lesions that become angular when bounded by major veins. Lesions often exude bacterial ooze in high humidity, but when dry they can be mistaken for other spots.

 

Angular Leaf Spot produces water-soaked lesions

Figure 3. Angular Leaf Spot. Image Credit: Andres Sanabria-Velazquez

4. Alternaria Leaf Spot

  1. Alternaria spp. produce small, brown to black circular to irregular lesions with concentric rings on melon and squash. In heavy infections, leaves may coalesce and blight, reducing yield and fruit quality.

Alternaria Leaf Spot

Figure 4. Alternaria Leaf Spot. Image Credit: Andres Sanabria-Velazquez

Why Submit Samples?

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 44691Samples may be hand-delivered:
    Requires coordination with Dr. Rotondo: (330-263-3721) | rotondo.11@osu.edu

-Current Disease Update for Ohio Vegetable Growers – 7/3/2025

As of today, no downy mildew has been detected in Ohio sentinel plots, according to the most recent update from 7/3/2025. These sentinel plots, designed for early crop disease detection, are located at:

Fremont (North Central Agricultural Research Station, Sandusky County)

Huron (Huron County)

Wooster (Wayne County)

Pike County (Piketon Research Station)

Sentinel plots locations

Figure 1. Cucurbit sentinel plot locations. Image Credit: Andres Sanabria-Velazquez

Weekly scouting and spore-trap monitoring are ongoing at all four locations. Despite favorable conditions and outbreaks in neighboring states, these Ohio plots remain negative so far for Pseudoperonospora cubensis activity.

 

Figure 2. Field visit to a cucurbit sentinel plot in Ohio for early detection of downy mildew and other vegetable diseases. From left to right: research team members conducting disease scouting and environmental monitoring.

Figure 2. Field visit to a cucurbit sentinel plot in Piketon, Ohio, for early detection of downy mildew and other vegetable diseases. From left to right: research team members conducting disease scouting and environmental monitoring. Image Credit: Andres Sanabria-Velazquez

However, powdery mildew has been observed in cucurbit crops across the state, and Phytophthora capsici has been confirmed in pepper plantings. Growers should maintain regular scouting and utilize pathogen-free seed, proper irrigation, and effective fungicide rotations as needed.

Figure 3. Early symptoms of powdery mildew on a cucurbit leaf. Image Credit: Francesca Rotondo

Figure 3. Early symptoms of powdery mildew on a cucurbit leaf. Image Credit: Francesca Rotondo

 

Figure 3. Advanced symptoms of powdery mildew on Zucchini leaf. Image Credit: Andres Sanabria-Velazquez

Figure 4. Advanced symptoms of powdery mildew on Zucchini leaf. Image Credit: Andres Sanabria-Velazquez

Figure 6. Microscopic view showing characteristic structures of powdery mildew

Figure 6. Microscopic view showing characteristic structures of powdery mildew. Image Credit: Francesca Rotondo

Figure 5. Typical symptoms of Phytophthora blight caused by Phytophthora capsici on pepper stem tissue. Image Credit: Francesca Rotondo

Figure 5. Typical symptoms of Phytophthora blight caused by Phytophthora capsici on pepper stem tissue. Image Credit: Francesca Rotondo

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

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