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 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.
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.
Figure 3. Angular Leaf Spot. Image Credit: Andres Sanabria-Velazquez
4. Alternaria Leaf Spot
- 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.
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:
- 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 44691Samples may be hand-delivered:
Requires coordination with Dr. Rotondo: (330-263-3721) | rotondo.11@osu.edu