Pathogen
Cucurbit powdery mildew is caused by the fungal pathogens Podosphaera xanthii and Erysiphe cichoracearum.
Host Crops and Plants
All cucurbits can be affected, however, cucumber and melons are less susceptible due to host resistance that has been incorporated in most commercial cultivars.
Identification
Cucurbit powdery mildew causes white, powdery fungal growth on leaves (Figure 1, Figure 2), stems, and petioles. Infection usually begins on older or shaded leaves of cucurbits. As the pathogen continues to colonize the plant, the leaves senesce early and fruit yield and quality can be affected. Speckling can be observed in the fruit of some cucurbits due to plant stress caused by powdery mildew infections.
Figure 1. Cucumber leaf infected with powdery mildew.
Dr. Lina Quesada, NC State Vegetable Pathology Lab
Figure 2. Squash leaf infected with powdery mildew.
Dr. Lina Quesada, NC State Vegetable Pathology Lab
Favorable Environmental Conditions for the Disease
The disease prefers moderate temperatures of 68-80°F, medium to high humidity (50-90%), dense foliage, and low light. Infections can occur under dry conditions of relative humidity as low as 50%.
Disease Transmission
The pathogen survives via conidia carried by wind over long distances.
General Disease Management
- Application of fungicides to protect the crop
- Use of resistant cultivars
- Adequate plant spacing
- Remove debris, weeds, and volunteers that may harbor the pathogen
Disease Control for Conventional Growers
Fungicides may be required for control of cucurbit powdery mildew. Before disease appears, apply fungicides on a 7 to 10-day interval. Alternate products with different modes of action or fungicide group to avoid generating fungicide-resistant strains.
For the latest fungicide recommendations for cucurbit downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.
Example products for cucurbit powdery mildew control:
| Active ingredient | Example product | Pre-harvest interval (days) | Fungicide group |
|---|---|---|---|
| Chlorothalonil | Bravo Weather Stik | 0 | M |
| Myclobutanil | Rally 40WSP | 0 | 3 |
| Pyraclostrobin + boscalid | Pristine | 0 | 11 + 7 |
| QuinoxyfenL | Quintec | 3 | 13 |
| Metrafenone | Vivando | 0 | U8 |
| Cyflufenamid | Torino | 0 | U6 |
| Azoxystrobin + difenoconazole | Quadris Top | 1 | 11 + 3 |
| L Labeled on melons, winter squashes, pumpkins and gourds only, do not use Quintec on summer squash or cucumber. | |||
Disease Control for Organic Growers
Organic growers have less chemical options that are effective; the only OMRI labeled active ingredients that have some efficacy against cucurbit powdery mildew are fixed copper formulations.
Disease Control for Home Gardeners
Products containing the active ingredients copper or chlorothalonil (the trade name of one product with chlorothalonil is known as ‘Daconil’) are the best and only effective products available to home gardeners. In addition, home gardeners should grow varieties with tolerance.
Useful Resources
- The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations
- The Extension Plant Pathology Portal provides information on crop disease management
- The Southeastern US Vegetable Crop Handbook provides information on vegetable disease management
- The USDA Fungus-Host Distributions Database provides information about reported hosts for fungal and oomycete pathogens
Acknowledgements
This factsheet was prepared by the NC State University Vegetable Pathology Lab.
Publication date: May 29, 2018
Revised: Jan. 9, 2023
There is an alternate Spanish language version of this document here: Añublo polvoriento en cucurbitáceas
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.
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