NC State Extension Publications

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.

Cucumber leaf infected with powdery mildew

Figure 1. Cucumber leaf infected with powdery mildew.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Figure 2. Squash leaf infected with powdery mildew.

Figure 2. Squash leaf infected with powdery mildew.

Dr. Lina Quesada, NCSU 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 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

Acknowledgements

This factsheet was prepared by the NCSU Vegetable Pathology Lab.

Author

Associate Professor, Vegetable Pathology
Plant Pathology

Publication date: May 29, 2015

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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