Pumpkin Downy Mildew Found in Surry County, North Carolina

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Written by Lina Quesada-Ocampo and Kimberly D’Arcangelo

Cucurbit downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis, has been confirmed on a pumpkin sample from a production field in Surry County on August 21, 2020. This is the first report of clade 1 P. cubensis isolates in North Carolina.

Small, circular chlorotic lesions on the upper surface of the leaves and gray sporulation on the underside were symptoms and signs observed of CDM in pumpkin (Figure 1). For confirmation, sporangia typical of P. cubensis were observed on the underside of the leaf using a dissecting microscope (Figure 2).

Circular, yellow lesions on surface of the leaves

Figure 1: Circular, yellow lesions on surface of the leaves. (Kimberly D’Arcangelo, NC State Vegetable Pathology Lab)

Dark spores on the underside of the leaf

Figure 2: Dark spores on the underside of the leaf (Kimberly D’Arcangelo, NC State Vegetable Pathology Lab)

Pseudoperonospora cubensis has two types of isolates or clades that preferentially infect certain cucurbit crops. In North Carolina, clade 2 isolates preferentially infect cucumbers and cantaloupes, while clade 1 isolates preferentially infect squash, pumpkin, and watermelon.

Growers of all cucurbits are recommended to take immediate action to protect their crops with effective fungicides in North Carolina.

As this pathogen becomes resistant to fungicides quickly, it is critical to have a robust spray program that alternates effective modes of action and tank mixes with protective chemistries to reduce the build-up of fungicide-resistant isolates. Results from our annual fungicide efficacy trials in North Carolina have revealed differences in fungicide efficacy between clade 2 and clade 1 P. cubensis isolates, thus, please refer to our cucurbit downy mildew fact sheet for effective chemistries for each isolate type depending on your cucurbit crop.

The recent rains in the state create favorable conditions for the pathogen and we expect more fields to become infected as the weather continues to be wet and cool. If you think you may have CDM in your fields, please contact your local Extension agent and send physical samples and/or photos to the Plant Disease and Insect Clinic. Upon confirmation, please make a completely anonymous report to the Cucurbit downy mildew IPM website.

Management of CDM remains a community effort because it is an airborne pathogen that can travel from state to state through wind currents. We encourage growers and homeowners to actively scout cucurbit leaves for downy mildew throughout the season.

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