Cucumber Downy Mildew Found in North Carolina

— Written By and last updated by Mary Lorscheider
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Written by Mariana Prieto-Torres and Lina M. Quesada-Ocampo.

Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew (CDM), was confirmed on cucumbers in Wilson and Sampson Counties on June 1 by the NC State Vegetable Pathology Lab. Leaves presented approximately 40% disease (Figure 1) and 10% of the fields were affected.

Angular, yellow lesions on topside of the leaves that are bound by leaf veins

Figure 1: Angular, yellow lesions on topside of the leaves that are bound by leaf veins (Hunter Collins, NC State Vegetable Pathology Lab).

Chlorotic lesions, angular in shape, were observed on the upper surface of the leaves. Additionally, gray sporulation was present on the underside. These are all typical symptoms and signs of CDM. Distinctive structures of the causal agent (Pseudoperonospora cubensis), called sporangiophores, were confirmed on the underside of the leaf using a microscope (Figure 2).
The causal agent has two types of isolates or clades. Each one of the clades has a preference for infecting a particular cucurbit crop. In North Carolina, clade 2 isolates have a host preference of cucumbers and cantaloupes. Clade 1 isolates have a preference to infect squash, pumpkin, and watermelon.

Therefore, cucumber and cantaloupe crops are currently most at risk of CDM because clade 2 isolates have arrived in the state of North Carolina through air currents. Cucumber and cantaloupe growers are strongly encouraged to take immediate action to protect their crops with effective fungicides in North Carolina.

Pseudoperonospora cubensis clade 2 sporangia under a dissecting microscope

Figure 2: Pseudoperonospora cubensis clade 2 sporangia under a dissecting microscope (Mariana Prieto, NC State Vegetable Pathology Lab)

Because this pathogen has the potential of becoming resistant to fungicides rapidly, it is critical to have a strong spray program. Such a program should ideally alternate tank mixes with protective chemistries and effective modes of action to decrease the risk of fungicide-resistant isolates. Our annual fungicide efficacy trials in North Carolina showed differences on effectiveness in fungicides between clade 1 and clade 2 isolates, thus, please refer to our cucurbit downy mildew fact sheet for effective chemistries for each isolate type depending on your crop.

If you think you may have downy mildew in your crops, please contact your local Extension Agent and send physical samples and/or photos to the Plant Disease and Insect Clinic. Upon validation, please make a completely anonymous report to the Cucurbit downy mildew IPM website. Management of CDM remains a shared community effort since it is an airborne pathogen that can travel from state to state through wind currents. We encourage growers as well as homeowners to actively scout cucurbit leaves for downy mildew throughout the season.

Register with the CDM IPM PIPE to receive texts, emails and/or phone alerts when new disease outbreaks are reported near you.