Squash Downy Mildew Found in North Carolina
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Written by Mariana Prieto-Torres, Savithri Purayannur, Mike Adams, and Lina M. Quesada-Ocampo
Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew (CDM), was confirmed on butternut squash in Lenoir County on July 26 of 2021. The sample was part of a sentinel plot maintained by the NC State Vegetable Pathology Lab. The plot showed 10% disease incidence and leaves showed 2% disease severity. Samples were brought to the laboratory for DNA extraction and confirmation that isolates are clade 1 Pseudoperonospora cubensis via clade-specific quantitative PCR assays developed by our lab.
Small chlorotic lesions, circular in shape, were observed on the upper surface of leaves alongside with gray sporulation on the underside, which are typical signs and symptoms of CDM (Figure 1). Sporangiophore structures, which are distinctive of the causal agent (Pseudoperonospora cubensis), were observed on the underside of the leaf using a hand lens (Figure 2).
The cucurbit downy mildew pathogen has two types of isolates or clades, each of which has a preference for infecting specific cucurbit crops. In the state of North Carolina, clade 2 isolates have a preference to infect cantaloupes and cucumbers, while clade 1 isolates preferentially infect pumpkin, squash, and watermelon. Currently, both clade 1 and clade 2 have arrived in North Carolina through air currents. Therefore, growers of all cucurbits are recommended to take immediate action to protect their crops with effective fungicides in North Carolina.
Pseudoperonospora cubensis has the potential of becoming resistant to fungicides rapidly. Because of this, it is critical to have a strong spray program. This program should ideally alternate effective modes of action along with tank mixes with protective chemistries to decrease the build-up of isolates that are fungicide-resistant. Our annual fungicide efficacy trials in North Carolina revealed differences on fungicide effectiveness between the two different clades, thus, please refer to our cucurbit downy mildew fact sheet for effective chemistries to be used, according to your cucurbit 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 effort within the community, since it is an airborne pathogen that can travel from state to state through air currents. We encourage growers and also homeowners, to actively scout cucurbit leaves for downy mildew throughout the season.
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