Downy mildew, caused by the oomycete Pseudoperonospora cubensis, has been identified on cucumber crops in Eastern North Carolina as of June 10, 2025. Disease symptoms were observed in commercial fields, with a confirmed disease severity of approximately 10% (Figure 1).
July 7, 2025 update: Downy mildew is suspected on cucumber in Rowan County, but has not yet been confirmed. Regardless, growers in the central part of the state should be applying fungicides as directed below.
Initial symptoms included yellow, angular lesions on the upper surface of the leaves, which are characteristic of the disease (Figure 2). A gray to dark purple fuzzy growth was evident underneath the leaves, corresponding to the sporulation of the pathogen. Microscopic diagnostics with a hand lens confirmed the presence of P. cubensis sporangia, verifying active infection.
In North Carolina, the pathogen occurs in two genetically distinct groups, or clades. Clade 2 affects cucumbers and cantaloupes, while Clade 1 is associated with watermelon, squash, and pumpkins. The current outbreak is attributed to Clade 2, which tends to arrive in the region early in the season via wind borne spores from the south. This puts cucumbers and cantaloupes at high risk.
To limit crop damage, it is critical to begin or adjust fungicide applications immediately. Choose products proven effective against Clade 2 downy mildew and rotate among different fungicide classes to reduce resistance development. North Carolina’s annual fungicide trials continue to demonstrate variable effectiveness depending on clade and host, so consult the latest cucurbit downy mildew fact sheet for updated chemical recommendations.
Downy mildew pathogens can adapt quickly to fungicides, making resistance management a top priority. Avoid repeated use of single-mode-of-action fungicides. Integrating protectants with systemic chemistries provides the best chance for disease suppression.
If you suspect downy mildew, contact your local Extension Agent promptly. The NC State Plant Disease and Insect Clinic accepts both digital photo submissions and physical plant samples for diagnosis. Confirmed cases can also be reported anonymously to the Cucurbit Downy Mildew IPMpipe website, a critical tool for tracking the disease’s spread across the U.S.
Growers and homeowners alike should scout cucurbit leaves frequently and look for early signs of downy mildew. Early detection and rapid response are essential to minimize losses.
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