By Dr. Lina Quesada-Ocampo
Cucurbit downy mildew was reported today (June 6, 2017) in commercial cucumber fields in Duplin County, North Carolina and Horry County, South Carolina and confirmed by the Quesada Lab at NCSU.
Symptoms were typical of downy mildew, including angular chlorotic lesions on the leaves and a grayish-brown “downy” appearance on the abaxial surface. Structures of the oomycete Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, were identified using dissecting and compound microscopes, confirming the disease.
Cucumber leaf from Duplin County, North Carolina collected on June 5, 2017, adaxial (left) and abaxial (right) side. Angular, chlorotic lesions were present and in some cases, the lesions have started to turn necrotic and merge into one large infected area
Cucumber leaf from Horry County, South Carolina collected on June 5, 2017, adaxial (left) and abaxial (right) side. Angular, chlorotic lesions were present and in some cases, the lesions have started to turn necrotic and merge into one large infected area
The disease appears to be in early stages in North Carolina and only 5% of the field was affected; however, individual leaves show about 40% disease severity. In South Carolina, disease was more advanced with 100% of the field affected at approximately 50% disease severity. Growers are recommended to take immediate action to protect cucurbit crops in NC. Typically, cucumbers, melons, and watermelons are more susceptible to the disease than squash and pumpkin.
For more information about the disease and how to control it see our Cucurbit Downy Mildew fact sheet, previous alerts, and results from our demonstration trials. Control recommendations are also available in the Cucurbit downy mildew IPM pipe website, where you can register to receive text, email and/or phone alerts when new disease outbreaks are reported. Upon detection, growers should use intensive spray programs every 5-7 days, especially if weather conditions are conducive (wet and cool). P. cubensis can become resistant to fungicides quickly and it is important that growers alternate products in their fungicide management programs and tank mix with a protectant upon every application. This will protect the few chemistries that are still highly effective in controlling cucurbit downy mildew.
If you think you have cucurbit downy mildew in your cucurbits please contact your local Extension Agent and send photos and/or physical samples to the Plant Disease and Insect Clinic. Physical samples are always strongly preferred. If cucurbit downy mildew is confirmed in your samples by an expert, please make sure a report is sent to the Cucurbit downy mildew IPM pipe website or allow permission for us to do so.
We encourage commercial growers and homeowners to be thorough in checking cucurbit leaves for downy mildew and reporting this disease to the Cucurbit downy mildew IPM pipe website. These reports are crucial to warn growers of an outbreak, and serve as valuable data for research of this pathogen, which contributes to developing disease management strategies.