Pseudoperonospora cubensis is responsible for cucurbit downy mildew (CDM), which was detected in Sampson County on cucumbers on June 4, 2024, with a disease incidence of around 5%.
On the upper side of the leaves, chlorotic lesions were noted (Figure 1), while the underside exhibited the characteristic dark sporulation of CDM (Figure 2). These observations align with the typical symptoms and signs of the disease. Microscopic examination confirmed the presence of sporangia, the microscopic structures of P. cubensis, on the underside of the leaves. P. cubensis consists of two clades, each with host preference among cucurbit crops. In North Carolina, Clade 2 targets cucumbers and cantaloupes, while Clade 1 prefers watermelon, squash, and pumpkin. Currently, cucumber and cantaloupe crops are particularly vulnerable to CDM due to the arrival of Clade 2 isolates in North Carolina via air currents. Growers of these crops are strongly advised to take immediate protective measures using effective downy mildew fungicides.
Given the potential for P. cubensis to develop resistance to fungicides, it is essential to implement a robust spray program that alternates fungicides with different modes of action and includes protective chemistries. This approach reduces the risk of resistance. Annual fungicide efficacy trials in North Carolina have demonstrated differences in effectiveness between the two clades. Therefore, consult our cucurbit downy mildew fact sheet for information on effective chemistries for your specific crops.
If you suspect downy mildew in your crops, contact your local Extension Agent. You can send physical samples or photos to the Plant Disease and Insect Clinic. Upon confirmation, consider anonymously reporting to the Cucurbit downy mildew IPMpipe website. Managing CDM is a community effort due to its ability to spread over long distances via wind currents. Both growers and homeowners should regularly inspect cucurbit leaves for signs of downy mildew.
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