Cucurbit downy mildew (CDM) has been confirmed on cucumbers in Center Maryland and on the Eastern Shore. It was recently confirmed on cantaloupe in New Castle, Delaware. It was found on butternut squash and cucumber in Lancaster County, PA, on August 18, 2023. CDM has been confirmed on pumpkin, butternut, and spaghetti squash in Northern New Jersey.
Cucurbit downy mildew is a significant disease that affects all cucurbits. Commercially important species of cucurbits include:
- Watermelon (Citrullus lanatus).
- Muskmelon (Cucumis melo).
- Cucumber (Cucumis sativa).
- Squash (Cucurbita pepo, Cucurbita moschata).
- Pumpkin (Cucurbita maxima).
The causal agent is the fungal-like organism (oomycete) Pseudoperonospora cubensis. CDM falls into two separate clades: Clade I and Clade II. Clade I predominately infects watermelon, pumpkin, and squash, and Clade II predominately infects cucumber and cantaloupe. Research suggests that isolates in Clade II can quickly become resistant to specific fungicides.
Most fungicides labeled for the control of CDM are at risk for resistance development because of the specific modes of action. Growers should scout their cucurbit fields every week, note the efficacy, or lack thereof, seen in the field, and incorporate using as many different chemical groups as possible to help mitigate fungicide resistance development. Loss of efficacy in the control of CDM has also been documented in FRAC code 4 (mefenoxam), FRAC code 11 fungicides (azoxystrobin), FRAC code 28 (propamocarb HCL), and FRAC code 43 (fluopicolide) in the mid-Atlantic region.
For more information on the specific fungicides recommended for CDM control on cucurbit crops, please see the 2022/2023 Mid-Atlantic Commercial Vegetable Production Recommendations. Always read and follow the label, as not all fungicides are listed for all cucurbit crops, and they might have a limited number of applications.