Hello,
Downy mildew on cucumber foliage was confirmed today from a commercial pickling cucumber field in Adams County. This is the first confirmed diagnosis of this disease in Wisconsin in 2024. This disease is caused by a water mold pathogen, Pseudoperonospora cubensis which is not known to overwinter here in WI. Rather, the pathogen can be introduced into the region on infected transplants, or in air currents carrying the spores of the pathogen. Alternatively, the pathogen can be overwintered regionally if cucurbit host plant material is maintained living (and infected) through the winter cold months.
Because this downy mildew is on cucumber, it is likely of the Clade 2 or Similarity group 2 status (see below) and can move from cucumber to just cantaloupe and Buffalo gourd – but will not move to pumpkin, watermelon, acorn squash, or butternut squash. Once a field is infected, the foliage quickly becomes chlorotic (losing green color), and ultimately necrotic (dead) and is characterized by having angular edges to the lesions due to the vein limitations of the cucurbit leaves. The underside of the leaves develop gray-purplish masses of pathogen spores (sporangia) that form when weather is warm and wet. Spores are produced roughly 4-5 days after the initial infection.
Loss of foliage can result in a decline of plant productivity and protection for the fruit resulting in reduced yield and quality – and susceptibility to other secondary infections following sunscald damage. Most cultivars are susceptible to downy mildew and may require use of fungicides (listed below) to maintain the crop to target harvest.
CUCURBIT DOWNY MILDEW: IDENTIFICATION AND MANAGEMENT
Please feel free to contact me with further questions.
Best regards,
Amanda
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Amanda Gevens
Chair, Professor & Extension Specialist
Administrative Director of the Wisconsin Seed Potato Certification Program
Fritz Friday Chair for Vegetable Production Research
Plant Pathology Department
1630 Linden Drive Room
University of Wisconsin-Madison
Madison, WI 53706
gevens@wisc.edu/608-575-3029
Potato & Vegetable Pathology Research and Extension
https://vegpath.plantpath.wisc.edu/
