According to Clemson Plant Pathologist Anthony Keinath in the The South Carolina Grower, cucurbit downy mildew was observed at the Coastal Research and Education Center on Sept. 13. It was found on cucumber and butternut squash that was transplanted Aug. 15.
Symptoms on both cucumber and squash means that both strains of cucurbit downy mildew are here.
Downy mildew appears to be developing rapidly, as shown below by the range of symptoms seen the same day in a 0.5-acre planting. At the same time, more than half of the plots had no symptoms…yet. Growers should check three to five areas in a field before they conclude that they do not have downy mildew…yet.
Downy mildew prefers warm but not hot weather. The long dew periods in the fall provide plenty of moisture for its development, even during a dry period.
All cucumber and cantaloupe crops should be sprayed weekly for downy mildew from now until one week before the last harvest. Crops that are most at risk of yield and quality losses are pickling cucumber (yield) and muskmelon/cantaloupe (sweetness). Downy mildew can reduce yields of any cucurbit, even zucchini, if it affects more than 50% of the leaf area.
Fungicide recommendations have not changed in the past two years. The only organic-approved product recommended for downy mildew is fixed copper.
Note that not all of squash-watermelon strains actually infect watermelon, as some will infect just squash. The watermelon fungicide guide includes fungicides in the spray program to protect crops against downy mildew. These applications are enough until downy mildew is seen in a field. Then a downy mildew-specific fungicide should be applied every week.Source: The South Carolina Grower