Planting Cucumbers Early Avoids Downy Mildew and Increases Yield without Fungicides

From Clemson Plant Pathologist Tony Keinath

Growers who have flexibility in deciding when to plant cucumbers should plant them within 1 month of the earliest recommended planting date for their region. Planting dates for commercial growers are in the Southeast Vegetable Crop Handbook (Pg 75). Planting dates for home gardeners are in fact sheet HGIC 1256, Planning a Garden. Plants can be direct-seeded or transplanted.

Early symptoms of downy mildew on June 8 on the first true leaf of this cucumber plant. Photo from Tony Keinath.

Early planting works because cucumber downy mildew typically shows up between May 15 and June 15. In one of my 2021 experiments at the Coastal REC in Charleston, cucumbers susceptible to downy mildew seeded on March 24 yielded twice as much as cucumbers seeded on April 15 and not sprayed for downy mildew or May 12 and sprayed with fungicides for 6 weeks. Yields were the same for sprayed and non-sprayed early seeded cucumbers. They were ready to harvest on May 19 before downy mildew appeared on May 25.

Blue bars show percentage control. The cucumbers seeded April 15 and not sprayed had no control (maximum disease). Yield in cartons per acre is the value shown in the orange bars multiplied by 10 (or 206 to 1648 for worst to best).

Even though some of the early season cucumbers froze on April 3 and had to be replaced with transplants, the excellent yields of cucumbers that set fruit before the heat of summer more than paid the cost of the transplants. Early plants can be protected with row covers if nighttime temperatures of 35°F or lower are predicted.

Susceptible cultivar ‘Speedway’ seeded March 24 and treated with a downy mildew-specific fungicide alternated with chlorothalonil starting on May 18 looked almost disease-free on June 8. Photo from Tony Keinath.

For late spring planting between mid-April and mid-May, a resistant cultivar must be planted if conventional fungicides will not be used. ‘Bristol’ slicing cucumber and ‘Citadel’ or ‘Peacemaker’ pickling cucumbers have the best resistance and yielded well in North and South Carolina, even without fungicides.

For more information see this Clemson News Release.

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