Downy Mildew

Downy mildew is not a true fungus. It belongs to the oomycetes, a group of water molds more closely related to brown algae than to the fungi they superficially resemble. On cucurbits, the primary pathogen is Pseudoperonospora cubensis; brassicas and basil are affected by distinct Peronospora species, though the symptoms are similar enough to cause confusion in the field.

The disease requires free moisture on leaf surfaces to germinate spores and initiate infection. Temperatures between 59°F and 68°F are optimal for sporangia production, and a single overnight dew period can trigger rapid spread across a planting. Warm, dry afternoons slow sporulation but do not reverse established infections. Because spores travel on wind currents, a regional outbreak can reach a garden within days of first appearance at nearby farms. The Cucurbit Downy Mildew Forecasting Network at cdm.ipmpipe.org tracks these spore-cloud movements in near real time and is the most reliable early-warning tool available to cucurbit growers.

The most cost-effective management combines resistant varieties with cultural controls: drip irrigation instead of overhead watering, wide plant spacing, and trellising to improve airflow. Preventive copper or phosphorous-acid sprays are warranted when spore advisories indicate regional pressure, but they work best applied before symptoms appear. Resistance in cucurbit varieties shifts frequently as new pathogen races emerge, so checking current seed catalog ratings each season is more reliable than replanting the same variety year after year.