Downy Mildew on Swiss Chard: How to Identify and Control It

Downy mildew caused by Peronospora farinosa is a fungal-like disease that affects beet-family plants including swiss chard. It thrives in the cool, wet, humid conditions of spring and autumn and can spread rapidly through a bed once established. Recognising it early and responding quickly is the key to saving your crop.

How to Identify Downy Mildew

Downy mildew on swiss chard produces angular, pale yellow patches on the upper surface of leaves, bounded by the leaf veins. On the underside of the same patches you will see a pale grey to purple-grey downy growth—this is the sporulating fungal material. The spots enlarge and may merge as the disease progresses, causing leaves to turn brown and die. Unlike powdery mildew, which appears as a white powder on the top surface, downy mildew always has the diagnostic grey fuzz on the underside in humid conditions.

Conditions That Cause Downy Mildew to Spread

The disease spreads via airborne spores and requires high humidity (above 85%) combined with cool temperatures (around 10–18 °C) to germinate and infect. Overhead watering that leaves foliage wet overnight is a major contributor in garden settings. Dense plantings trap moisture and reduce air circulation, creating ideal conditions for the pathogen. Weather you cannot control; plant spacing and watering technique you can.

Immediate Response: Remove Affected Leaves

As soon as you spot symptoms, remove affected leaves cleanly—do not compost them as spores remain viable. If the infection is widespread, remove entire plants and destroy them. Do not water from above; switch to base watering to keep foliage dry. Increase plant spacing by thinning if the bed is crowded. These steps will not cure infected tissue, but they slow the spread to healthy leaves significantly.

Copper-Based Fungicide Sprays

Copper fungicide (copper hydroxide or Bordeaux mixture) is approved for organic use and provides some protection against downy mildew when applied preventively or at the first sign of disease. It forms a protective barrier on leaf surfaces that prevents spore germination. Apply in the evening, covering both leaf surfaces. Repeat every seven to ten days or after rain. Avoid using repeatedly over many seasons as copper can accumulate in soil.

Prevention Through Good Growing Practice

Space plants at least 25 cm apart to allow air movement between leaves. Water at the base in the morning so any moisture on foliage evaporates during the day. Practice crop rotation, keeping swiss chard out of the same bed for three to four years to break the soil-borne part of the disease cycle. In regions with persistent downy mildew pressure, choose resistant varieties—seed catalogues sometimes note resistance ratings for beet-family crops.