Cercospora leaf spot identification & organic treatment
Cercospora beticola (and related species)
How to identify cercospora leaf spot
A fungus that produces distinctive small circular spots with tan-gray centers and reddish-purple borders on beet, chard, and spinach leaves.
Symptoms
- Small (1/8-1/4 in) circular spots with tan-gray dead centers and a reddish-purple to brown border
- Spots may have tiny black specks in the center under a hand lens
- Heavily spotted leaves yellow and die from the outside in
- Young leaves are usually spared until infection is severe
Host crops
Beet, Swiss Chard, Spinach.
How it spreads
Seedborne and survives in old leaf debris; spread by splashing water and wind-driven rain.
Favorable conditions
Warm, humid weather (75-85°F) with heavy dew or frequent rain.
Progression
- Older leaf spotting (Mid-season, humid weather) — Spots appear on outer leaves first.
- Spread inward (Ongoing wet weather) — New leaves become infected as spores splash upward.
- Defoliation (Heavy pressure) — Repeated leaf loss reduces root/leaf yield.
Organic treatment for cercospora leaf spot
- 1. Remove spotted leaves · low effort
Strip and discard heavily spotted outer leaves.
When: Weekly
- 2. Copper fungicide · medium effort
Apply an OMRI-listed copper spray every 7-10 days once spots appear.
When: Onset through harvest
- 3. Water at soil line · low effort
Avoid overhead watering, especially late in the day.
When: All season
Prevention
- Rotate beets/chard/spinach out of a bed for 2-3 years.
- Use disease-free seed.
- Space plants for airflow.
Easy to confuse with
Downy mildew on spinach (fuzzy purple-gray underside, angular spots), Sunscald spotting (no defined ring pattern).