Cercospora leaf spot: how to recognise and treat it

Disease

Symptoms

Many small, round spots with a pale brown to off-white centre and a red to reddish-purple margin.
Spot centres turn grey as the fungus sporulates; tiny black dots are visible in them with a hand lens.
The centre of a spot may drop out, leaving a hole; spots also appear on leaf stalks.
Spots merge and whole leaves die, causing heavy loss of foliage in warm, humid weather.

Remove badly spotted leaves; if the disease is present, cut chard or spinach once and clear the planting rather than cutting for regrowth.
Copper-based fungicide, applied preventively before symptoms become severe; a mix with a Bacillus amyloliquefaciens biofungicide works better than either alone. Follow the label.

A protectant fungicide labelled for Cercospora leaf spot on beet, applied preventively; follow the label dose and pre-harvest interval.
The fungus has become resistant to strobilurins such as azoxystrobin in some areas, so alternate fungicides with different modes of action.
On pansy, zinnia and hydrangea, thiophanate-methyl or myclobutanil sprayed every 7–14 days while the weather favours the disease; check they are approved in your country and follow the label.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Rotate beet, chard and spinach with crops outside the beet family for 2–3 years; the fungus survives up to two years in the soil.
Bury or remove crop residues after harvest and destroy volunteer plants and weeds that carry the fungus.
Start with disease-free or hot-water-treated seed.
Use drip irrigation or water at midday so leaves dry quickly; avoid overhead watering that keeps leaves wet overnight.

Figures checked against these sources.