Cercospora leaf spot

Cercospora beticola and relatives

Small round spots with pale tan centers and a distinct purple-red rim, riddling chard and beet leaves in a humid summer until they fall apart.

Fungus Serious Rain splashAirborne sporesSeed-borne Spreads to neighbors

What you'll see

Round spots three to five millimeters across with an ash-gray or tan center and a sharp reddish-purple border.
Centers that dry, crack and drop out, leaving the leaf shot-holed.
Spots running together until the whole leaf collapses, starting with the oldest outer leaves.
Tiny dark pinpoints in the center of a spot, visible with a lens — the fungus fruiting.
On hydrangea, the same purple-rimmed spots on the lower leaves from midsummer, and early leaf drop.

Why it happens

A fungus that survives on infected leaf debris in the soil and, in beet and chard, on the seed itself. Spores splash and blow onto the lower leaves first, and each new spot produces the next round within a fortnight in warm weather. It is a different fungus from the septoria that spots tomatoes, though the two look alike at a glance.

Warm and humid — days above 25 °C with nights above 16 °C and long leaf wetness. Crowded rows, overhead watering and a bed that grew chard last year are what tip it over.

Treating it

Pick off the worst leaves as soon as the spots appear and bin them rather than composting.
Harvest chard and beet leaves hard from the outside; a plant kept picked outruns the disease better than one left standing.
Thin the row and stake or space to get air through the canopy.
Water at the base in the morning and mulch to stop the splash from the soil.
Clear the bed entirely once the leaves are more spot than leaf; the roots of beetroot are usually still perfectly good.

Keeping it away

Rotate beet, chard and spinach on a three-year cycle and clear all debris at the end of the season.
Buy treated seed or hot-water-treat your own, since the fungus travels on it.
Space plants at the label distance and resist the urge to squeeze a row in.
Grow tolerant chard and beet cultivars, which differ markedly.
Keep the hydrangeas open and pruned so the lower leaves dry, and clear fallen leaves in autumn.

Who gets it

Beetroot, chard and spinach beet above all; also hydrangea, sweetcorn, carrot, celery and many ornamentals.