Cercospora leaf spot

What you see

Round spots 3-5 mm with a pale grey centre and a distinctly reddish-purple border. As they age the centres fall out, leaving the leaf shot-holed. Older leaves die back while the plant keeps producing new ones from the crown.

Conditions

Warm and humid, above 25C with high night humidity - a late-summer disease on this coast. Splash and wind carry it.

Treatment

Remove affected outer leaves. Copper protects new growth. Harvest chard leaf by leaf rather than letting old foliage sit and sporulate.

Prevention

Wide spacing, morning watering at the base, two-year rotation, and clearing old beet and chard leaves rather than leaving them on the bed.

Months: July, August, September, October

Plants

Swiss chard, Beetroot, Spinach

Remedies

Bordeaux mixture (copper), Bacillus subtilis biofungicide

The thing people get wrong

Chard is a cut-and-come-again crop and this disease exploits exactly that: every old leaf you leave standing is a spore factory aimed at the new ones in the middle. Harvesting the outer leaves hard and regularly - even leaves you do not need - is a genuine disease control measure, not just tidiness.