Cercospora leaf spot
A fungal leaf disease that causes round grey spots with reddish-purple borders on beetroot, chard, spinach and many ornamentals.
Also known as: Frogeye leaf spot
Symptoms and identification
Small round spots with ash-grey centres and a dark red, purple or brown ring appear on older leaves first. Spots join together, the leaves yellow and die, and badly hit plants lose most of their foliage, which reduces the size of roots and yields.
Causes
Spores spread by wind, rain splash and tools in warm, humid weather with long leaf wetness. The fungus lives over winter on crop debris and sometimes on seed.
Organic treatment
Remove and bin spotted leaves. Spray a copper-based fungicide at the first sign and repeat every 10 to 14 days in humid weather.
Prevention
Space plants for airflow, water early in the day at soil level, clear crop debris after harvest and rotate beet-family crops. Buy certified disease-free seed.
Chemical treatment (use with care)
Apply a fungicide labelled for Cercospora on that crop, following the label for rates, repeat intervals and harvest wait.