Alternaria leaf spot
A fungal disease that creates dark brown spots with concentric rings, like a target, on the leaves of cabbage-family crops, carrots and flowers.
Also known as: Target spot, Black spot of brassicas
Symptoms and identification
Round brown to black spots with concentric rings appear on older leaves first, sometimes with a yellow halo. Spots grow and join, leaves yellow and fall, and on brassica heads and pods, dark sunken spots cause decay. Carrot leaves show dark spots and dying tips.
Causes
The fungus survives on seed and crop debris and spreads by wind and rain splash in warm, humid or wet weather. Stressed or old plants are most susceptible.
Organic treatment
Remove and bin spotted leaves promptly. Copper or Bacillus-based fungicides used at first sign of spots may slow the spread.
Prevention
Buy clean seed, rotate crops for at least 2 to 3 years, water at the base and clear debris after harvest. Give plants room for airflow.
Chemical treatment (use with care)
Use a fungicide labelled for Alternaria on that crop, following the label for interval and harvest wait.