Bacterial leaf spot
Small, dark, water-soaked spots on leaves and fruit caused by bacteria that spread rapidly in warm, wet weather.
Also known as: Bacterial spot, Xanthomonas leaf spot
Symptoms and identification
Tiny water-soaked spots that turn dark brown or black, often angular and surrounded by a yellow halo. Spots merge, leaves yellow and drop, and fruit develops raised, scabby spots. The spots do not show the rings typical of fungal blights.
Causes
Bacteria such as Xanthomonas are carried on seed and plant debris and spread by rain splash, overhead watering, hands and tools, especially in warm, humid monsoon weather.
Organic treatment
Remove and bin affected leaves as soon as spots appear. Copper-based sprays (allowed in organic growing) applied every 7–10 days in wet weather slow the spread but do not cure infected leaves.
Prevention
Use healthy seed and transplants, water at the base, and avoid working among wet plants. Rotate crops and clear away infected debris at the end of the season.
Chemical treatment (use with care)
Use a copper-based bactericide labelled for bacterial spot on that crop, following the label.