Gardening glossary
Leaf spot
Leaf spot describes a symptom rather than a single disease. Dozens of fungal genera (Septoria, Cercospora, Alternaria, Colletotrichum, Phyllosticta) and several bacterial groups (Xanthomonas, Pseudomonas) produce discrete lesions on foliage. Accurate identification matters because fungal and bacterial leaf spots respond to different controls.
Fungal leaf spots typically have well-defined edges, a darker margin, and sometimes concentric rings or tiny dark fruiting bodies (pycnidia) in the centre that look like black pepper under a hand lens. Bacterial leaf spots are often angular because they spread within the leaf veins, frequently surrounded by a yellow halo, and can appear water-soaked or greasy when wet. Holding a suspect leaf to the light helps reveal the translucent halo characteristic of bacterial infection.
Both types thrive in warm, wet, humid conditions and spread through splashing water, contaminated tools, infected seed, and insect movement. Indoors they appear on plants kept too wet, crowded, or with poor airflow. Outdoors they peak after long rainy periods.
Cultural controls are the first line of defence: water at the soil line rather than overhead, space plants for airflow, prune affected leaves with clean shears and bag them (do not compost), rotate vegetable crops on a three-year cycle, and avoid working in the garden when foliage is wet. For fungal leaf spots, copper or sulfur fungicides and biofungicides containing Bacillus subtilis provide protection when applied before symptoms spread. Bacterial leaf spots are harder to treat chemically — copper sprays slow but rarely eliminate them, so cultural prevention is essential. Replace severely affected annuals and choose resistant cultivars at the next planting.