Leaf Spot and Leaf Blotch

Leaf spot and leaf blotch are not a single disease but a broad group of symptoms caused by many different fungi (such as Alternaria, Cercospora, Septoria and Ascochyta species) and some bacteria (such as Pseudomonas and Xanthomonas species). Each pathogen tends to favour particular host plants, so the disease found on tomatoes, roses, turf or stone fruit is usually caused by a different organism, even though the visible symptoms look similar.

On the leaf surface, fungal leaf spots typically appear as small, roughly circular lesions, often brown, tan or grey with a darker margin, sometimes surrounded by a yellow halo. As the disease develops, spots may merge into larger irregular blotches, and tiny black or dark specks (fruiting bodies) can sometimes be seen within older lesions with a hand lens. Bacterial leaf spots tend to look more water-soaked at first, with angular or irregular margins that follow the leaf veins, and lesions may ooze slightly in humid weather. In turf, leaf spot appears as small purple-brown to straw-coloured spots on individual grass blades, which can expand and cause thinning or dieback of patches when severe.

These diseases occur throughout Australia and are most active during periods of extended leaf wetness — humid, warm weather in spring and autumn for many fungal leaf spots, and warm wet conditions for bacterial spots. Spores or bacteria spread by wind, splashing rain or irrigation water, and by contaminated tools, hands or plant material. Infection is worse where plants are crowded, poorly ventilated, or where leaves stay wet for long periods overnight.

Management relies on reducing leaf wetness and pathogen build-up rather than any single fix. This includes watering early in the day at the base of the plant rather than overhead, spacing plants for airflow, removing and disposing of fallen or infected leaves and debris, avoiding working among wet plants, and practising crop rotation in vegetable gardens. In lawns, raising mowing height, improving drainage, avoiding excess nitrogen, and not mowing when the turf is wet all help reduce spread. Resistant or tolerant varieties are available for many crops and turf species and are worth choosing where leaf spot is a recurring problem.