Leaf spot
Fungal or bacterial infections that cause brown, black or purple spots on leaves, often with a yellow halo, leading to early leaf drop.
Also known as: Black spot, Early blight, Cercospora leaf spot
Symptoms and identification
Round or irregular spots on the leaves, ranging from tiny dots to patches several centimetres wide. Spots are often brown or black with a yellow ring, and may join up. On roses, black spots with fringed edges appear and leaves turn yellow and fall. On tomatoes, lower leaves show brown spots with target-like rings.
Causes
Fungal spores and bacteria spread through rain splash, overhead watering, infected tools and fallen leaves. Long periods of wet foliage, high humidity, crowded plants and the monsoon season all favour leaf spot diseases.
Organic treatment
Remove and bin spotted leaves as soon as you see them, and clean tools afterwards. Spray neem oil or a copper-based organic fungicide every 7–10 days during humid spells, covering both sides of the leaves. Thin out dense growth to let air move through the plant.
Prevention
Water the soil, not the leaves, and do it in the morning so foliage dries quickly. Space plants well and prune for airflow. Remove fallen leaves regularly, mulch to stop soil splashing onto lower leaves, and rotate vegetable crops each season.
Chemical treatment (use with care)
Where the disease keeps coming back, use a fungicide labelled for leaf spot on the crop, such as one based on mancozeb or copper oxychloride, following the label. Begin at the first sign of disease and alternate products to avoid resistance.