Spots on leaves: five causes and the one-page decision path

listLikely causes

• Fungal leaf spot: brown or black spots, often with a yellow halo and a concentric ring pattern, spreading in humid still air and worse on leaves that stay wet.
• Bacterial leaf spot: dark, water-soaked spots with a yellow halo that spread fast and often follow a leaf that was damaged or watered from above.
• Overwatering oedema: small brown or corky bumps on the undersides of leaves, in lines along the veins, caused by roots taking up water faster than the leaves can transpire.
• Direct sun through glass: pale bleached patches with brown papery centres, on the side facing the window, following the shape of a water droplet or a gap in the curtain.
• Cold water droplets: pale rings and spots where cold water landed on a leaf, most obvious on hairy-leaved plants like African violet.
• Pest damage: fine pale stippling from spider mites, silvery scarring from thrips, or sticky honeydew with black sooty mould from scale and aphids.

listWhat to do

• Look at where the spots are. If they are only on the window side and follow the shape of the light, it is sun or cold-water damage, not disease.
• Look at the underside. Small corky bumps in rows along the veins are oedema, which is a watering problem, not an infection.
• For fungal or bacterial spots, remove the affected leaves, stop all overhead watering, and increase air movement immediately. Most cases stop spreading with those three changes alone.
• Do not mist a plant with leaf spots. Water on the leaf surface is how fungal spores germinate.
• If spots continue to spread after two weeks of dry foliage and good air, treat with a copper-based fungicide for bacteria or a potassium bicarbonate spray for fungi. Do not use them preventively.
• Water at the base with room-temperature water and keep the foliage dry, particularly in the evening.
• Space plants so that leaves do not touch, and remove any fallen leaves from the surface of the mix.

differentialConfusable conditions

presentationWhat you are seeing

• Discoloured areas on the leaf surface: brown, black, yellow, pale or translucent, sometimes with a halo, sometimes with concentric rings, sometimes raised and corky.
• The distribution matters more than the appearance. Spots on one side of the plant, in a pattern that follows the light or the watering, are environmental. Spots scattered across all leaves, spreading over days, are infectious.

aetiologyWhy it happens

• Fungal spores need a film of water on the leaf for four to eight hours to germinate. Overhead watering in the evening, in a warm still room, is the ideal incubator.
• Bacteria enter through wounds and spread in water, which is why bacterial spot often follows a plant that was splashed, trimmed with dirty tools, or damaged by thrips.
• Oedema is mechanical. When the roots take up water faster than the leaves can transpire — usually after a heavy watering in cool humid conditions — cells in the leaf swell and burst, forming corky scar tissue in rows along the veins.
• Sun through glass acts as a lens when there is a water droplet on the leaf, concentrating energy into a spot small enough to burn within minutes.

managementWhat to do

• Identify where the spots are and whether they are spreading. This takes two minutes and rules out half the causes.
• For anything that looks infectious: remove the affected leaves, stop wetting the foliage entirely, and improve air movement. Reassess in two weeks.
• For oedema: reduce watering frequency, raise the light level, and increase air movement. The corky spots are permanent scars on those leaves, but new leaves will come through clean once the balance is right.
• For sun or cold-water damage: move the plant out of the direct sunbeam, use room-temperature water, and stop misting in bright light.
• Never apply fungicide as a preventive. It is ineffective against the environmental causes, and unnecessary use builds resistance in the pathogens that do exist.