Brown Spots with Yellow Halos on Leaves — Fungal Leaf Spot Explained

Photo: Rasbak, CC BY-SA 3.0, via Wikimedia Commons

Round spots with a defined edge, often ringed in yellow or purple and pale in the middle, are fungal leaf spot. The ring is the plant walling the infection off — which tells you the plant is fighting successfully, and that the spots themselves will never heal.

Fungal, bacterial, or neither

• Fungal — round, sharply defined, often with a tiny dark dot in the centre (the fruiting body). Spreads slowly, worst on lower and older leaves.
• Bacterial — angular rather than round, because it is stopped by leaf veins. Often looks water-soaked and greasy, and spreads fast in warm wet weather.
• Sun or chemical damage — bleached and papery, follows the shape of a water droplet or the exposed side, and does not expand after the event.

Why it starts

Nearly always splash. Spores sit in old debris on the soil surface, water hits them, and they land on the lowest leaves — which is why the bottom of a plant is affected first and worst. Crowding and long leaf-wetness periods do the rest.

Treatment

1. Remove spotted leaves as soon as you see them, and clear fallen debris from the soil surface. Do not compost either.
2. Stop wetting the foliage. Water the soil, in the morning, so anything that does get wet dries within the hour.
3. Improve spacing and airflow. Leaf-wetness duration is the single variable that decides whether spores germinate.
4. Copper or a broad-spectrum fungicide if it keeps advancing. This protects healthy tissue rather than curing existing spots.

What to expect

Existing spots never disappear — judge success by whether new leaves stay clean. If a plant loses more than about a third of its leaves, expect weak growth for the rest of the season, and consider replacing badly hit annuals rather than nursing them along.

Not sure that's what you have?

Symptoms overlap, and the wrong treatment wastes weeks. Photograph the affected leaf with Sherleaf and you get the diagnosis, a severity rating and a treatment plan in about ten seconds.