Identium
Spots on Leaves: Fungal, Bacterial, or Not a Disease at All
Halo, texture and edge. Three things separate an infection from a splash of cold water.
A spot appears on a leaf and the instinct is to reach for a fungicide. Most of the time it is not an infection, and treating it as one does nothing except cost money.
Three observations sort it out.
1. Is there a yellow halo?
A distinct yellow ring around a dark spot usually means an infection, because the plant is isolating the damaged area and pulling resources out of the tissue around it.
No halo, and the spot has a hard edge that stops abruptly, generally means physical or chemical damage rather than something living.
2. What is the texture?
Dry, papery, sometimes with tiny black dots in the centre. Fungal. Those dots are fruiting bodies. Fungal spots tend to be round with a defined edge and often concentric rings.
Wet, greasy, water-soaked looking, sometimes smelly. Bacterial. Bacterial spots frequently have angular edges, because they spread until they hit a leaf vein and stop, which gives them straight sides.
Dry and crisp with no halo and no dots. Physical. Sunburn, cold damage, a splash of cold water on a warm leaf, or a chemical burn from a spray or fertiliser.
3. Where is it and how is it spreading?
Lower leaves first, spreading upward, is typical of fungal problems, since spores splash up from the soil surface.
Random distribution across the plant, or only on the side facing a window or a vent, points at environmental causes.
Confined to new growth points at a pest or a nutrient issue rather than a disease.
The common ones
- Round brown spots with yellow halos and concentric rings, lower leaves first. Fungal leaf spot. Remove affected leaves, improve airflow, stop wetting the foliage.Fungal
- Angular dark spots that look water-soaked, edges following the veins. Bacterial leaf spot. Remove affected leaves immediately, isolate the plant, do not mist.Bacterial
- Powdery white coating on the leaf surface. Powdery mildew. Thrives in humid stagnant air. Improve airflow and remove the worst leaves.Fungal
- Fuzzy grey mould on stems, flowers or damaged tissue. Botrytis. Cut out affected parts, reduce humidity, increase airflow.Fungal
- Black sooty film wiping off the leaf surface. Not a disease. Sooty mould growing on honeydew from sap-feeding insects. Find the pest.Pest
- Sunken brown patches with a dark border on lower leaves, plant wilting. Root rot showing above ground. Check the roots now.Root problem
- Pale dry patches only on the side facing a window. Sunburn.Physical
- Small brown or black spots after a cold night or a cold water splash. Cold damage.Physical
What to actually do
Remove affected leaves. This is the main treatment for almost all of it. Cut the leaf off at the base with clean scissors, do not just tear the spot out. Bin them, do not compost.
Isolate the plant if you suspect bacterial, which spreads readily on splashing water and on your hands.
Stop wetting the leaves. Water at the soil. Stop misting. Water sitting on foliage overnight is how most of these get established in the first place.
Improve airflow. Space plants apart, open a door, run a small fan in the room. Stagnant humid air is the common factor behind nearly every fungal problem indoors, and this is the single most effective preventative measure.
Sterilise your scissors between plants with rubbing alcohol. Several of these transmit on blades.
When to reach for a fungicide
Rarely, indoors. For a houseplant, removing affected leaves plus fixing airflow and watering resolves the great majority of cases without any product.
A copper based fungicide is reasonable for a persistent fungal problem on a plant you care about. Nothing available to a home user cures a bacterial infection, so for those the answer is removal and prevention only.
And if the spots keep appearing on new growth after you have removed everything affected, stop treating the leaves and check the roots. Leaf symptoms are often the visible end of a root problem.
Prevention, which is mostly airflow and water discipline
Water the soil not the plant. Space plants so air moves between them. Do not mist. Remove dead leaves and fallen debris from the soil surface, since that is where spores overwinter. Quarantine new plants for two weeks.
Fungal and bacterial spots look similar enough that people treat one as the other for weeks. Photograph the affected leaf close up, including the edge of a spot, and Identium will tell you which you are looking at and whether it spreads to your other plants.
Frequently asked questions
How do I tell fungal from bacterial leaf spot?
Texture and edge shape. Fungal spots are dry and papery, usually round with a defined edge, and often have tiny black dots in the centre or concentric rings. Bacterial spots look wet and greasy, and frequently have angular edges because they spread until they meet a leaf vein and stop.
Should I cut off leaves with spots?
Yes, for almost all of them. Removing affected leaves at the base with clean scissors is the main treatment for both fungal and bacterial spots, and it works better than any spray. Bin the leaves rather than composting them, and sterilise the scissors between plants.
What causes leaf spots on houseplants?
Most often water sitting on the leaves combined with stagnant air, which lets fungal and bacterial problems establish. Many spots are not diseases at all: sunburn, cold damage, a splash of cold water on a warm leaf, and chemical burn from sprays all produce spots with no halo and a hard edge.
Does misting cause leaf spots?
It contributes. Misting raises humidity for only a few minutes but leaves water sitting on foliage, often overnight, which is exactly the condition fungal and bacterial spots need to establish. Watering at the soil and improving airflow does more for the plant than misting ever does.
Do I need a fungicide for houseplant leaf spots?
Rarely. Removing affected leaves, watering at the soil rather than the foliage, and improving airflow resolves most cases indoors. A copper based fungicide is reasonable for a persistent fungal problem, but nothing available to home users cures a bacterial infection, so those need removal and prevention instead.