Treatment Plans
Stop leaf spots
Why do these spots appear on leaves, and how can you save your plant?
The problem
Leaf spots are a symptom of disease caused by a fungus or, less often, a bacterium. These spots, often brown or black with a yellow halo, are actually areas of necrotic (dead) tissue. Infection almost always starts because of one key factor: moisture lingering on foliage. Poor air circulation makes matters worse by preventing leaves from drying quickly after watering or misting.
- The fungi responsible include genera such as Colletotrichum (anthracnose) and Wilsonomyces (shot hole disease).
- Spots may enlarge, merge, and cause premature leaf drop.
- Without intervention, the disease weakens the plant by reducing its capacity for photosynthesis.
The real culprit is not water itself but how long a leaf stays wet.
How infection works
Imagine a fungal spore, invisible to the naked eye, landing on a leaf. If the leaf is dry, nothing happens. But if a thin film of water remains for several hours, generally 4 to 8 hours, the spore germinates. It then penetrates the leaf tissue. Once inside, the pathogen multiplies, killing plant cells to feed. This process creates the visible spot. Each spot can then produce thousands of new spores, ready to infect other leaves at the slightest contact with water.
What to do now
- Isolate the infected plant immediately to prevent spores from spreading to other plants.
- Use clean pruning shears or scissors to cut off all spotted leaves. Put them in the trash, never in the compost.
- Disinfect your cutting tool with 70% alcohol or diluted bleach between cuts to avoid transmitting the disease.
- Improve air circulation around the plant. Move it away from other plants or a wall.
- Change your watering technique: water only the potting mix, at the base of the plant, without wetting the foliage.
- To protect the remaining healthy leaves, spray a sulfur-based fungicide, a gentle, effective option for indoor use. Apply it in the evening, away from direct sun.
Prevention
- Always water at the base of the plant, never over the foliage.
- Encourage good ventilation by spacing plants sufficiently apart.
- Always remove dead or yellowed leaves from the surface of the potting mix.
- Inspect foliage once a week to act at the first signs.
Common problems
- Anthracnose · Brown or black spots, often sunken and bordered by a halo.
- Shot hole disease · Small brown spots drop out, leaving clean-edged holes in leaves.
FAQ
Will spots disappear from leaves I haven't removed?
No. The spots are areas of dead tissue and cannot heal or turn green again. Treatment aims to prevent new spots from appearing and the disease from spreading to healthy leaves.
How can I distinguish fungal from bacterial spots?
Fungal spots are often dry, sometimes with concentric rings or tiny black dots, the fungus's fruiting bodies. Bacterial spots often look **water-soaked or oily** and may be angular because leaf veins limit their spread. Fortunately, control measures, such as cutting off leaves and avoiding moisture, are the same for both.
Why is removing affected leaves crucial?
Each spot is a veritable spore factory. Removing infected leaves eliminates the main source of contamination and prevents the disease from spreading extensively to the rest of the plant and its neighbors.
Can I use Bordeaux mixture on my houseplants?
Bordeaux mixture is a copper-based fungicide that is very effective but potentially harsh. Some houseplants with delicate foliage may react badly, with burns. It is better suited to the garden. Indoors, a **sulfur-based fungicide** is often a safer and equally effective alternative.
Is baking soda an effective treatment?
Baking soda has a mild fungistatic effect: it can hinder spore germination by changing the pH of the leaf surface. However, it is **not a powerful curative treatment** for an established infection. It is better used preventively, but its effectiveness remains limited compared with dedicated fungicides.
Can this disease kill my plant?
Untreated, a severe infection can seriously weaken a plant by destroying much of its foliage, limiting photosynthesis. Young plants and cuttings are particularly vulnerable. Prompt intervention is therefore essential.