Anthracnose on Monstera: recognize Colletotrichum and stop it

Anthracnose is one of the two main fungal diseases on indoor Monstera (the other being bacterial leaf spot). Caused by Colletotrichum fungi, it thrives in humid and stagnant conditions and can make a plant lose all its leaves in weeks if nothing is done.

This article teaches you to recognize it precisely, apply an effective protocol, and identify the point where rescue propagation becomes preferable to treatment.

Precise identification

The visual signature is fairly characteristic but sometimes confused with other pathologies. Look for these markers.

Circular spot with diffuse edge. Anthracnose starts as a small dark point (dark brown to black) on a leaf, generally older and lower on the plant. The edge is not sharp, it fades into healthy tissue, unlike mechanical damage which has precise borders.

Concentric rings. As it grows, the spot develops alternating darker and lighter rings. This is the visible expression of successive fungal pushes. The “bull’s eye” structure is highly diagnostic.

Yellow halo around. A diffuse yellow ring surrounds the spot, marking the active attack zone where chlorophyll is being destroyed.

Dry texture, sometimes dotted. The spot surface is dry, not wet. In advanced stage, tiny black points (acervuli, fungal reproductive structures) appear in relief in the darker zone.

Visible spread in 48-72 h. Photograph the spot, wait two or three days, photograph again. Visible growth confirms active infection.

Confusions to avoid

Several other problems give dark spots. The most frequent are bacterial leaf spot (watery, sharper angular borders, vivid yellow halo), sunburn (dry spot, sharp edge, light halo, stops growing, always on window facing side), thermal shock (clean necrosis without halo, after a cold night or move), and armored scale insect (small 2-4 mm “spot” in relief, fixed, actually an insect).

See also our article on black spots on Monstera which details the three major cause families.

Conditions favoring anthracnose

The fungus is everywhere. Its spores arrive by air, clothes, tools, or new plants. It becomes problematic only when three conditions meet:

High persistent humidity on leaves. A regularly misted or showered plant without quick drying creates the perfect microclimate. Poor air circulation. Spores need standing water to germinate. Moving air dries them before they install. Weakened plant. A badly watered, over fertilized, or stressed Monstera (recent repotting, moving) resists less well.

If you treat without correcting these conditions, anthracnose returns.

Treatment protocol

Step 1, immediate isolation

Move the plant from any shared zone with other green plants. Spores spread by water projection and contact.

Step 2, cut affected leaves

Cut every leaf showing one or more active spots. Do not try to save a half affected leaf. The spore is already in all its tissues. Cut the petiole at its base, as close to the stem as possible. Disinfect your shear with alcohol between each cut. Non negotiable. A contaminated shear transmits the fungus directly to the next wound. Place leaves in a sealed trash bag and put in the city trash (not compost). Spores survive domestic composting.

Step 3, immediate humidity reduction

Stop all misting. If you watered frequently, space out by at least a week. Substrate must dry between waterings. Aerate the room one hour daily (no cold drafts) or install a small fan at low speed pointed at the plant from 2 m away.

Step 4, copper based fungicide treatment

The copper based fungicide (diluted Bordeaux mixture, or commercial products based on copper hydroxide) is the standard. Available at garden centers in plant treatment.

Prepare per instructions (typically 5 g of Bordeaux mixture in 1 L of water). Spray all leaves, both sides, to light dripping. Spray the substrate surface too. End of day, away from children and pets. Let the plant dry 24 h in a ventilated room.

Step 5, second application at day 10

Renew the application 10 days later, same method. If new spots appear between, you missed some affected leaves at first pass. Do a full reinspection. A third application at day 20 is useful for severe infestation.

Step 6, surveillance for 1 month

Inspect the plant weekly for a month after the last application. A new spot means the protocol must resume. Without new occurrence, the plant is out of danger.

When to switch to rescue propagation

If anthracnose reaches the main stem (soft dark zones spreading over 5 cm or more along the stem), healing becomes unlikely. The fungus has colonized the vascular system.

Rescue propagation means taking a healthy section above the infected zone and starting a new plant. Identify the highest perfectly healthy zone (firm stem, vivid green, clean aerial roots). Cut 5-10 cm above with a freshly disinfected shear. Let the cut heal in air for 24 h. Place the cutting in a vase of clean water at bright indirect light, or directly in a pot of light well draining substrate. Watch root appearance (1-3 weeks in water) before planting in soil. The mother plant, too affected, goes in a sealed bag.

Lasting prevention

Three habits prevent return.

Do not mist unless strictly necessary, and never in evening. Prefer a room humidifier saturating air without wetting leaves.

Aerate regularly. A window ajar 30 minutes daily or an auxiliary fan at low speed prevents fungal spores from settling.

Disinfect your tools with alcohol after every pruning or handling of a sick plant. A contaminated shear is the number one spread vector.

If you hesitate between anthracnose and another fungal disease, the copper protocol is generally compatible with both main candidates. But precise identification helps tailor prevention.