Black spots on Monstera: disease, parasite or physiological?

Black spots are rarely aesthetic, never good. But they don’t all mean the same thing. Before reaching for fungicide or insecticide, you need to know what you’re dealing with: three major families, three different protocols, and a panic protocol can do more harm than a clear-headed diagnosis.

Three cause families

Your first observation: is the spot changing within 48 h? If yes, it’s urgent.

Family 1. Mechanical causes

“Stable” black spots are almost always mechanical. They don’t evolve, stay dry and sharply defined. Possible causes:

• Cold shock: the plant touched a cold window in winter, or got hit by a draft below 10 °C. Tissue necroses.
• Thermal shock: moving from a warm room to a cold one, or vice versa.
• Chemical burn: fertilizer spilled on the leaf, cleaning product nearby.

What to do: nothing urgent. Cut the dead part, identify the source (cold window, draft), correct it. The leaf will survive if most of it is still green.

Family 2. Fungal causes

Fungi love three conditions: high humidity, mild temperatures, stagnant air. If you mist heavily without ventilation, you create exactly their preferred environment.

Typical look:

• Soft black spots, sometimes with a brown center and yellow halo.
• Often circular, growing.
• Several leaves may be affected at once.
• Sometimes a down or microscopic dots on the spot.

The two most frequent diseases on Monstera are anthracnose (Colletotrichum) and bacterial leaf spot (Xanthomonas), often confused by eye.

Protocol:

1. Isolate the plant. Fungal spores spread by contact and air.
2. Cut all affected leaves (don’t keep a leaf with a black spot, the spore is already inside).
3. Disinfect your shears with alcohol between cuts.
4. Reduce humidity: stop misting, reduce watering.
5. Improve ventilation without cold drafts.
6. Treatment: copper-based fungicide, sprayed at end of day on all leaves, twice 10 days apart.

If the disease reaches the main stem, switch to “rescue propagation”: take the healthy part above the infected zone and start a new plant.

Family 3. Sucking parasites

If the “black spots” are really tiny dots, grouped or aligned, look very close: they’re probably parasites or their droppings.

Scale insects: small black, brown, or white bumps fixed on veins or the underside of leaves. They don’t move, they don’t look like insects at first glance.

Thrips: tiny black dots (1 mm), often paired with silvery or bronze streaks on the leaf. They jump when disturbed.

Spider mites: barely visible to the naked eye, accompanied by a fine web between leaves or underneath. Typical symptom: discolored leaves, fine punctures.

Protocol:

1. Magnifying inspection to identify precisely.
2. Shower the plant (whole, lukewarm water, gentle jet) to physically dislodge the maximum.
3. Neem oil or diluted black soap, sprayed on both leaf faces, early morning or late evening.
4. Repeat every 7 days for 3 weeks: that’s the duration needed to break the insect reproduction cycle.
5. Continue weekly inspection for 2 months after visible disappearance.

The urgency threshold

You must intervene within 24 hours if:

• A spot doubles in surface between two observations 24 h apart.
• More than three leaves are newly affected in one week.
• The main stem shows a black or soft zone.
• The substrate smell turns swamp-like.

In those cases, fast visual diagnosis beats waiting, and rescue propagation is always a last-resort option that saves the lineage even if the individual is too far gone.