If your Swiss cheese plant (Monstera) has been thriving for years and suddenly develops brown spots spreading across many leaves, this isn’t normal aging or a seasonal dip. Based on the pattern, timing, and discussion from experienced growers, there are two realistic causes. One is far more likely than the other.
Let’s go step by step, without fluff.
Image from reddit.
The Most Likely Cause: Thrips Infestation
Despite how healthy the plant was for years, Monstera are thrips magnets. A long-established plant is not immune, especially if:
- Any new plants entered the home recently
- Windows are opened frequently in warm months
- The plant is near kitchens or high-traffic areas
Why this looks like thrips
- Brown, necrotic patches spreading quickly
- Damage appearing on multiple leaves at once
- Black specks on leaves (thrips frass, or droppings)
- Progression over weeks, not months
Thrips damage often gets mistaken for fungal issues or nutrient burn, but once it’s on nearly every leaf, it’s usually advanced infestation, not environment.
This is why several top commenters immediately identified thrips and advised isolation and treatment.
What About Salt Buildup or Old Soil?
Salt buildup can cause leaf tip burn, especially if:
- The plant hasn’t been repotted in 2–3 years
- Fertilizer was used regularly
- Soil is compacted and doesn’t flush well
However, salt damage usually shows as:
- Uniform browning at tips only
- Slower progression
- No black speckling on leaves
In your case, the speed and spread strongly point to pests first, soil second.
What You Should Do (In Order)
1. Isolate the Plant Immediately
Thrips can fly. Keep it away from every other plant.
2. Confirm Thrips (Simple Test)
- Shake or tap a leaf over white paper
- Look for tiny, moving, rice-shaped insects
If you see movement, the diagnosis is confirmed.
3. Treat for Thrips (EU-Safe Approach)
If you’re in the EU (where systemics are banned):
- Use insecticidal soap
- Treat all leaf surfaces, especially undersides
- Repeat every 5–7 days for 3–4 cycles
Neem oil is not recommended unless you know exactly how to use it. It often clogs leaf pores and burns foliage.
If you’re in a country where it’s legal and safe to do so, systemic granules are effective, but only for indoor, non-edible plants and with full awareness of risks.
4. Remove Severely Damaged Leaves
- Cut off leaves that are mostly brown or necrotic
- Do not strip the plant bare
- The goal is to reduce pest load, not “clean it up”
5. Address Soil After Pest Control
Once thrips are under control:
- Repot in fresh, well-draining Monstera mix
- Flush soil thoroughly to remove any salt buildup
- Resume fertilizing only after new, healthy growth appears
What Not to Do
- Don’t assume it’s fungal without proof
- Don’t keep fertilizing
- Don’t repot before treating pests
- Don’t rely on misting
Bottom Line
This isn’t age, winter stress, or “sudden bad luck.”
A Monstera that crashes after years of stability is almost always dealing with thrips, sometimes combined with old, compacted soil. Treat pests first, refresh soil second, and the plant can recover surprisingly well.
If you want, next we can turn this into a Google Discovery–style article or a diagnosis guide for common Monstera leaf damage patterns.
