One thing worth knowing before you dig into the cause: once a pothos leaf has gone fully yellow, it isn't going back to green. Fixing the underlying issue stops more leaves from following it, but the yellow one is worth removing rather than waiting on.
Overwatering and root rot
Check whether the mix stays wet, whether water is trapped in the cover pot and whether the stem base is firm. Do not add water to a saturated pot. Continuing decline, soft tissue or a bad smell warrants a careful root inspection.
Root colour alone is not a verdict. Keep firm tissue and remove clearly soft, decaying roots with clean tools if rot is present. Repot the healthy remainder in fresh, well-drained mix when needed. Do not leave living roots completely dry as a treatment. Extensive decay or a collapsing stem needs specialist help.
Natural aging (not actually a problem)
An isolated older leaf gradually declining while the rest stays healthy may be normal turnover. Photograph it and check the whole plant again before changing care. If more leaves begin yellowing or growth deteriorates, reassess rather than assuming age.
You can remove a fully spent leaf for tidiness. There is no need to repot or fertilise solely because one old leaf has faded.
Underwatering
If the root ball is dry and the pot light, water thoroughly and drain. Check that the centre becomes moist; water can run around the edge of shrunken compost without rewetting it.
Do not keep soaking an already wet pot because a damaged leaf still looks poor. Record moisture checks as well as watering dates.
Too little light
Review light if yellowing accompanies weak, sparse new growth. Increase indirect light gradually if the position is dim. A pale leaf alone does not establish a light problem, and a fixed distance from a window does not suit every room.
Too much direct light
Possible scorch deserves a check if damage appeared after stronger sun and mainly affects the exposed side. Filter harsh sunlight while retaining useful daylight. Appearance alone cannot exclude other causes.
Scorched tissue will not repair itself; watch later growth and whether new damage stops.
Nutrient deficiency
Yellowing between veins or across older leaves can make nutrition worth checking, but does not identify a particular deficiency. Review feeding history, root health and growing conditions before adding supplements.
For a healthy plant in active growth, follow the houseplant fertiliser label. Do not add iron, magnesium or extra feed simply to test a guess. Persistent symptoms may need a closer assessment of the mix and roots.
Pests
Inspect leaf undersides and stem joints for insects, cottony deposits, fine webbing or sticky residue. Take a close photograph if identification is uncertain.
Keep an infested plant apart from others. Choose a treatment labelled for the identified pest on indoor plants, and follow dilution, repeat-treatment and sensitivity precautions. Do not improvise a stronger mixture.
Fertilizer burn
Review recent feeding, dilution and any fertiliser already in the compost. Deposits may support a salt-buildup concern, but yellowing does not prove it.
Pause extra feeding while investigating. Leaching may help when a pot drains freely; repeatedly flushing saturated mix with damaged roots can worsen conditions. Correct drainage or unsuitable mix first.
Transplant stress
A recent repot can cause stress, but is not a reason to ignore continued decline. Check drainage and how the new mix dries. A much larger container may stay wet longer than expected.
Keep suitable light and warmth and avoid unnecessary further disturbance. Worsening wilt, soft stems or spreading damage needs fresh investigation, not a fixed waiting period.
What to watch next
Record moisture checks and recent changes, and compare photographs weekly. Look for stable leaves and healthy new growth, not damaged tissue turning green again. Rapid collapse or continuing spread needs closer assessment from a plant specialist.
Reference material
Reference material consulted for this independently written guide. The publishers listed have not reviewed or endorsed it.
- University of Wisconsin Extension — Pothos
- NC State Extension — heartleaf philodendron
- University of Maryland — watering
- University of Maryland — root rots
- University of Maryland — symptom checks