Plant diagnosis
Why is my tomato turning yellow?
Warm-season fruiting crop from the Andes — needs 6-8 hours of direct sun, consistent water, and steady feeding.
The 4 most likely causes
The cause of tomato yellow leavesusually narrows to one of the items below, ranked by how often we see each in Growli's diagnostic chats. Work down the list — most readers find their answer in the top two.
- Underwatering or letting it dry too long (Most likely)
Underwatering looks similar to overwatering at first — both produce limp, dull leaves — but the soil tells the truth. If the soil is dust-dry several centimetres down, water deeply. Tomato prefers deep watering 2-3 times per week, more in heat. - Nutrient deficiency (nitrogen or iron) (Possible)
If tomato has not been repotted or fed in a year or more, the older leaves can yellow uniformly while the newest growth stays green — a classic sign of nitrogen depletion. Yellow leaves with green veins on new growth point to iron or manganese deficiency. A balanced liquid feed during the growing season usually resolves both. - Fungal disease (early blight, septoria, mildew) (Likely)
Tomato is vulnerable to several fungal diseases that show up first as yellowing or browning lower leaves — early blight on tomatoes and peppers, downy mildew on cucurbits, rust on beans. Water at the soil line, mulch to stop spore splashback, and rotate crops between seasons. - Aphids on new growth (Likely)
Aphids cluster on the softest new shoots of tomato, sucking sap and curling new leaves as they go. Look closely at the growing tips and undersides of the youngest leaves. A blast of water followed by insecticidal soap clears most infestations.
How to diagnose in 60 seconds
Run these quick checks before you change anything — the right fix depends on what you find.
- Stick a finger 3-4cm into the soil — is it dry, damp, or soggy? Damp-to-soggy with yellow leaves is the overwatering signature.
- Are the yellowing leaves the oldest ones at the base, or the newest at the tips? Old-leaf yellowing is usually water or nitrogen; new-leaf yellowing is usually iron or root damage.
- Look at the back of a yellow leaf with strong light — any speckling, webbing, or sticky residue? That points to pests, not water.
- Tip the plant out and look at the roots. Firm white roots = healthy; brown mushy roots = root rot, the real cause of the yellowing.
The fix — step by step
This is the recovery sequence Growli walks users through for tomato with yellow leaves. Work through the steps in order; skipping ahead is the most common reason a plant fails to bounce back.
- Stop watering and check the roots. Don't add more water yet. Unpot tomato and look at the rootball — firm white roots mean you have time; brown mushy roots mean you need to act today.
- Trim damaged roots and yellow leaves. Cut off any soft brown roots with clean scissors. Remove fully yellow leaves at the base — they won't green back up. Leave half-yellow leaves alone for now; the plant is still pulling nutrients out of them.
- Repot into fresh dry mix. Repot into fresh, well-draining mix in a pot one size up. For tomato, pick a spot with 6-8 hours of direct sun.
- Reset the watering rhythm. Water deeply once, then wait. For tomato, that means deep watering 2-3 times per week, more in heat. Use a finger or a moisture meter — never a calendar.
- Resume feeding only after recovery. A stressed plant cannot use fertiliser and the salts will worsen the damage. Wait for at least one round of healthy new growth (4-6 weeks) before resuming a half-strength liquid feed during the growing season.
When this can't be saved
Most cases of tomato yellow leaves are recoverable, but a few red flags point to a plant that has gone past the point of return. If you spot any of these, consider propagating a clean cutting and starting over.
- Every leaf has yellowed simultaneously and the stem feels soft at the base — root rot has likely consumed the plant.
- New leaves emerge yellow and crispy and never green up — the growing tip is damaged.
- The soil smells sour or sulphurous even after a thorough drying period.
Prevention
For tomato, the single biggest preventative is matching its native rhythm: deep watering 2-3 times per week, more in heat, 6-8 hours of direct sun, and a free-draining pot with a working drainage hole. Group tomato with plants of similar needs so you can water them as a batch rather than guessing per-pot. Set a calendar reminder to feed during the growing season but never feed a sick or freshly-repotted plant — wait for healthy new growth first.