Some yellowing is completely normal — a tomato plant sheds its oldest leaves as it grows. The question that matters is which leaves turned yellow first, and how quickly it spread.
✅ Quick Answer
- Is this usually serious?
- Sometimes. Slow yellowing of the lowest leaves is normal ageing. Fast yellowing that climbs the plant is not.
- Most common causes
- Nitrogen shortage, especially in pots and old soil
- Overwatering and waterlogged roots
- Natural ageing of the lowest leaves
- Early blight or septoria leaf spot (yellowing with dark spots)
- Magnesium or iron deficiency (yellow between green veins)
- First thing to check
- Look at where it started — bottom leaves, top leaves, or all over?
- What NOT to do
- Don’t respond to every yellow leaf with more fertiliser. Overfeeding causes its own yellowing.
👁 Look At Your Plant
Work through these before you decide what’s happening. Each one narrows it down.
- Bottom leaves only, working slowly upward? Likely ageing or nitrogen.
- Yellow between the veins while the veins stay green? Think magnesium or iron.
- Yellow patches with brown or black spots and concentric rings? Suspect fungal leaf disease.
- Whole plant pale and limp with soggy soil? Overwatering and root suffocation.
- New growth pale while old growth stays green? Iron or sulphur, usually a pH or root issue.
- One side of the plant only? Check for root damage, a blocked dripper or a stem canker.
Most Likely Causes
Nitrogen shortage
Tomatoes are hungry. In containers and long-used beds the nitrogen runs out mid-season, and the plant pulls it out of its oldest leaves to feed new growth. The result is uniform pale yellowing that starts at the bottom and creeps up. Growth also slows and the plant looks generally thin.
Overwatering and poor drainage
Roots need air. Constantly saturated soil suffocates them, and drowned roots can’t take up nutrients — so the plant yellows even in rich soil. Pots without working drainage holes, saucers left full of water, and heavy clay beds are the usual offenders.
Normal ageing
The lowest leaves are shaded, old, and no longer earning their keep. A yellow leaf or two at the base of a healthy, fruiting plant is completely unremarkable.
Early blight and septoria leaf spot
Both start on lower leaves during warm, humid, wet weather. Early blight makes brown spots with concentric rings inside a yellow halo. Septoria makes many small tan spots with dark edges. Both climb the plant if the weather stays wet.
Magnesium and iron deficiency
Magnesium shortage shows as yellowing between the veins on older leaves, leaving a green skeleton. Iron shortage does something similar but on the newest leaves. Both are common in containers and in soils where pH is off.
🔧 What To Do Now
- Confirm drainage first: water the pot and make sure it runs out the bottom within a minute or two. Tip out any saucer.
- Remove yellowed lower leaves that touch the soil — they’re the launch pad for fungal spores.
- Feed with a balanced liquid fertiliser if growth is slow and the plant is uniformly pale.
- Mulch and water at the base, not over the foliage, to reduce fungal spread.
- Improve airflow by thinning crowded growth if plants are packed together.
- If spots are spreading fast in wet weather, remove affected leaves and consider an appropriate registered fungicide, following the label.
❌ What NOT To Do
- Don’t pull off half the plant’s leaves in one afternoon because they look imperfect.
- Don’t pile on high-nitrogen fertiliser during fruiting — you’ll get a magnificent green bush and few tomatoes.
- Don’t water overhead in the evening if fungal disease is already present.
- Don’t assume Epsom salts fix all yellowing. They only help genuine magnesium shortage.
💚 Can The Plant Recover?
Nutrient and watering-related yellowing usually improves within one to two weeks of correcting the cause — though the yellow leaves themselves won’t re-green. Fungal leaf diseases can be slowed and managed to the end of the season, but not cured. Keep the fruit coming and clear the debris at season’s end.
Filed under Tomato Leaf Problems. General gardening information — climate, soil, variety and local pests vary, so see our disclaimer.