Yellow Leaves on Tomato Plants: 8 Causes and How to Fix Them

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Your tomato plant's leaves are turning yellow. Don't panic - this is one of the most common tomato problems, and it's usually fixable once you identify the cause.

Yellow leaves can mean many different things. The key to diagnosis is where the yellowing appears and what pattern it follows.

Quick Diagnosis Guide:

Bottom leaves only: Usually normal aging or nitrogen deficiency
Yellow between veins (veins stay green): Magnesium or iron deficiency
Entire plant yellowing: Overwatering, root problems, or disease
Yellow spots or patches: Fungal disease or pest damage
New growth yellow: Iron deficiency or soil pH problems

1 Overwatering

The most common cause of yellow tomato leaves in containers. Roots sitting in waterlogged soil can't absorb oxygen, leading to root rot.

Symptoms: Lower leaves turn yellow first. Leaves may feel limp or soft. Soil stays wet for days. You might see fungus gnats.

Fix: Let soil dry out between waterings. Ensure containers have drainage holes. Water deeply but less frequently. Consider self-watering planters that prevent overwatering.

2 Nitrogen Deficiency

Nitrogen is mobile in plants, so deficiency shows in older leaves first as the plant moves nitrogen to new growth.

Symptoms: Oldest (bottom) leaves turn uniformly pale yellow, then fall off. Plant growth slows. Stems may look thin and weak.

Fix: Apply a balanced fertiliser or one higher in nitrogen. Add compost to soil. For quick results, use a diluted liquid fertiliser.

3 Magnesium Deficiency

Common in container plants and in acidic soils. Magnesium is needed for chlorophyll production.

Symptoms: Yellowing between leaf veins while veins stay green (interveinal chlorosis). Starts on older leaves. Leaves may curl upward.

Fix: Dissolve 1 tablespoon Epsom salts in 4 litres of water and apply to soil. Can also spray diluted solution on leaves for faster uptake.

4 Early Blight (Fungal Disease)

A common fungal disease that thrives in warm, humid conditions. Spreads through water splash.

Symptoms: Brown spots with concentric rings (target pattern) on lower leaves. Yellow halo around spots. Leaves eventually die and fall off.

Fix: Remove affected leaves immediately. Water at soil level, not on leaves. Improve air circulation. Apply copper fungicide preventatively. Mulch to prevent soil splash.

5 Septoria Leaf Spot

Another common fungal disease. Often appears after wet weather.

Symptoms: Small circular spots with dark borders and light grey centres. Tiny black dots in spot centres. Starts on lower leaves.

Fix: Remove and destroy infected leaves (don't compost). Water at base only. Space plants for airflow. Apply fungicide if severe.

6 Underwatering / Heat Stress

Inconsistent watering causes stress. Tomatoes are particularly sensitive to drought-flood cycles.

Symptoms: Leaves wilt during hot days. Lower leaves turn yellow and crispy. Blossom end rot may appear on fruit. Plant looks stressed overall.

Fix: Water consistently - tomatoes prefer even moisture. Mulch to retain soil moisture. Self-watering containers eliminate this problem by providing consistent water supply.

7 Normal Leaf Aging

Sometimes yellow leaves are just... old leaves. This is normal and not a problem.

Symptoms: Only the oldest, lowest leaves turn yellow. Plant otherwise looks healthy. New growth is green and vigorous.

Fix: No action needed. Remove yellow leaves to improve airflow and appearance. This is normal plant behaviour, especially mid-season.

8 Root Problems

Root-bound plants, compacted soil, or root damage can all cause yellowing.

Symptoms: General yellowing despite proper watering and feeding. Roots circling the pot. Plant stunted. Water runs straight through or pools on surface.

Fix: Repot into larger container with fresh soil. Gently loosen root ball when transplanting. Ensure good drainage.

Prevention is Easier Than Cure

Most yellow leaf problems come from inconsistent watering or poor drainage. The easiest prevention:

Consistent moisture - Not too wet, not too dry
Good drainage - Roots need oxygen
Proper feeding - Balanced fertiliser every 2-3 weeks during growing season
Air circulation - Space plants, remove lower leaves touching soil

Grow Healthier Tomatoes

Garden Stack's self-watering system provides the consistent moisture tomatoes need - eliminating the overwatering/underwatering cycle that causes most problems.

See Garden Stack