🍅 Why Are My Tomato Leaves Turning Yellow?

Yellowing leaves on tomato plants are a very common issue, and the cause can range from something completely harmless to a problem that needs addressing quickly. The key is to look at where the yellowing starts, how fast it spreads, and the overall health of the plant.

Here’s a clear, practical guide to help you diagnose and fix the problem.

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🌱 1. Normal Leaf Ageing (Nothing to Worry About)

If lower leaves near the base of the plant are turning yellow while the rest of the plant looks healthy and green, this is often completely normal.

Why it happens

• Older leaves naturally die off
• The plant redirects energy to flowers and fruit

What to do

• Remove yellow leaves to improve airflow
• Focus on general plant care
• No feeding or treatment needed

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💧 2. Overwatering (Most Common Cause)

Tomatoes dislike sitting in wet soil, especially in pots or poorly drained ground.

Signs

• Yellowing starts from the lower leaves
• Leaves may feel soft or limp
• Soil feels constantly wet

What to do

• Reduce watering frequency
• Allow soil to dry slightly between waterings
• Improve drainage if growing in containers or heavy soil

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🌿 3. Underwatering

A lack of water can also cause yellowing, particularly in hot or windy weather.

Signs

• Leaves turn yellow, then dry and curl
• Plant may wilt during the day
• Compost feels dry below the surface

What to do

• Water deeply and consistently
• Mulch to retain moisture
• Avoid shallow, frequent watering

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🧪 4. Nitrogen Deficiency

Nitrogen is essential for healthy green growth.

Signs

• Older leaves turn pale yellow
• Slow, weak growth
• New growth may be small

What to do

• Feed with a balanced fertiliser
• Use tomato feed once flowering begins
• Avoid overfeeding, which can cause other problems

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🌱 5. Magnesium Deficiency

Very common in tomatoes grown in pots or grow bags.

Signs

• Yellowing between leaf veins
• Veins remain green
• Often affects older leaves first

What to do

• Apply Epsom salts as a foliar spray or soil drench
• Use a fertiliser containing magnesium
• Ensure regular feeding

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🌡️ 6. Temperature Stress

Sudden temperature changes can shock tomato plants.

Signs

• Yellowing without obvious pests or disease
• Growth slows after cold nights or heatwaves

What to do

• Protect plants from cold with fleece
• Vent greenhouses in hot weather
• Keep watering consistent

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🐛 7. Pests Causing Yellowing

Sap-sucking pests weaken plants and cause leaf discolouration.

Common culprits

• Aphids
• Whitefly
• Spider mites

Signs

• Speckled or patchy yellowing
• Sticky residue on leaves
• Visible insects under leaves

What to do

• Check leaf undersides regularly
• Remove pests by hand or wash off with water
• Use suitable organic treatments if needed

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🦠 8. Disease Problems

Less common, but more serious.

Possible diseases

• Early blight
• Fusarium wilt
• Verticillium wilt

Signs

• Yellowing spreads upwards
• Leaves may develop brown spots
• Plant may collapse

What to do

• Remove affected leaves promptly
• Avoid overhead watering
• Rotate crops yearly to reduce risk

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🧠 Key Takeaway

Yellow leaves on tomato plants are usually a watering or feeding issue, not a disease. Start by checking moisture levels, then feeding, before assuming the worst.

Catching the cause early keeps plants healthy, productive, and cropping well all season.

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