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When tomato leaves turn yellow, the first thing to check is the location of the discolouration and the overall growth pattern of the plant. Yellowing is rarely a single-cause issue; it is typically a symptom of physiological disorders caused by environmental imbalances, nutrient deficiencies, or, less commonly, viral infections. By distinguishing whether the yellowing is occurring between the veins, on the oldest leaves first, or across the entire plant, you can narrow down the possibilities before deciding whether treatment is needed.
Table of Contents
Checking for nutrient deficiencies
Nutrient shortages are a primary cause of yellowing, especially in container-grown tomatoes where compost nutrients are depleted quickly. The pattern of yellowing provides the most significant clue.
Magnesium deficiency
This is a common issue for tomatoes. Check for yellowing specifically between the veins of older leaves. This interveinal chlorosis may sometimes include reddish-brown tints. If this is only present on older foliage and the plant remains otherwise vigorous, it is often a minor issue. However, if it spreads, it may be caused by a lack of magnesium in the soil or an over-application of potassium-rich fertilisers, which can block magnesium uptake.
Nitrogen deficiency
If the leaves are a uniform pale yellow and the plant appears spindly or stunted, nitrogen deficiency is likely. This typically begins with the older leaves and gradually spreads to the younger foliage. Nitrogen is easily leached from the soil by rain or washed out of containers during watering.
Phosphorus deficiency
While less common, phosphorus shortage can cause older leaves to turn a dull yellow, often preceded by purple discolouration. This is frequently seen in young plants when soil temperatures are too low at the time of planting, hindering root uptake.
Evaluating environmental and physiological factors
Not all yellowing is caused by a lack of food. Physiological disorders occur when the plant cannot process water, light, or temperature correctly.
Temperature fluctuations
Tomatoes require warmth and generally will not thrive if temperatures drop below 12°C. In early summer, a combination of cold nights and warm days can lead to leaf curling and discolouration. This is caused by an accumulation of carbohydrates that the plant cannot process due to the cold. New growth may improve as conditions become milder, but persistent or spreading symptoms need further investigation.
Water and humidity imbalances
Check for lumps or nodules on the foliage, a condition known as oedema. This occurs when the plant absorbs more water than it can transpire, often due to excessive root moisture or very high atmospheric humidity. While this may cause localised yellowing or distortion, retain functioning leaves where possible; remove only badly damaged or diseased foliage when appropriate.
Watering consistency
Erratic watering can stress the plant and mimic nutrient deficiencies. If the soil becomes too dry, roots cannot absorb available nutrients. When recovering a dry plant, avoid flooding the soil; instead, bring moisture levels back up gradually.
Identifying chemical contamination
Tomatoes are exceptionally sensitive to hormone-based weedkillers. Contamination can occur even through vapours from loosely capped bottles stored in a greenhouse or through the use of contaminated garden compost containing treated lawn clippings.
Symptoms to check:
- Twisted or distorted leaves.
- Significantly reduced leaf area.
- Yellowing accompanied by abnormal growth patterns.
If the source of the chemical is removed, mildly affected plants may produce healthier new growth, although recovery is not guaranteed. However, if the residues are present in the soil or compost, the symptoms may persist.
Distinguishing between deficiency and virus
It can be difficult to distinguish between chemical damage, nutrient deficiency, and viral infections without professional testing. However, certain markers can help you diagnose the problem.
Viral markers
Check for mosaic patterns (lighter green patches), a “fern-leaf” appearance where the leaf blade is greatly reduced, or bubbly, distorted surfaces. Unlike nutrient deficiencies, plants do not recover from viral infections. If you notice marbling or bronzing on the fruit alongside yellowed, distorted leaves, a virus is more likely than a deficiency.
Comparison table for diagnosis
| Symptom | Likely Cause | Primary Check |
|---|---|---|
| Yellow between veins (old leaves) | Magnesium Deficiency | Check potassium feed levels |
| Uniform pale yellow (old to new) | Nitrogen Deficiency | Check watering/leaching |
| Mosaic/mottled yellow patterns | Possible virus or other stress | Check for fruit marbling |
| Distorted/twisted yellow leaves | Weedkiller/Virus | Check nearby chemical storage |
Practical remedies for yellowing leaves
Once you have identified the cause, apply the specific remedy. Avoid applying general fertilisers if you suspect a specific micronutrient issue, as over-feeding one element can block another.
Correcting magnesium levels
If the leaf pattern and growing history suggest magnesium deficiency, check excessive potassium feeding first and use a horticultural product only at its labelled dose. Yellow leaves alone do not establish a need for Epsom salts.
Improving nitrogen and phosphorus
- Containers: Use a general-purpose liquid feed to replenish nutrients that have washed away.
- Ground: Apply a mulch of organic matter, such as garden compost or manure, to provide a steady release of nitrogen. For phosphorus issues in cool weather, delay planting until the soil is warmer (late May or early June).
Managing environmental stress
To prevent temperature-related yellowing and curling, use a maximum-minimum thermometer in greenhouses to monitor fluctuations. Use ventilation and white greenhouse paint to prevent overheating during the day, and consider fleece for outdoor plants to protect against cold snaps.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Maximum-minimum thermometer: Used to monitor and manage the temperature fluctuations that cause leaf curling and physiological stress in tomatoes. Ensure it is suitable for greenhouse environments and provides a clear reading of the lowest and highest temperatures reached. Search Amazon for Maximum-minimum thermometer.
- Magnesium sulphate: Used as a soil additive or foliar spray to treat interveinal yellowing caused by magnesium deficiency. Verify the product is pure magnesium sulphate suitable for horticultural use. Search Amazon for Magnesium sulphate.
Related garden guides
- How to Grow Tomatoes in the UK: A Comprehensive Guide (2026)
- How to organise a small greenhouse for several crops
- More greenhouse & growing guides
Sources and further guidance
- Tomato Leaf Problems | RHS Advice
- Nutrient deficiencies | RHS Advice
- Tomato Viruses: Identification & Management | RHS Advice
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.