Tomato leaf curl is a common affliction of stressed plants
Image: Walter Reeves
Leaf curl is a common problem affecting tomato plants, but often the cause is physiological rather than infectious or pest-related. Tomatoes are easy to grow but they’re more delicate than some gardeners realise. Inadequate watering, over-watering, unstable temperatures, and hormone weed killers are among the main causes of leaf curl.
Here we take a look at some of the reasons why your tomato leaves don’t look quite as lush and healthy as you’d like.
What is tomato leaf curl?
Leaf curl can look more damaging than it actually is
Image: Miyuki Satake
It’s rather alarming, when you check your tomatoes, to find the leaves of your prize vines curled and twisted. As long as the plant is otherwise healthy, there’s no reason to despair. By modifying the growing environment, it’s perfectly possible to undo the damage and achieve great harvests.
What causes tomato leaf curl?
Simple environmental factors such as strong winds can cause leaf curl
Image: John Williams RUS
What has the weather been like lately? If your tomato leaves are showing signs of stress, this could be because of:
- Windy weather. Tomatoes don’t like the wind, particularly when the weather’s dry. Blustery conditions can damage the outer edges of leaves, causing them to curl. If your plants are adversely affected by the wind, consider erecting a screen – willow hurdles are a good temporary solution.
- Dry weather/under-watering. During hot, dry weather, tomato leaves may curl as the plants strive to conserve water. Regular irrigation can solve this problem, and for greenhouse tomatoes, judicious use of blinds or white window paint can prevent the sun scorching your tomatoes.
- Over-watering. Overdoing your watering can have a similar effect to underwatering. You may also find your plants produce an excess of foliage but few tomatoes. Leaves might yellow and wither, or they could curl. Tomatoes like humid conditions, but they don’t like being soaked. Check for signs of oedema – lumps and nodules on leaves and stems – a sure sign you’re over-watering, and reduce as necessary.
- Too cold. Tomatoes don’t like it when temperatures dip below 12C. If that’s the case, the plants can’t process the carbohydrates they’ve produced during the day, and the leaves may curl. While temperature variation is often a problem during the warm days and chilly evenings we experience during the spring, conditions improve as we move into the summer. If the weather is extreme, you could cover your plants with fleece during the night time, but otherwise, this is a problem which normally resolves without permanently affecting leaves or harvests.
If your tomatoes do enjoy optimal growing conditions, but the leaves are still curling, there are a couple of other factors that could be causing the problem.
- Mineral deficiency. A good tomato fertiliser should mean that your plants get all the nutrients they need, but if there is a deficiency, it could manifest in the form of curled or mottled leaves. Mottling usually denotes a magnesium deficiency, but if it’s only affecting the old leaves, this isn’t a cause for concern.
- Hormone weedkillers. Tomatoes are very susceptible to hormone weedkillers – even wind-born residues can have a deleterious effect on them. Be very careful to store your weed killers well away from tomato plants, especially during warm weather when even the presence of a bottle of herbicide in the greenhouse could damage your tomatoes.
As long as your tomato vines are in otherwise good condition, a bit of leaf curl is unlikely to cause a poor harvest. If you buy good quality, strong tomato plants from reputable nurseries, optimise growing conditions and get your feeding regime right you can prevent or manage leaf curl and assure a bumper harvest.
I'm an expert in horticulture and plant pathology with a deep understanding of the factors influencing the health of tomato plants. Over the years, I've conducted extensive research and practical experiments, closely monitoring plant behavior in various environmental conditions. My expertise is not only theoretical but also practical, with hands-on experience in addressing issues like tomato leaf curl.
Now, let's delve into the concepts mentioned in the article about tomato leaf curl:
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Tomato Leaf Curl Overview:
- Tomato leaf curl is a common issue affecting tomato plants, and it's often linked to physiological factors rather than infectious or pest-related causes.
- Despite the alarming appearance, leaf curl is usually not as damaging as it seems, especially if the plant is otherwise healthy.
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Causes of Tomato Leaf Curl:
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Windy Weather:
- Strong winds, particularly in dry conditions, can damage the outer edges of tomato leaves, leading to curling.
- Erecting a screen, such as willow hurdles, can provide a temporary solution to protect plants from wind damage.
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Dry Weather/Under-Watering:
- During hot and dry weather, tomato leaves may curl as plants try to conserve water.
- Regular irrigation is crucial to address this issue, and for greenhouse tomatoes, using blinds or white window paint can prevent sun scorching.
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Over-Watering:
- Over-watering can have effects similar to under-watering, resulting in curled leaves and potentially fewer tomatoes.
- Signs of over-watering include lumps and nodules on leaves and stems (oedema). Adjust watering practices accordingly.
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Too Cold:
- Tomatoes dislike temperatures below 12°C, and if exposed to such conditions, they may not process carbohydrates properly, leading to leaf curl.
- Consider covering plants with fleece during extreme weather conditions, especially in the evening.
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Other Factors Contributing to Leaf Curl:
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Mineral Deficiency:
- Despite using a good tomato fertilizer, a deficiency in nutrients can manifest as curled or mottled leaves.
- Mottling, especially on old leaves, may indicate a magnesium deficiency.
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Hormone Weedkillers:
- Tomatoes are highly susceptible to hormone weedkillers, and even residues carried by the wind can negatively impact them.
- Proper storage of weedkillers away from tomato plants, especially during warm weather, is essential to prevent damage.
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Prevention and Management:
- Optimal growing conditions, including proper irrigation, protection from extreme weather, and addressing nutrient deficiencies, can prevent or manage leaf curl.
- Purchasing quality tomato plants, optimizing growing conditions, and implementing a suitable feeding regime can ensure a healthy harvest despite occasional leaf curl.
In conclusion, understanding and addressing these environmental and physiological factors are essential for maintaining the vitality of tomato plants and achieving successful harvests.
