There are several potential causes of leaf curl (also known as leaf roll) on tomato plants, and all indicate some form of plant stress. Essentially, the plants are going into self defence mode, and it is thought that by curling their leaves they can handle their conditions better, particularly in terms of excess/lack of water.
- They are suffering from cold spells, particularly at night. Cold weather can cause lower leaves to curl upwards. This is more common for tomatoes grown outside.
- They’ve suffered from irregular watering, going from very dry spells to very wet spells. This can cause leaves to curl, and can also lead to blossom end rot when the tomato fruits ripen (the bottom of the tomatoes start to rot, turning brown/black).
- They’ve been overwatered. Too much water can be just as stressful as too little water, and the plants will do what they can to repel the excess moisture.
- They’ve been pinched out hard – a sudden harsh pruning can lead the plants to curl their leaves, particularly around fruiting time. If you’ve suddenly noticed and removed a lot of sideshoots, this may well have caused the plants to go into their self-defence mechanisms!
- They’ve been infected by pesticides blowing in. This is unusual if you’re growing them in a greenhouse or polytunnel, but could be a potential cause if they’re growing outside.
- They have a viral disease – this is pretty uncommon, but there is a viral infection that can be carried by whitefly. In this instance, the foliage will get really quite curly and production levels will be affected. However, as we say, it’s not as common and it is far more likely that any curly leaves are brought about by the other points listed above!
Will it affect the crop?
Usually the crop is not badly affected, unless the leaf curl is caused by a viral disease (see point 6 above), so just keep looking after the plants, watering, feeding and pinching out new sideshoots before they grow too big.
How to prevent it in the future
- Keep plants nicely watered (not too much and not too little!)
- Try to keep them nice and warm (which is why they are usually happier in a greenhouse/polytunnel).
- Pinch out sideshoots while they are still small as this causes less stress to the plant.
- Keep aphids under control – companion planting (marigolds and basil are good) and regular checking and hosing off will help.