Whether you’ve grown a lemon tree from seed or bought one in a pot, they’re a lovely way to add a touch of Mediterranean charm to a sunny patio. And in colder climes, they’ll easily brighten up a glazed room with their zesty, yellow fruits. But if the foliage also turns yellow, it’s a sign that there’s a problem. Our guide explains what to do.  Spotted black or white leaves on your lemon instead? We’ve got tips for that, too.

The cause and solution to yellowing lemon tree leaves

‘Yellowing citrus leaves is most likely to be a sign of overwatering,’ explains John Negus. This is particularly common in winter, when many gardeners keep these fruit trees indoors. ‘Unless your plant is in a very warm room then it is likely that its growth has slowed due to cooler temperatures, so it will not need nearly as much water.’ Overwatering plants – particularly indoor ones – is a mistake that’s often made by gardeners. You may need to water your lemon tree as little as every two to three weeks, John says, fertilizing it each time. If you are giving it more than this, the compost may be becoming waterlogged, which prevents the roots from functioning correctly and stops valuable nutrients from reaching the plant as they should. ‘The way to determine whether a citrus plant needs watering is to feel the top inch or so of the compost,’ he continues. ‘If it is dry, then apply water – if not, then the plant probably has sufficient moisture available around the roots.’ If you’re growing these fruits in pots, whether indoors or as part of your outdoor kitchen garden, ensure there are drainage holes in the bottom. Confusingly, yellow leaves can also be caused by underwatering and drought. The way to tell if this is the issue is if the foliage also appears shriveled. 

Why have my lemon tree leaves turned black?

Yellowing isn’t the only type of discoloration lemon tree foliage can face. If the leaves have turned black, the problem is sooty mold. This is a disease caused by scale insects – a common plant pest. These tiny, white, lozenge-shaped creatures exude honeydew (a kind of syrup) that attracts the sooty mold, explains John. ‘This then blackens and weakens the leaves.’ Tackle the problem by carefully wiping the leaves, top and underside, with a soft, damp cloth to remove the scale insects and sooty mold. It may also be worth spraying the plant with a bug killer, he adds. Once you’ve eradicated the pests, the sooty mold will stop infecting your plant. 

Why have my lemon tree leaves turned white?

What about if your lemon tree has white leaves? The likely cause is mildew, which luckily, is easy to deal with.  ‘Plants that are growing in humid air conditions and are dry at the roots are most vulnerable. If the lemon tree is in a warm living room, it is likely to need more water,’ says John. ‘To restore the plant to good health, it might be worth removing the worst affected leaves so they are not a source of infection for the plant. It may also be worth spraying with a fungicide if the infection is bad,’ he adds. Another cause for pale lemon tree leaves is a red spider mite infestation. In this case, the foliage may also have a mottled appearance, and be covered in webbing. The leaves will fall off earlier than they would on a healthy plant.  These pests thrive in hot, dry conditions, so misting the plants can help to tackle them. Biological control can also be a useful deterrent for plants grown in a greenhouse.