What are the black spots on hardy palm leaves

The short answerBlack spots on hardy palm leaves are almost always a fungal leaf-spot disease rather than a sign the plant is dying. There is no specific cure, so the goal is prevention: feed with potash instead of a high-nitrogen fertiliser to toughen the leaves, and thin out crowded growth so air can circulate and the foliage dries quickly after rain.

Exotic foliage garden plants have never been more popular, following on from the huge interest in houseplants, but many hardy palms are presenting with black spots on their leaves. These elongated oval blotches – like the ones on the Chamaerops humilis (European fan palm) in the main picture here – are not too disfiguring in small numbers, but where there are lots of them the effect can be to make the plants look very sickly.

The shape of the marks seems to follow the shape of the leaf blades, stretched along the veins which transport the sugars and water in the plant. The oldest leaves usually show the most symptoms, but by the end of the growing season you are likely to see at least a few spots on most foliage.

What’s causing the spots on palm leaves

In common with many other plants, the foliage of palms is liable to infection by fungal diseases. These take advantage of the high sugar content and relatively thin cell walls of the leaves. There are many types that are referred to as ‘leaf spot fungi’, often specific to particular plants. They spread by microscopic spores which are released from the fungal colonies, mostly during the growing season from spring to autumn.

These spores need moisture on the leaf surface in order to develop into a new fungal colony. This means that infection is most effective during periods of high humidity. Spores produce tiny tube-like growths which tap into the surface of the leaf to extract sugars that power further growth of the fungus. The plant tissues which these sugars are removed from may initially turn yellow, before becoming brown or even black as the cells die. This leaves behind distinctive ‘spots’, often with a ‘halo’ of yellow around their edges.

How can leaf spot be prevented

Whilst there are still a number of general fungus killers or ‘fungicides’ available to treat certain infections, there is nothing specific for leaf spot on palm leaves. As with all plant pests and diseases, it is important to opt for prevention as the first line of defence against infection. This involves being careful with the nutrients that are given to plants and, in some cases, careful pruning.

Giving plants a fertiliser which contains lots of nitrogen will make them more prone to infection. Nitrogen promotes vigorous, lush growth which is easily attacked by fungi or fed on by insects. By contrast, the nutrient potassium will toughen up the tissues of plants making them more resistant to infection. Additional potassium can be given in the form of high-potash liquid ‘feeds’ (such as tomato fertiliser) during the growing season, or as a solid top-dressing of sulphate of potash. The latter is usually applied once at the start of the growing season in early spring and then followed up with another application in mid-summer.

Pruning is also useful in tackling such leaf spot diseases. Thinning out the growth of branches or, in the case of palms, the entire leaves, will allow more air to circulate through the plant so that the leaf surfaces can dry out between rain showers. In addition, air circulation will help prevent the build up of humidity around the foliage. In both instances, such pruning reduces the humidity levels that are essential for the development of the fungal spores.

And it’s not only the palm that may need pruning. Adjacent plants can be thinned out, too, so that they don’t trap moist air around the palm leaves. Such techniques are regularly practiced to help limit plants diseases, as well as pest infestations. They are the basis of good plant cultivation and gardening.

Frequently asked questions

Will leaf spot kill my palm?

No. A few spots are barely noticeable, and even a heavy outbreak that makes the plant look sickly rarely threatens its life. The aim is to stop it spreading rather than to rescue a dying palm.

Is there a spray or fungicide for it?

There is nothing specific to leaf spot on palms. A few general fungus killers exist, but the dependable approach is prevention through feeding and airflow, not spraying.

What should I feed a palm to prevent it?

Avoid high-nitrogen fertilisers, which push out the soft, lush growth that fungi attack most easily. Instead use a high-potash feed such as tomato fertiliser through the growing season, or a top-dressing of sulphate of potash in early spring and again in midsummer, to toughen the leaves.

Why do the same leaves keep spotting?

The oldest leaves usually show the most, and damp, still air lets the spores take hold. Thinning crowded leaves on the palm, and on any plants pressing in around it, lets air move through so the foliage dries quickly after rain and the humidity stays low.