What are the black spots on hardy palm leaves

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.