Tar spot of maple

Symptoms

The disease affects maples, especially Norway maple and sycamore maple. In summer, large, round, raised, glossy black spots up to 1–2 cm across appear on the upper side of the leaves, deceptively resembling drops of tar or hardened lacquer and often surrounded by a lighter, yellowish halo. The damage is mainly aesthetic, although with heavy infection the leaves drop somewhat earlier. Interestingly, the spots appear mainly where the air is clean — the fungus is sensitive to sulphur dioxide pollution and is therefore regarded as a natural indicator of good air quality. The symptom develops in a characteristic way: the first sign, visible as early as late spring or early summer, is a blurred, pale-green to yellowish patch of discolouration, still without any black structure. Only within it, in the second half of summer, does a thick crust build up — at first matt, later glossy black (the fungal pad, the so-called stroma), slightly raised, with a faintly radial or wrinkled surface. The spots are hard and permanently fused with the blade — they cannot be rubbed off with a finger or washed off with water, which is the key feature distinguishing them from a superficial, sooty film. On the underside of the leaf, opposite the spot, the tissue is sometimes lighter, yellowish. The number of spots ranges from single to a dozen or more per leaf; at high density they may almost touch. The disease is monocyclic — no secondary infections occur during the season, and the mature fruiting bodies (apothecia) form in the black stromata only on the fallen leaves over winter and in spring discharge ascospores that infect the young, developing leaves. Infection does not permanently weaken the tree — even with numerous spots, growth and overall condition remain undisturbed.

Treatment

Chemical control is usually unnecessary, as the disease does not threaten the life of the tree. The most effective measure is raking up and removing or composting the fallen leaves in autumn, because the fungus overwinters precisely in the leaves on the ground and infects the new leaves from there in spring. Breaking this cycle markedly reduces the severity of the spotting in the following season. The effectiveness of sanitation follows directly from the pathogen's biology: the disease is monocyclic, so the only source of infection in spring is the spores from the previous year's overwintered leaves — there are no summer secondary infections that could rebuild it after clearing. The leaves really must be removed from the site: carted away, buried deep, or composted in a heap that will manage to break them down before the spring spore discharge; leaves left loose on the soil surface will still be a source of infection. The operation is carried out in autumn, after leaf fall, at the latest before bud break. It must be said honestly, however, that for a tree growing among other maples the spores drift in from the neighbours' uncleared leaves, so raking under a single tree is sometimes only partly effective. Chemical control of a large landscape tree is in practice unfeasible and disproportionate to the purely aesthetic damage; fungicide sprays are considered at most on young, valuable or nursery trees (see the methods below), and even then only exceptionally.

Prevention

The foundation is site hygiene — systematically clearing the fallen leaves from under the crown, especially where the disease occurred the previous year. An airy, sunny site on which the leaves dry quickly further limits the development of the fungus. Airiness can be improved by thinning the crown — on maples, however, pruning is done in full leaf or in late summer, not in late winter, because these trees bleed heavily (sap flow) after pruning during the period when the sap is rising. It is worth avoiding wetting the leaves and watering that soaks the crown, because for the spores to germinate in spring the fungus needs moisture on the young leaves. A healthy tree with a good water supply tolerates the earlier drop of some leaves without harm, so care for its condition (watering in drought, balanced fertilising) is a real element of prevention. In areas with a large proportion of maples it is worth coordinating the autumn leaf clearing with the neighbours — the inoculum knows no plot boundaries. Finally, an honest note: since the fungus retreats where the air is polluted, the presence of spots is more a sign of clean surroundings than of neglect — on a mature ornamental tree simply tolerating the disease is fully justified.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.