Maple Tar Spot Identification Guide

A Fungus You Identify Standing Up

Most of this field guide asks you to look at the ground. This one asks you to look at a tree. From midsummer into autumn, sycamore, Norway maple and field maple carry conspicuous glossy black blotches on their leaves, each ringed by a diffuse yellow-green halo, looking exactly as if someone had flicked tar at the canopy. They are visible from a moving car, and on a badly affected tree a single leaf may carry half a dozen.

The first test takes two seconds. Take a leaf in your hand and rub the black spot firmly with a thumb. It does not smudge, it does not lift, and nothing comes off on your skin. The spot is fungal tissue fused into the leaf, not a deposit sitting on top of it, and that single observation removes most of the things it gets confused with.

Reading the Structure of a Spot

Each black blotch is a stroma: a compacted crust of fungal tissue embedded in and raised above the upper leaf surface. Diameter is usually 5 to 20 mm, occasionally more where two spots merge, and the outline is irregularly rounded to lobed with a slightly scalloped edge.

Go through it deliberately:

Surface. Glossy, coal-black, slightly domed above the plane of the leaf, and finely wrinkled. Later in the season fine sinuous ridges and pale hairline cracks appear across it, and these become important the following spring.

Margin. The black area is bordered by a diffuse chlorotic zone, yellow-green fading into normal leaf colour. In early summer that yellow zone appears on its own, before any blackening, so a June leaf showing pale yellow-green blotches with no black centre is often this fungus at an early stage.

Underside. Turn the leaf over. The corresponding area on the lower surface is pale, slightly raised or faintly discoloured, but it is not black. Blackening is an upper-surface phenomenon, and a spot that is black on both faces is something else.

Section. If you can cut a spot with a razor, the black crust sits on and within the upper epidermis and the outer leaf tissue, with leaf structure still recognisable beneath it.

The Seasonal Sequence Is Part of the Identification

Rhytisma acerinum cannot be fully identified from one visit, and understanding its calendar is what turns a guess into a record.

In spring, ascospores released from last year's fallen leaves land on newly expanded foliage and infect it. Through June and early July, pale yellow-green blotches develop. From July to September the fungus builds the black stroma inside those blotches, thickening and hardening it. In autumn the leaf falls with the spot intact and lies on the ground.

Over winter the stroma matures on the fallen leaf. In spring the black crust cracks open along wavy, lip-like slits, exposing a pale hymenium beneath, and long thread-like ascospores are shot into the air to reach the emerging leaves above. The cycle then repeats.

The practical consequence is that the best confirming visit is in April, not August. Turn over damp overwintered maple leaves under an affected tree and look for black spots whose surfaces have split into sinuous open slits. That structure, an apothecium opening by wavy lips in a black stroma on a maple leaf, is as close to a definitive field character as this fungus offers.

The Spore Print Question

There is no cap, no gills, no pores and no basidia, so a conventional spore print is not possible and its absence should not worry you: this is an ascomycete, and it discharges spores forcibly from asci rather than dropping them.

The equivalent procedure is easy enough. In early spring, collect an overwintered leaf bearing split-open stromata, lay it in a covered dish on damp tissue to keep the humidity high, and suspend a glass slide or a slip of dark paper a centimetre above the spots. Over several hours, discharged ascospores accumulate as a faint pale film that you can then examine.

Under a microscope the confirmation is unambiguous: club-shaped asci each containing eight filiform, thread-like ascospores roughly 60 to 80 µm long and only 1.5 to 2.5 µm wide, tapering at one end and sheathed in gelatinous material, mixed with slender paraphyses that are hooked or curved at their tips. Nothing else on a maple leaf produces that combination.

Host, Habitat and Distribution

The fungus is restricted to maples, genus Acer. In Europe the overwhelming majority of records are on sycamore, Acer pseudoplatanus, with Norway maple, Acer platanoides, and field maple, Acer campestre, also affected. It occurs across Europe and has become widespread in North America.

Habitat is simply where the host grows: woodland edges, hedgerows, parks, gardens, street trees and self-sown saplings. Note one ecological quirk that is genuinely useful in the field. The fungus depends on overwintered fallen leaves to complete its cycle, so it is scarce where leaf litter is removed each autumn, which is why street trees in tidy city centres are often clean while rural and suburban sycamores a few miles away are heavily spotted. It is also historically associated with cleaner air, having been reduced by sulphur dioxide pollution and having returned as air quality improved, which is why it is often cited as an indicator species.

Separating the Look-Alikes

Rhytisma punctatum, the speckled tar spot, shares the host genus but builds its stroma differently. Instead of one solid black blotch, you get a cluster of many tiny black dots, each under a millimetre, scattered within a single yellow leaf patch. Counting dots against solid area is the easiest separation in the whole group.

Rhytisma americanum occurs on red and silver maple in North America and is essentially indistinguishable in the field from acerinum. Host species and microscopic detail are the practical separators, so record the maple carefully.

Rhytisma salicinum has the same architecture but grows on willow. Host genus decides it.

Sooty moulds and honeydew fungi are black but entirely superficial. They form a thin, powdery or filmy layer spread irregularly over the leaf surface and along the veins, they have no defined margin and no yellow halo, and they wipe off on a finger.

Leaf spots caused by Phyllosticta, Septoria and similar fungi are brown to grey rather than glossy black, often sunken and papery, frequently with a dark ring around a pale centre, and under a lens the centre carries pinprick fruiting bodies. They are matt, not lacquered.

Insect frass, scale insects, tar and road grime all scrape or wipe away; the stroma is anchored and takes leaf tissue with it if you try.

Guignardia aesculi produces brown blotches with yellow margins, but on horse chestnut, and they are brown and papery rather than black and glossy.

What to Record

Note the host maple, identified from bud, twig and leaf shape rather than assumed. Note whether the spot is a single solid blotch or a group of dots. Note the month and whether the leaf is still on the tree or overwintered on the ground. Photograph the upper surface, the corresponding lower surface and, in spring, a close-up of the split-open stroma. That set answers every question a reviewer is likely to ask.