Maple Tar Spot

Maple tar spot is caused by Rhytisma acerinum, an ascomycete in the Rhytismataceae. The fungus never forms a mushroom; instead it builds a stroma, a flattened crust of black fungal tissue, within the tissue of a living leaf. The result is one of the most instantly recognisable fungal signs in temperate woodland.

Spots begin in early summer as pale yellow-green flecks. Through midsummer a raised black area develops in the centre of each, expanding until, by late summer, the leaf carries one or several glossy, coal-black blotches with slightly irregular margins and a persistent yellow-green halo. The black surface is often faintly ridged or wrinkled in fine concentric or radiating lines.

The fungus is highly host-specific to maples. On sycamore (Acer pseudoplatanus) and Norway maple (Acer platanoides) it makes large single blotches up to 15-20 mm across, while the related speckled tar spot (Rhytisma punctatum) makes clusters of tiny black dots on a yellow patch. Because the stroma is so conspicuous and so long-lasting on fallen leaves, tar spot is often the first fungal structure a beginner learns to name.

The spots

Mature stromata are 5-20 mm across, jet black, distinctly glossy as if a drop of tar or bitumen had dried on the leaf, and raised a fraction of a millimetre above the surface. The outline is rounded but irregularly lobed, and there is nearly always a sharply defined yellow to yellow-green halo of living tissue around each spot, most visible while the rest of the leaf is still green.

Surface texture

Under a lens the black surface shows fine wrinkling in concentric or radiating folds. In spring, on leaves that have overwintered on the ground, these folds split open along narrow slits to expose the pale hymenium beneath, which is the apothecial stage.

Leaf and tree

Spots occur on the upper leaf surface predominantly, with a fainter brown counterpart beneath. Any number from one to several dozen may occur on a single leaf. Only leaves are affected; twigs, buds, bark and fruit remain clear, which is a useful check.

Spores

No spore print applies. Asci are cylindrical and contain eight very long, thread-like, colourless ascospores, each with a gelatinous sheath that helps them stick to newly opened leaves in spring.

The fungus occurs on maples of many species: sycamore, Norway maple, field maple, sugar maple, red maple and silver maple, along with several ornamental Acer. It is common in woodland, parkland, street plantings, hedgerows and gardens throughout Europe and temperate Asia, and closely related Rhytisma species occupy the same niche across North America.

Spots first become visible in early to midsummer and darken through August and September, remaining conspicuous on the leaf as it colours and falls. The critical phase is the following spring, when overwintered leaves on the damp ground produce apothecia and release ascospores just as buds break. Damp, sheltered sites with heavy leaf accumulation and still air produce far more spotting than open, windy or regularly cleared ground.

Rhytisma acerinum is a biotrophic leaf parasite for most of the season and a litter saprotroph in its final phase, a two-stage life history typical of the Rhytismataceae. It colonises the leaf shortly after bud burst, grows quietly for weeks, and only then forms the visible black stroma in the leaf tissue.

After leaf fall the fungus continues to develop within the fallen leaf, maturing its apothecia over winter using the leaf as a substrate. In spring the stroma splits and shoots thread-like ascospores upward, where wind and rain splash carry them to the new canopy. Because the fungus is sensitive to certain air pollutants, particularly sulphur dioxide, its abundance has long been used as an informal air quality indicator: tar spot became scarce in industrial cities during the twentieth century and returned as emissions fell.

Speckled tar spot (Rhytisma punctatum) is the closest relative and forms clusters of many tiny black dots within a yellow patch rather than a single solid blotch, chiefly on bigleaf and vine maples. Rhytisma salicinum produces very similar black spots but occurs only on willows, and Rhytisma andromedae on bog rosemary, so the host tree separates them at once.

Other black leaf marks can mislead. Sooty mould forms a superficial black film that can be wiped off with a damp finger, whereas a tar spot stroma is embedded in the leaf and cannot be rubbed away. Anthracnose lesions on maple are dull brown to grey-brown, dry and often follow the veins, without the lacquered gloss or the yellow halo. Insect frass and honeydew deposits sit on the surface and are irregular, sticky rather than glossy and hard.