Tar spot of maple: what the extension services record

This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists tar spot as regulated or notifiable. Each section names its source, all read on October 9, 2026. The full list of diseases is on the tree diseases page.

Cause and hosts

The University of Connecticut and the Cornell Plant Disease Diagnostic Clinic both say tar spot of maple is caused by several fungi of the genus Rhytisma, and both say the name comes from the spots looking like tar on the leaf. Natural Resources Canada names Rhytisma acerinum on a maple leaf and lists red maple, silver maple and sugar maple on its page. Cornell names red, silver, striped and Norway maple. Connecticut says maples and several other tree species show the black spots in late summer and fall.

The spot forms the sources describe

Form	What is recorded	Source
Raised tar spot on native maples	Prominent raised spots 0.25 to 0.5 inches (6.35 to 12.7 mm) across, caused by Rhytisma americanum, called the most common tar spot on native maples in North America.	University of Connecticut
Speckled tar spot	Spots that stay small, about 1 mm across, caused by R. punctatum.	University of Connecticut
Giant tar spot	Many small black spots that later merge into one large black spot of 1 to 1.5 inches (2.54 to 3.81 cm), possibly with a yellow border and less raised and rippled; caused by R. acerinum and common in the northeastern United States on Norway and other European maples. First described in Europe by Persoon in 1794.	University of Connecticut
Red and silver maple	A black spot develops in each yellow spot by mid-July to early August, growing until by late summer it looks like a spot of tar, with a surface that may show wavy ripples.	Cornell Plant Disease Diagnostic Clinic
Striped and Norway maple	20 to 50 pinhead spots appear in late July or early August; on striped maple they do not enlarge much, on Norway maple they grow and merge into a black mass up to 1.5 inches across with a surface that is not rippled.	Cornell Plant Disease Diagnostic Clinic

Cornell adds that the fungus can also attack maple seeds, and shows tar spot on the winged samaras.

Timing and disease cycle

Cornell says the first symptoms usually show in mid-June as pale yellow spots under 1/8 inch across, which enlarge and deepen in colour through the season. Connecticut gives the first sign as a small yellow or pale green spot, expanding over the summer, with black spots on the upper surface in mid to late summer. Natural Resources Canada describes oval or irregular yellowish-green spots on the upper leaf surface that spread, become raised and blacken over time.

All three agree on the cycle. The fungus overwinters in the black spots on fallen leaves. Cornell says that in spring, just as new leaves unfold, the fungal tissue on the ground ripens, the spot surfaces split and minute needle-like spores escape. Connecticut says the spores are produced during damp weather and are dispersed to new leaves by wind and splashing rain. Cornell says spores that land on a susceptible host may germinate, penetrate the leaf tissue and start a new cycle.

Harm to the tree

Connecticut says that numerous spots look unattractive but the disease does not typically cause the host tree any harm, and that tar spots occasionally cause somewhat earlier leaf drop that it calls negligible in its effect because it comes late in the season. Cornell says tar spot alone is rarely serious enough to threaten the health of trees, that heavy infections can cause early leaf drop, and names Norway maple in some upstate New York towns as a case where early leaf drop from tar spot is a particular problem. Cornell also says research has shown that the fungus does not cause long-term damage to the host. Natural Resources Canada calls severe infections with early leaf drop extremely rare and says the main effect is on how ornamental trees look, seldom on their health.

Treatment and reporting

This page gives no treatment steps, product names or timings. None of the pages names a quarantine or a report line. For maple profiles on this site see red maple, silver maple and sugar maple.