Tar spot of maple

What is tar spot of maple?

Tar spot of maple is caused by fungi in the genus Rhytisma. Rhytisma acerinum causes giant tar spot, mostly on Norway maple and other European maples. Rhytisma americanum causes the common tar spot on native maples like silver and red maple, and Rhytisma punctatum causes tiny speckled spots.

Infection happens in spring as new leaves come out, but nothing shows at first. UMass says pale green or yellow spots appear after leaves reach full size. By mid to late summer, shiny black raised spots form in the middle, looking like drops of tar.

UW-Madison says plainly that tar spot does not kill trees or shrubs and does not usually cause serious leaf drop. UConn says it does not typically harm the tree. Heavy infections can make leaves drop somewhat early. UMass notes it has been reported to cause poor survival and even death of seedlings and saplings in shaded understories.

Symptoms and signs

• Pale green or yellow spots on maple leaves in late spring or early summer
• Raised, shiny black spots that look like drops of wet tar by mid to late summer
• On Norway maple, many small black dots merging into one large spot up to about 1.5 inches
• On native maples such as silver and red maple, smaller raised spots, often about 1/4 to 1/2 inch across
• Some early leaf drop in late summer or fall in bad years

Tar spot of maple identification: what to look for

• Symptoms: Pale green to yellow spots appear on leaves in late May to June, after leaves reach full size.
• Signs: Raised, shiny black fungal structures (stromata) form by mid to late summer and look like wet tar.
• Symptoms: Norway maple: many small black spots that merge into one large spot up to about 1 to 1.5 inches across, sometimes with a yellow border (UConn). Native maples: smaller raised tar spots, often 1/4 to 1/2 inch across.
• Timing: Spores come from last year's fallen leaves in spring during damp weather. Black spots show in mid to late summer.

Disease cycle

The fungus overwinters in fallen infected leaves, which tend to land with the black spots facing up. In spring, at or shortly after bud break, the spots release spores in damp weather. Wind and splashing rain carry them to new maple leaves. Spots show up after the leaves reach full size. Rainy weather in late May and early June leads to worse outbreaks (UMass).

When to look: Spores fly in spring. Look for yellow spots in early summer and black tar spots from midsummer into fall.

How to manage tar spot of maple

Cultural control

• Rake up and dispose of fallen maple leaves every fall. This is the main control.
• Burn (where allowed), bury, or hot-compost the leaves. UW-Madison says a compost pile should reach about 140°F and be turned.
• Improve air flow around crowded trees. UMass says early leaf drop is most common in heavily shaded spots with little air flow or where many Norway maples grow together.

Chemical control

Fungicides are not recommended for landscape trees. UMass says they are not recommended, and UW-Madison says treatment is rarely, if ever, warranted. UW-Madison notes copper products, alone or with mancozeb, are labeled for tar spot in Wisconsin, and that if treatment were used it would take three sprays: at bud break, when leaves are half expanded and when they are fully expanded. UConn says spring sprays can be cost-prohibitive. Use only products labeled for this use; the label is the law, and large trees need a licensed applicator (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Clean up maple leaves each fall.
• Spores travel on local air currents (UMass), so leaves under nearby maples can be a source too.

When to call a pro

You usually do not need a pro for tar spot. Call an ISA Certified Arborist only if the maple also shows dieback, bark damage or other decline, which point to a different problem.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about tar spot of maple

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