Unsightly Tar Spot on Maples Again This Year

Homeowners and nurseries may be concerned about circular (1 cm diameter) tarry black spots on the leaves of their maple trees again this year. This is a symptom of a leaf disease on maples called "Tar Spot". The disease appears to be worse this year compared to last year; however, it primarily affects the aesthetic appearance of the trees and is rarely serious. Severe infections observed this year reduced the leaves photosynthetic ability and will result in early leaf drop. Although experts think this is only a cyclical event, trees weakened by this disease from repeated infections over several years may become more susceptible to other disease and insect pests.

Although the disease is usually noticed in August or September, the actual infection took place way back in spring. In fact, the disease is most severe in years that had a cool wet spring such as 2003. Tar spot on maples is caused by a couple of fungi in the genus Rhytisma. These fungi over-winter in the thick black tar looking stroma on fallen infected leaves. During cool rainy springs, such as the one experienced this past year and the year before, spores that form in the stroma are rain-splashed or blown to the newly expanding leaves, which they infect.

The pathogens mainly infect Norway, red, silver and sugar maples. They do not infect other trees like pines, spruce, oak or birch. The disease first appears in early summer as a round light yellowish- green spot but usually goes unnoticed. On red maple and silver maple, a single black spot eventually develops in each yellow spot during late July or early August. Eventually these round spots will turn shiny, black and appear thickened (Figure 1). Infected leaves appear as if tar had been tripped on them. The black tar spot may appear rippled or wavy upon close inspection. On Norway maple, the spots enlarge and eventually coalesce to become a larger black mass up to 2 cm in diameter.

So what can you do to reduce these unsightly spots? Nurseries and homeowners with infected maple trees should rake all of the diseased leaves and burn, bury or compost them this fall. Pruning trees to improve air circulation through the tree canopy will facilitate quick drying of leaves and help reduce infection and disease severity next spring.

Figure 1. Tar spot lesion on infected Norway maple leaf

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