Maple Tar Spot

Symptoms may begin to show in late spring as small pale yellow spots, but often go unnoticed until late summer as the darker coloration of the spots become more apparent (image: Tar spot is a common foliar disease on Norway maples and can be particularly pronounced in years with extended wet spring weather.  Fortunately, the damaged caused is generally considered to be a cosmetic problem, rather than detrimental to tree health.  The tar spot is typically seen on maple trees and is caused by several related fungi (Rhytisma acerinum, R. americanum and R. punctatum) that produce black lesions on the upper surface of leaves.  While tar spot can affect many species, it is most common on Norway, silver and red maples.

Symptoms  The first symptoms of infection begin to show in late spring through early summer as small pale yellow spots, and often go unnoticed until mid to late summer as the darker coloration of the spots become more apparent.  Development and size of the spots varies by pathogen and range from tiny spots to larger dark lesions 1” or more in diameter.

Heavy infections can cause early leaf drop which often causes concern as lawns and landscapes become littered with leaves prior to the autumn raking season.  Late summer leaf drop is typically not significant enough to harm the host tree.

Management  Current research has shown that the tar spot fungi does not cause long term damage to the host tree. The most effective management practice in home landscapes is to rake and dispose the leaves as they fall.  The black spots contain fungal spores that will survive the winter season. Disposing infected leaf litter helps to reduce available inoculum on site.  In the following spring, wind and rain release the spores to infect the newly emerging foliage.  Composting leaves may not be effective, because home compost piles seldom reach temperatures high enough destroy fungal spores.

Size of the spots varies by the species of pathogen and range in from tiny specks to larger dark lesions 1” or more in diameter (image UMass).

Application of fungicides may be desired when high levels of infection become unacceptable, however control of the disease is difficult.  If fungicides are used, applications may be needed most years and the cost of treatment may outweigh the benefits.

If your have concerns about the health and vitality of your foliage, consult one of BTSE’s Certified Arborists to discuss plant health care strategies to best care for your trees.