Question: we have several maple trees in our neighborhood afflicted with this black spot (see attached photo). What can we do to protect our maple trees?

Anwer: while it is difficult to confirm a diagnosis from photos alone it looks and could be a fungal disease called tar spot. The most common species are Rhytisma acerinum and R. punctatum. Symptoms first appear in late spring or early summer as infected leaves develop light green or yellow-green spots. During mid to late summer these produce black tar-like raised structures on the upper surface of leaves within the yellow spots. R. acerinum causes larger spots that are 0.5 to 2 cm in diameter; R.punctatum causes many small punctate spots that are smaller (about 1mm in diameter). Spots caused by R. punctatum are sometimes called speckled tar spots.

Tar spot is a common disease on silver maple and is primarily considered to be a cosmetic problem rather than a serious threat. The Purdue Plant and Pest Diagnostic Laboratory has additional information on tar spot including more photos and best treatment, Tar Spot on Maple.

To confirm a diagnosis, you can submit a sample directly to the Purdue Plant & Pest Diagnositc Lab (PPDL) or find a certified arbroist to come to your property and access your tree.

Resources:
Large Spots on Maple Leaves that Look Like Tar, Purdue Extension News
Find an Arborist, International Society of Arboriculture
Diseases in Hardwood Tree Plantings, The Education Store, Purdue Extension’s resource center
Fifty Trees of the Midwest app for the iPhone, The Education Store
Native Trees of the Midwest, The Education Store
Shrubs and Woody Vines of Indiana and the Midwest, The Education Store
Tree Installation: Process and Practices, The Education Store
Planting Your Tree Part 1: Choosing Your Tree, Video, Purdue Extension – Forestry & Natural Resource (FNR) YouTube Channel
Investing in Indiana Woodlands, The Education Store
Forest Improvement Handbook, The Education Store
ID That Tree, Purdue Extension-FNR YouTube playlist

Purdue Plant and Pest Diagnostic Laboratory

Diana Evans, Extension and Web Communication Specialist
Purdue University Department of Forestry and Natural Resources