Q. We have several maple trees in our neighborhood afflicted with this black spot (see attached photo) last year. We're looking for information on cause, prevention, health risk factors (to the trees), treatment and long-term prognosis for the infected trees.
A. While I can't confirm a diagnosis from this photo alone, a fungal disease called tar spot would be a good guess. 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 at http://www.ppdl.purdue.edu/PPDL/weeklypics/1-19-09.html.
To confirm a diagnosis, you can bring a sample to your Purdue Extension office in your county. Find contact information at http://www3.ag.purdue.edu/counties/. Or you may submit a sample directly to the PPDL. See http://www.ppdl.purdue.edu for instruction.
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Purdue Plant Pest and Diagnostic Lab
Writer: Rosie Lerner, Purdue Extension, Consumer Horticulturist