SPRINGFIELD - If you’re seeing spots on maple trees, you are not alone. Tar spot outbreaks are becoming more common and its distinct black spots can leave home gardeners worrying about their tree’s health.
“I recently traveled to Northern Illinois where nearly every Norway maple that I came across had tar spot symptoms,” says Travis Cleveland, a University of Illinois Extension plant pathologist.
This late-summer fungal disease is caused by fungi in the genus Rhytisma that is easily identified by its raised, black spots to develop on the upper surfaces of affected leaves.
The symptoms initially appear in mid-June as small, pale yellow spots. By mid-July, the yellow spots expand and a thick, raised, black stomata starts to form.
While the black spotted leaves look sickly, this disease usually does not affect the tree’s overall health. Severe cases may cause some premature defoliation.
Illinois Extension horticulture experts and the University of Illinois Plant Clinic have had a lot of questions about tar spot this summer.
“Tar spot outbreaks have been more frequent in recent years,” says Cleveland, who also works at the plant clinic. “Likely due to moist spring weather with above average rainfall.”
Trees that are damaged by tar spot every year tend to be in moist, sheltered areas which is an ideal environment for the pathogen. Tar spot fungi overwinter on infected leaf debris and in the spring, the wind carries the spores to developing leaves.
Since this disease is cosmetic, management practices are not usually needed. Raking and destroying infected leaves in the fall or early spring will help limit infections.
Fungicides with the active ingredient Mancozeb or Copper Hydroxide can be used to protect newly developing leaves from infection. Apply the spray when leaf buds are opening and re-apply two more times at 10-day intervals.