By Joe Boyle
Sue Lehr, public works director for the city of Hickory Hills, said fielding calls about falling leaves and how to dispose of them is expected as autumn makes its arrival.
But a phenomenon known as maple tar spot has some residents concerned and Lehr has had to assure them that they don’t have to worry. Taking some simple steps will help to limit the problem that has not only been occurring in Hickory Hills, but other southwest suburban communities as well.
Suburban residents who walk along tree-lined streets have noticed that leaves have fallen on lawns and sidewalks long before the beginning of fall. Many of the leaves have changed to a yellow color. While it is unusual to see discolored leaves on the ground during the summer, black dots that appear on the leaves is what has drawn the attention of residents.
“Our office has received a lot of calls,” Lehr said. “But the dark spots do not mean their trees are dying. We have had several mild winters and wet springs. It’s just an aesthetic problem. Clean up your leaves and hope for the best next year. But it will not kill the trees.”
Lehr said the discolored trees have been a common sight and it is due in part to the mild winter conditions. When trees are infected with maple tar spot, known scientifically as Rhytisma fungi, the leaves will have round, light green to yellowish green areas about a half to one inch in diameter that eventually blacken. Fungal spores can occur over winter. But Lehr emphasizes that fungicide treatments are usually not necessary.
“Just make sure you clean up the leaves really well,” Lehr said. “But don’t mulch them. That will just allow the spores to remain in the ground and eventually get attached to the trees. The way the weather has been, the leaves don’t have a chance to break down naturally.”
The maple tar spot is a more visible problem for maple trees. According to officials at The Morton Arboretum in Lisle, the yellow spots on leaves become noticeable in late summer and soon expand into black blotches. By the end of September, the black spots are at full size and my even appear to be rippled or deeply grooved like fingerprints. The fungi will not harm the trees and shed when the leaves fall.
Unfortunately, maple tree tar spot is spread on the wind, which means that trees can become infected again next year if the spores hitch a ride on the right breeze, an official at Morton Arboretum noted. Mild winters followed by wet springs can help exacerbate the problem.
Officials at The Morton Arboretum said that black spots on the leaves that contain the fungal spores can survive winters. However, raking and destroying leaves will reduce the number of spores that survive to the next spring. They said that to eliminate these spores, everyone should rake and destroy the leaves from their trees. Composting may not completely destroy spores because home compost piles seldom heat up enough to really kill fungal spores, according to The Morton Arboretum.
“The past three years this type of weather has been pretty prevalent in the Midwest,” Lehr said. “The best way to take care of this problem is to clean up all your leaves from the ground and dispose of them. Encourage your neighbors to do the same. If some of your neighbors don’t, the spores could travel to your tree. Again, just clean the leaves up and hope for the best.”
Fungicides are available to treat tar spots in the spring, but are seldom recommended since tar spots do little damage to trees, according to Morton Arboretum officials. The cost of treatment may outweigh the benefits.
Residents can contact the Plant Clinic at The Morton Arboretum for current recommendations at (630) 719-2424 or firstname.lastname@example.org.