Weird spots appearing on maples (tar spot)
Editor’s note: We’ve heard from readers that many maples are showing signs of tar spot on their leaves. The following information was written for A Pocket IPM Scouting Guide for Woody Landscape Plantsby MSU Extension’s Diane Brown. Her recommendations indicate the best solution for home gardeners is to rake and destroy the leaves in the fall.
Cause: Fungus Rhytisma acerinum, causes large spots, up to nearly an inch in diameter; R. puntatum causes much smaller spots, about pinhead size.
Hosts (both fungi): Norway, silver, red, mountain, bigleaf and sugar maples; boxelder.
R. acerinum only: Norway, Amur and hedge maples.
Symptoms: As maple leaves develop to full size, light to
yellowish green spots develop in the infected areas of the leaves. The
area becomes yellow, with numerous small, raised, black spots forming
within the yellow area. As late summer and early fall approaches, the
black spots coalesce to form a large, irregular, shiny, raised spot with
the appearance of wet tar, called a stroma. Severely infected leaves
may fall prematurely. How it’s spread: The disease cycle is similar for
both species of the fungus that cause tar spot. In early spring, sticky
spores are released from fruiting bodies on diseased maple leaves lying
on the ground, and travel in the air to developing maple leaves. Within a
month or two, light green spots develop on infected leaves. The tarlike
spots don’t appear until late summer or fall. After overwintering, the
tarlike lesions on fallen leaves produce sexual spores that infect young
maple leaves and continue the cycle of infection for another season.
Management: In recent years, tar spot caused by R.
acerinum has been increasing in frequency and severity in Michigan. The
fungus overwinters on fallen, diseased maple leaves. Rake up and destroy
maple leaves in autumn to reduce the amount of inoculum for the
following spring. In the home landscape, raking up fallen leaves may be
sufficient to manage the disease. In nursery settings, protective
fungicide applications may be warranted as leaves develop in spring;
consult your county Extension office for currently recommended products.