Tar spots are one of the most common fungal infections of maple trees in our area. Like most leaf affecting fungus it all starts in the spring and symptoms tend to be more common on trees growing in moist, sheltered locations. Spores are released, from the infection on last years leaves and from the soil, just as the new foliage from this year is developing. Any time a spore manages to find its way to the soft delicate tissue of a brand new leaf it can germinate and a new spot is born.
They first appear as a slightly faded yellow mark but will soon become a raised black blotch and remain an eyesore for the rest of the year. Fortunately, for your tree that’s all it is, an eyesore. Tar spot cause nothing more then cosmetic damage to the leaves of your tree. In severe cases, premature leaf drop can occur.
Although absolutely terrifying in appearance to the concerned tree lover, this fungus is merely using the leaves as a vehicle for reproduction. To help control the infection, rake up and destroy infected leaves as they fall since tar spot fungi overwinter on fallen leaves and in the soil. This will reduce the number of overwintering spores that can infect new leaves the following spring. Properly trimmed trees can also help. This allows more sunlight and airflow throughout the tree to reduce moisture. If you would like more options, talk with an ISA Certified Arborists about other possible treatments.
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