Norway Maple Tar Spot Control

Fungal Spores

• Fungi hailing from the genus Rhytisma are responsible for precipitating tar spot in many types of maple trees. Rhytisma acerinum is the one that most often infects Norway maple trees, according to the Maine Forest Service. It first develops in springtime, when tiny yellowish dots appear on the surface of the foliage. These areas turn blackish during the summer months, looking like small drops of tar on the leaves. In reality, the black spots are stromata, structures capable of producing reproductive spores. These spores can infect trees the following year, even after the leaves they are on fall off. Those leaves that have multiple tar spots often fall off prematurely after turning brown.

Weather Conditions

• The combination of the warm weather that arrives with spring and the usual accompanying damp conditions are ideal for tar spot stromata to reproduce. Once warm weather commences, if damp weather ensues, the tar spots activate on the fallen leaves around the Norway maple. The spots will absorb water, causing them to open and eject their minute spores into the air. Winds then carry them onto the new leaves, starting the cycle over once more. Dry springs are good news for Norway maples when it comes to tar spot, as there are fewer spores around.

Fungicides

• One method of controlling Norway maple tar spot is to spray the trees with an effective fungicide. However, this can become a costly proposition, as several applications of fungicide are necessary when rainy spring and early summer weather washes away one coating of the chemicals after another. You must spray the Norway maples early in springtime to control the tar spot disease, usually early in May before any infections take place on the newly emerging foliage.

Rake and Dispose

• The most effective manner of controlling Norway maple tar spot is to rake up any leaves that fall beneath the tree and get rid of them. Since the fungus survives the winter in these dead leaves, by removing them, you make it impossible for them to infect the next year’s leaves. Bag the leaves or burn them, if legal in your area. Do not mulch them with a mower or toss them into compost piles, since the spores will survive and possibly infect the trees. In neighborhoods where many homes feature Norway and other types of maples, unless everyone rakes and disposes of their leaves, the tar spot may remain to infect trees.