Maple leaf tar spot disease

Trees have been on the minds of many gardeners this year.  It seems like our landscape trees are under assault from all directions.  Some of you may have noticed maple trees are losing leaves early, and many of those leaves are marked with large black spots, which look like someone splashed paint or tar on them.  The name for this condition, Maple Leaf Tar Spot disease, actually reflects that. But relax, the tree will not die from Maple Leaf Tar Spot disease.

Maple Leaf Tar Spot is caused by at least two types of fungus in the genius Rhytisma. The fungus over winters on fallen maple leaves.  In the spring during wet weather the fungus sheds spores into the wind, which then infect maple leaves as they open.  The fungus has been on the leaves all summer, in midsummer yellow spots could have been seen on maple leaves with close inspection, but its late summer when the fungal disease really gets noticed. In late summer Maple Leaf Tar Spot Disease matures into large black spots.  If leaves are heavily infected they usually turn yellow or brown and fall from the tree early.

Gardeners are usually quite alarmed when the leaves start falling early and they notice the spots.  However the trees health is not greatly impacted at this point in time, even if it loses many leaves.  Maple Leaf Tar Spot disease only affects maple trees, including box elder trees, which are actually in the maple family.  It can affect any type of maple although some species are less susceptible than others.

There is nothing gardeners can do at this stage to stop the disease or leaves being shed.  The fungus actually attacked the tree in early spring and any sprays applied now are useless.  Next year fungicide sprays can be applied in early spring as the maple leaves emerge from the buds.  It requires a through spraying from top to bottom and repeat sprayings every 10-14 days until leaves are mature size.  Most homeowners don’t have the capability to do this for large trees, although you could protect smaller trees.  Since Maple Leaf Tar Spot Disease is primarily a cosmetic problem spraying isn’t recommended, except for those in the business of selling trees, where looks count.

What homeowners can do is rake up any maple leaves from infected trees and remove them from under the tree.  This will help prevent the fungus from over wintering under the tree and infecting leaves in the spring.   But since the fungal spores are windborne if there are other maple trees close by your tree may still become infected.

The infected maple leaves can be composted, shredded and applied to lawns and gardens where there are no maples, or if your community allows it, burnt.  There is no worry about the infected leaves being applied under other types of trees or other types of leaves being under maples.  Leaves are valuable for the stored nutrients they contain and should be returned to the soil.

If your tree was heavily infected it can benefit from deep watering if you have a dry fall and after a hard freeze the trees can be fertilized.  Root growth takes place for many weeks after trees shed their leaves and appear dormant.  Apply about 3 pounds of a tree fertilizer per 1,000 square feet of root zone. The root zone is measured from the trunk to about three feet beyond the drip line of the tree, all around the tree.  Fertilizers formulated as 18-6-12 or 12-4-4 or something similar is best.  For maple trees look for a fertilizer that contains iron and manganese.

There are many ways to apply fertilizer.  Read and follow the label directions for the fertilizer you purchase.   Don’t get fertilizer on hard surfaces, where it might wash away and contaminate surface water.  Don’t fertilize trees in the fall before they have shed their leaves and we have had a hard freeze. 

Maple trees can get other diseases at the same time they have Maple Leaf Tar Spot disease and some of these could be more harmful to the tree.  But if your tree looked good for most of the season and then suddenly developed black spots on the leaves and early leaf drop you can relax.  Think of it as a bad case of acne for the tree. Your tree will be fine.