Amur maple (Acer ginnala) is a 15- to 20-foot-tall deciduous tree growing in U.S. Department of Agricultural plant hardiness zones 3 through 8. It produces clusters of yellowish white fragrant blooms in the spring, and its green foliage changes to shades of red in the fall. Amur maple has few serious problems, but can develop leaf spots that negatively affect the appearance of the tree.
Bacterial Leaf Spot
Also known as bacterial blight, bacterial leaf spot occurs during long periods of rainy spring weather. Infected leaves develop black or brown angular or irregularly shaped spots on the surface, and cankers or lesions can appear on stems. When bacterial leaf spot occurs, remove branches and leaves with severe dieback or blight to help prevent the disease from spreading. However, you must prune diseased plant tissue during periods of dry weather. If bacterial blight is only affecting the leaves, the maple tree will probably survive. Unfortunately, the tree will probably succumb to the disease once it becomes systemic and cankers forms on the trunk, according to the University of California Statewide Integrated Pest Management Program website.
Fungal Leaf Spot
Yellowish or brown patches or dots that range in size are the first symptoms of fungal leaf spot. Fungal leaf spot is rarely a severe problem that results in long-term damage. However, this fungal disease -- which is prominent in moist conditions -- can cause infected leaves to fall from the tree branches. The University of California Statewide Integrated Pest Management Program website states that fungicide treatment is typically not needed and in most instances, fungal leaf spot can be tolerated.
The fungal pathogen causing tar spots typically presents itself as yellowish spots on the leaves. As the disease progresses, these spots enlarge and begin to develop a black tar-like appearance. This fungal disease spreads via spores carried by wind and -- like most other fungal pathogens -- will overwinter on diseased leaves that have fallen to the ground. Thankfully, tar spots are not considered a serious problem and are more of an annoyance than a threat to maple trees.
No matter what leaf spot disease you are dealing with, prevention is practically the same -- good sanitation practices and proper cultural care. Most fungal pathogens causing leaf spots overwinter on infected leaves that have fallen to the ground, which will lead to future infections. By raking and destroying all fallen plant matter from around the Amur maple tree, you will reduce the chance of leaf spots as well as other fungal diseases. Also, avoid over wetting the leaves with overhead irrigation. Fungal pathogens thrive in moist conditions; and when the surface of the leaves stay wet for an extended period, the chance of leaf spots developing increases. Instead, water near the trunk or base of the tree.