See:
Maple (Acer spp.) - Bacterial Leaf Spot and Dieback
Maple (Acer spp.) - Nectria Canker
Cause The OSU Plant Clinic has found several fungi from maple branch cankers including Cytospora sp., Diplodia mutila, Nectria spp., Phoma sp. and Phomopsis sp. The fungi Cytospora chrysosperma and Stegonsporium pyriforme have been reported from Washington and Idaho. Many of these fungal cankers occur through injuries from freezing, sunburn, drought, or pruning cuts, but under ideal conditions they can infect uninjured bark. It is suspected that trees with Verticillium wilt are more prone to canker development.
Several tree squirrels will strip the bark from late spring to early summer using the tender cambial layer for food and/or shredded bark for lining of nests.
Symptoms Cytospora - Portions of a twig or branch dieback in the spring or summer. Stems and branches develop elongate, shallow cankers where the bark in centers usually contains many small, gray-to-white, pimple-like reproductive bodies of the fungus. Symptoms may also be seen on small-diameter trunks.
Squirrel damage may show as sudden dying of whole branches where dead brown leaves remain attached for many months. Long stretches of bark may be seen removed from the lower portion of these branches. If the branch has been girdled then branches will die quickly. If the branch has not been girdled then distal leaves may wilt, have marginal burning, and/or symptoms of nutrient deficiency. Strands of bark may be seen hanging from the stripped areas.
Cultural control
- Avoid injuring or weakening trees, especially during construction operations.
- Prune out and destroy cankered or dead branches. Prune about 1-foot below the cankered area.
- Infrequent deep watering during extended drought periods may be helpful. Do not focus waterings near the trunk.
- Paint the southwest side of tree trunks with white latex paint to avoid winter injury.
References Voglmayr, H., and Jaklitsch, W.M. 2008. Prosthecium species with Stegonsporium anamorphs on Acer. Mycological research 112:885-905.
Rice, I.Y. 1977. Distribution and Behavior Study of Diurnal Tree Squirrels in Portland, Oregon, with Emphasis on the Western Gray Squirrel (Saiurus grieeus griseus Ord) and the Western Fox Squirrel (S. niger rufiventer E. Geoffroy St.-Hilaire). PhD Dissertation, Portland State University.