Many red maples have recently shown stress symptoms throughout Illinois. Callers report red leaves near the top of the tree, smaller than normal leaves, some branch decline, and overall poor appearance.
It is likely that various problems are to blame. One possibility, Verticillium wilt, has been confirmed on a red maple this season at the Plant Clinic. That fungal pathogen can infect any of the maples and causes decline, dieback, wilting, and branch death. Verticillium wilt was discussed in issue no. 8. Vascular streaking of the branches is an excellent diagnostic tool for identifying this disease. Laboratory cultures can verify the fungus.
Most samples we saw were not infected with Verticillium. They had small leaves with a red cast. Stem growth was only 1 inch annually for the last 3 years–a clear indication of stress. Michael Dirr in Manual of Woody Landscape Plants says that red maple is very tolerant of soils but prefers slightly acidic, moist soils. He adds that chlorosis shows on foliage of trees in high-pH soils. The actual limiting element may be iron or manganese, but the cause is the high-pH soil that ties up these elements.
Another problem some may be seeing is potato leafhopper feeding injury. In issue no. 4, Raymond Cloyd discussed this insect: “Feeding, especially on maples, results in stunted tree shoots and leaves that curl downward, with brown edges.” There are three to five generations of this insect per year, so damage may still be occurring.
If your red maple is showing stress, look for potato leafhoppers on the newest leaves. Look for vascular streaking of the wood, and send to a lab to confirm Verticillium wilt. Have a soil pH test run to determine if your site is appropriate and if you need to supplement nutrients; refer to Report on Plant Disease (RPD) no. 603, “Iron Chlorosis of Woody Plants.” Sometimes, spraying an iron or manganese chelate (available at garden centers) on the foliage quickly shows which element is limiting. Try to provide additional water to this species during drought stress. The tree does best on moist sites. Consider a general tree fertilizer in the fall. By then, you will know if the fertilizer should be acidic, based on your soil test.
Another problem we occasionally saw was a rapid decline of an entire red maple tree. In many cases, this was caused by a graft incompatibility. Red maples available in the trade now are usually produced on their own roots, eliminating the possibility of graft incompatibility.