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, depending on the tree in question. Verticillium wilt is one possibility. That fungal pathogen can infect any of the maples and cause decline, dieback, wilting, and branch death. Verticillium wilt was discussed in issue no. 6. Vascular streaking of the branches is an excellent diagnostic tool for identifying this disease. Laboratory cultures can prove the fungus is present.
Most of the samples that we have seen were not infected with Verticillium. They had small leaves with a red cast. Stem growth on these samples was only 1 inch annually for the last 3 years--a clear indication of stressed conditions. Michael Dirr, in Manual of Woody Landscape Plants, says that red maple is very tolerant of different soil conditions but prefers slightly acidic, moist soils. He also 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. Drought stress aggravates the chlorosis problem.
Another problem on this species is potato leafhopper–feeding injury. In issue no. 7, Raymond Cloyd discusses this insect and the injury it can cause on many species, including maple. Feeding, especially on red maples, results in stunted tree shoots and leaves that curl downward, with brown to black edges. There are three to five generations of this insect per year, so damage still may be occurring.
What do you do 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 whether your tree is in an appropriate site and whether you need to supplement nutrients. Refer to Report on Plant Disease (RPD), 603, “Iron Chlorosis of Woody Plants.” Sometimes spraying an iron or manganese chelate (available at garden centers) on the foliage quickly identifies which element is limiting. If the foliage is deficient in these micronutrients, it turns darker green within a week of spraying. Try to provide more 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 whether the fertilizer should be an acid fertilizer, based on your soil test.
One other problem we occasionally saw in the past was a very 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.