Many red maples have recently shown stress symptoms throughout Illinois. We’ve seen red leaves near the top of the tree, smaller than normal leaves, some branch decline, and overall poor appearance on Plant Clinic samples. There are many possible causes of decline, and here are a few to consider.
Verticillium wilt is a possibility. That fungal pathogen can infect any maple and cause decline, dieback, wilting, and branch death. Verticillium wilt was discussed in issue no. 8 of this newsletter. Vascular streaking of the branches is a quick diagnostic tool for identifying this disease. Laboratory cultures take about 10 days but can prove the fungus is present.
Many samples contain small leaves with a red cast. The amount of stem growth on these samples is only about 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 has a moderate growth rate and grows 10 to 12 feet in 5 to 7 years. He adds that it is very tolerant of soils but prefers slightly acidic, moist soils. He states 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 on this species is potato leafhopper feeding injury. In issue 10 of this newsletter, Raymond Cloyd discussed this insect and the injury it can cause on many species, especially red maple. Feeding, especially on maples, results in stunted tree shoots and leaves that curl downward, with brown edges. There are three to five generations 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, if it’s present, send to a lab for confirmation of 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), no. 603, “Iron Chlorosis of Woody Plants,” for information on how to do this. Sometimes spraying an iron or manganese chelate (available at garden centers) on the foliage quickly answers the question of which element is limiting. Try to provide additional water to this species in periods of drought stress. The tree does best on moist sites. Consider a general tree fertilizer in the fall. By then, you will know whether to use an acidic fertilizer, based on your soil test.
Rapid decline of an entire red maple tree could be caused by a graft incompatibility. Red maples available in the trade now are usually produced on their own roots, eliminating this possibility.