River Birch Mouse Ear

The following is excerpted from an article in Georgia Green Industry Association’s Journal, August 2003. The author of the article is Dr. John Ruter from the University of Georgia.

THE PROBLEM: “Mouse ear (leaf curl, little leaf, squirrel ear) has been a problem in container grown river birch (Betula nigra) since the early to mid 1990’s. . . . The disorder is easy to detect in nurseries as the plants appear stunted and may appear to have been “sheared” into their stunted form. The leaves are small, wrinkled, often darker green in color, commonly cupped, and have necrotic margins. Interveinal chlorosis is generally lacking in symptomatic leaves. New growth also has severely shortened internodes which gives a witches-broom appearance.

“ . . . Other researchers and I have never detected any pathogens or eriophyid mites which could be causing the problem. I have seen problems on plants from #1 containers to #25’s. . . . One grower has reduced the problem by wrapping aluminum foil around above-ground containers. This reduces heat stress on the plants and keeps roots alive and growing. The problem appears to be correlated with plants that have been in containers for too long and are rootbound. The problem also occurs in plants grown using the pot-in-pot system.

“ . . . One very good grower I work with generally only has the problem on his border rows near the road where the sprinklers do not provide enough overlap to adequately water the plants. This generally occurs later in the summer when the roots have filled the container. Containers on border rows are also exposed to increased solar radiation.

“ . . . I have seen the problem on Dura-Heat, Heritage and several seedlings from several sources. . . . I have only encountered one case of plants growing in the field where the problem occurred and this was after plants were exposed to runoff from a pile of limestone . . . This leads me to believe there is something in the native soil that we are not supplying via the pine bark based substrates and highly refined fertilizers commonly in use today.”

THE SOLUTION: “Recent research in Georgia has indicated that the cause of mouse ear on pecans appears to be a nickel (Ni) deficiency induced by high levels of zinc and manganese in the soil. . . . I have applied Ni to river birch trees with severe mouse ear at a commercial nursery on June 9 and one week later certain treatments could be picked out of the block because they were producing normal leaves. At 16 days after treatment, up to two inches of new growth had occurred on certain treatments.

“The story begins to make sense. In the early 90’s we saw the introduction of new controlled release fertilizers which contained micronutients. We started using them in conjunction with the same old rate of micronutrients incorporated into the substrate. By adding too much manganese and zinc we may have induced a problem, similar to what has been seen in pecans. Also, back in the late 80’s we started recommending that superphosphate be dropped from container substrates due to its solublility and problems with eutrophication of waters. Nickel is a known contaminant in phosphate fertilizers. Doubling the micro’s and dropping an inadvertent source of Ni may have contributed to the problem.

Being a heavy metal, research is needed to determine the safest methods of application and the lowest use rates possible. Keeping the root systems healthy and the pH down (5.0-5.5) may be beneficial. Development of commerical products are being considered.”

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North Carolina State University