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OAK WILT? FROST DAMAGE? OR A NEW MYSTERY DISEASE?
I stood belly to belly with a 150-year-old bur oak tree (Quercus macrocarpa), craning my neck to search its lofty limbs and branches for the slightest sign of life. Think about it - this creature of habit had been doings its leaf thing for more springs than Minnesota has been a state!
Bur oaks grow slowly (1 inch of trunk diameter every 12 years) and are not easily transplanted, which explains why few are selected by homeowners as back-yard shade trees. Very old bur oaks can live for 300 years and reach 100 feet in height.
On this spring day - a spring certainly no more or less severe than many before it - the bur oak in my yard stood motionless, its branches and twigs as bare as my bald head. Was it dead? No, because several yellow-green clusters of new leaves were managing to push through their terminal bud cases on branches close to the ground. Was it dying? It certainly looked that way.
Elsewhere on my lot, and scattered throughout the adjoining golf course, other, less-majestic bur oaks were also struggling to send forth their spring robe of green.
My initial suspicion was more depressing, just about, than a grown man can bear: oak wilt. Had this deadly epidemic spread underground through the interconnecting root systems of all of these brothers and sisters and first cousins?
Oak wilt has been killing Minnesota oak trees since the late 1800s. Red oaks can succumb in a single season, while white and bur oaks may take up to five years to die. Oak wilt is caused by a sapwood-inhabiting fungus similar to the fungus that causes Dutch elm disease. Disease transmission is most common through the roots but can also be spread by picnic beetles in the spring via wounds in a tree's bark.
I called DNR forester Al Olson, who offices in Waconia and watches over the forests of Carver, Scott and Hennepin counties. He stopped by later that afternoon and crawled around the upper limbs of one of my bur oaks, studying the bare twigs and cutting a few samples. He left scratching his head, somewhat puzzled by the situation, and promised to call in a couple of days when the lab results are complete. In recent days, Olson said, he had heard about similar problems with bur oaks in Plymouth and in Credit River Township in Scott county.
Several phone calls from homeowners the next day left me perplexed. Several reported oak leaves failing to appear, while others asked about barren ash and poplars. A drive to Chaska revealed other stands of equally aged bur oaks with the same problem.
I called Cindy Ash, University of Minnesota plant pathologist, to get her fix on the problem. "I doubt you've got oak wilt," she said quickly. "More likely, your trees all suffered bud damage on April 5, when the temperature dropped to -8ƒ F. It was particularly damaging to oaks and poplars."
My spirits rose as she spoke. Frozen terminal buds meant a momentary set back - not death, like in forever. Frozen terminal buds meant that leaves might still be forthcoming from lateral buds present on all branches.
I called Olson again, sharing this new diagnosis. "That makes more sense than oak wilt," he said. "I've seen oak wilt take out an entire stand of red oaks while it left the bur oaks untouched."
As this is written, I'm still waiting for the lab analysis. That test will reveal only that oak wilt is, or is not, present in the tissue of the sample. If oak wilt is not present, the April 5 hard freeze is the most logical explanation.
Meanwhile, remember the rules about trimming and pruning oaks - leave them alone during April, May and June, the period when oaks are most susceptible to infection from fungus-carrying insects.
Good luck with your trees. I'll write a postscript in a future column on how my bur oaks are faring.
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