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Fire and Wilt

In Sherburne County, a large oak stand suffered a serious fire in early May. Where the fire burned the strongest, bark was partially burned off the lower eight feet of the stem and these trees will obviously die. But in other areas, oaks were partially scorched and live sapwood exposed. Fresh scrapes and wounds to the oaks' sapwood were caused by falling trees and bulldozers. Will oak wilt get a foot hold in this stand because of all the wounding during the fire?

Not this year. In normal years this would have been a cause for concern and actions taken to prevent oak wilt infections. But spring this year was late and very cold, delaying the development of spore mats on infected oaks. According to Dr. Jennifer Juzwik, a North Central Forest Experiment Station researcher, oak wilt spore production was just about nil prior to May 12th in areas north of the Twin Cities. If you remember the jingle "Never prune is May or June", you realize that wounding an oak during this time frame can allow the overland spread of oak wilt by picnic beetles and other vectors. But if you don't have the spores, you won't get the disease even if the oaks are wounded.

On the day that the fire occurred and for 48 more hours, oak wilt spores were not present in the Sherburne County so the probability of overland spread was virtually nil. The oak wilt fungus is poor competitor on the surface of oak sapwood and if spores land on the sapwood 48 hours or more after the initial wound, the fungus is unable to infect the recent wound. Lucky for the landowner; oaks surviving the fire were not infected by oak wilt.