A homeowner in Washington County spotted an Ironwood tree this spring that had orange slime oozing down the bark. This orange gunk, coupled with white foam, occasionally appears on Ironwood trees as well as Birch trees. It's partly a result of orange-pigmented yeast growing on sap leaking from wounds, says Dr. Robert Blanchette, a tree pathologist with the University of Minnesota.
This orange yeast is just one of many microorganisms that grow on sap leaking from trees. Perhaps the most commonly seen organisms that grow on sap are sooty molds. Sooty molds are black fungi that feed on sugars in tree sap. Streaks of blackened bark colored by sooty mold often appear below tap holes in sugar bushes or below sapsucker holes drilled into sugar maples.
Additionally, there is a suite of organisms that feed on dead outer tree bark or use bark as a surface on which to grow. Several fungal species harmlessly feed on outer tree bark, creating a condition known as "smooth patch". White and bur oaks frequently have smooth patch. Lichens, a combination of a fungus and either algae or a type of blue bacteria, also are common on tree bark.
With all of the above cases, the organisms you see are not harming the tree. In the cases of orange yeast on birch and ironwood trunks or sooty mold on tree trunks, they indicate the tree has some sort of injury. In most cases, that injury is minor and not concerning.
Orange slime oozing down Ironwood bark.
Close-up of orange slime oozing down Ironwood bark.
Black sooty molds on Sugar maples.