What’s that orange goo?!

By Mike Hillstrom, forest health specialist, Fitchburg, Michael.Hillstrom@wisconsin.gov, 608-513-7690

What’s the orange goo on that tree?!

Should I fight or should I flee?

I bet forest health staff can ID!

Close-up of orange gelatinous gall growing on cedar caused by cedar apple rust.

The spore-producing, slimy, orange gall caused by cedar apple rust fungus.

Indeed, there are number of orange goos in the woods during spring. If it’s a lumpy mass of goo on a log, stump or mulch then it’s probably a slime mold. Slime molds are an amoeba-like group of organisms called myxomycetes. They move very slowly to eat bacteria and organic matter. Slime molds are harmless, so just leave it be and it will disappear on its own.

Orange slime mold on a large vine. Photo credit: Donna Olson.

Slime mold on a vine. Photo credit: Donna Olson.

If you see an orange gelatinous mass on a cedar tree, it’s some type of gymnosporangium rust, such as cedar apple rust. These fungi need two host plants to complete their life cycle. In the case of cedar apple rust, the fungi cause galls on cedar that produce slimy, orange, gelatinous appendages in the spring. To protect nearby fruit trees, galls can be pruned out.

And let’s not forget the orange goos covered in the May 5, 2020 forest health articles by Linda Williams. White pine blister rust produces orange spore-producing pustules, and the spore-producing stages of jack pine gall rust bear either liquid orange goo or orange/yellow powder.

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