Cast members of the new Ghostbusters film aren’t the only ones getting slimed – trees sometimes get slathered in slime flux as well. Many kinds of trees are subject to sludge assaults, with elms, apples, oaks, maples, and walnuts being among the more vulnerable species. Tree-goo, unlike the Psychomagnotheric Slime in Ghostbusters, is basically harmless. In fact, it can be beneficial. Also known as bacterial wetwood, slime flux is pretty much what it sounds like: wet nastiness that oozes from a bark crack, V-shaped trunk union, or pruning wound like an eternal fountain of fetid foam.

It is also perennial; once a tree has it, there’s no way to cure it. Sort of like herpes for trees, I guess. At its mid-season peak, slime flux can seem dire, and is often a source of concern for the homeowner. Although dramatic, slime flux is not even a disease, precisely. It can involve one or more of about a half-dozen different types of native soil bacteria, including Clostridium and Klebsiella. If these names ring a bell, it’s because a few species in those genera cause human illnesses like botulism, tetanus, pneumonia, and meningitis. Don’t worry – bacterial wetwood can’t make you sick any more than gardening or simply touching the ground can.

For bacterial wetwood to get started, three conditions must be met. The key requirement is a stressed-out tree, which is most always a result of root damage. This can be from excavation, soil compaction, the addition of fill, or deicing salt exposure. In our fast-changing climate, even so-called “natural” events like the severe droughts we had in 2016 and 2018 will gravely damage root systems. An opening in the bark is also necessary. This is easy to come by, as wounds caused by frost or drought cracks in the bark, storm damage, pruning, mowers, and wildlife are commonplace.

The third element is soil bacteria. Given that trees and soil go together, there are plenty of these available at every site. There is only one treatment for bacterial wetwood: chill out and do nothing, which is my favourite kind of solution. Bacterial wetwood is not as bad as it looks. Also, just because one tree has an infection, other trees are not at increased risk of developing one. Power-washing the slime flux or dousing it with disinfectants will hurt the tree and do nothing for the infection. In the past, arborists seem to have to modeled tree interventions after human first-aid, plastering stuff over pruning wounds and filling trunk cavities. And just as we might lance a boil, arborists would drill through a trunk or branch into the wetwood as a means of draining it. However, when an amazing concept called “research” was at last applied to tree care, it turned out all of those things were bad for trees.

We can’t fix bacterial wetwood, but we can take steps to minimise the risk. Prune only in the dormant season – cuts made during spring leaf-out and fall colour change are more likely to become infected. When pruning an infected tree, sterilize tools with rubbing alcohol or a 10% bleach solution between cuts. Maintain a mulch ring (but not a volcano) around trees to prevent injuries from “lawn-mower blight.” Help support a tree’s optimal health by not driving on or adding fill to its root zone, which is three times the branch length. Appropriate watering during dry spells can reduce stress (mainly for the tree), and mulching to its dripline helps conserve moisture.

Although unsightly, slime flux can be good for trees. It raises the pH of the surrounding bark from about 6.0 to between 7.0 and 8.0. It may not sound like much, but this is one to two orders of magnitude more alkaline than normal tissue. The significance of this is that a higher pH is less hospitable to many fungal marauders like Armillaria mellea, the most virulent of native tree pathogens.

ISA-Certified Arborist Paul Hetzler is looking for a Ghostbusters Proton Pack to neutralize bacterial wetwood.

Photo at top: Bacterial Wetwood. Credit: Penn State Department of Plant Pathology and Environmental Microbiology Archives, Penn State University, Bugwood.org