I came across a colorful agglomeration of slimy growth on a cut stem of wild grape (Vitis spp.) during a recent walk in a local park. With a little imagination, the shimmering mass took the form of a strange sea monster with a dripping nose, perhaps because of our high pollen count. Of course, I had a little fun with enhancing the effect.
I knew the true story behind the "sea monster" thanks to a diagnostic challenge last year. Participants in a Greater Cincinnati BYGLive! Diagnostic Walk-About came across a colorful display of bright orange viscous goo that appeared to be oozing forth from a flowering dogwood tree (Cornus florida).
We first thought that we were seeing bacterial slime flux. However, a check into the literature produced a sharp turn on the diagnostic pathway. One of the most rewarding things about plant problem diagnostics is not just achieving a successful diagnosis; it's also what we learn from the instructive twists and turns along the way.
Drilling Down on a Diagnosis
Leakage from the stems of trees and other woody plants can be caused by three things: slime flux associated with wetwood; foamy, alcoholic "frothy flux;" and short-lived but often color-enhanced spring sap-flows. The key to separating these sources of tree leakage rests with understanding both the origin of the leakage and the colonization of the fluids by various microorganisms.
Spring Sap-Flow: This condition occurs when sap seeps from recent wounds, or from wounds that have failed to close. While the sap originates deep within the stem, the focal point is the surface of an obvious wound such as a pruning cut, broken branch, or some form of deep stem damage like a lightning strike.
As the name implies, this occurs during heavy periods of sap flow in the spring. It's usually a fleeting condition that ceases when the sap flow ceases or once the wound closes. However, if the wound fails to close, the sap flow will return each spring.
The sap can become colonized by fungal yeasts that produce carotenoids which may impart strange colors to the sap including deep reds, oranges, and yellows. The colonization of the sap occurs under aerobic conditions, but the carotenoids have antioxidant properties to protect the yeast cells from reactive oxygen.
Despite the appearance that the colorful goo is oozing out of the stem much like toothpaste, the sap issuing forth from the stem is actually clear in color. It only looks like some otherworldly goop because the sap has been colonized by the yeast after it reaches the surface of the stem. The yeast also ferments sugar in the sap to produce alcohol; the fermentation produces bubbling in the ooze. The "yeasty" odors that are released are not unpleasant. They smell a bit like baked bread, or as my West Virginia ancestors would have said, it smells like money.
You can see the true nature of the sap in the image below. I took this picture a few days after cutting the grapevine stem to remove the portion covered in yeasty goop so I could take the pictures appearing with this Alert. The bottom line is that spring sap flow does not indicate there is a problem inside the tree.
Bacterial Slime Flux: The slimy, fluxing fluid is called bacterial slime flux because of its connection to bacterial activity. The exact nature of slime flux and the associated wetwood deep inside the tree is not clearly understood. However, it is known that this is a much more serious condition compared to spring sap flow. It is also known that the unsightly bacterial slime flux may last several months and recur for many years.
Wetwood is discolored water-soaked wood that may include the xylem heartwood and sapwood. The affected wood has a high concentration of solutes that draws water through osmosis from the sapwood. The resulting "wet wood" has a much higher moisture content than even the sapwood making the affected zone anaerobic.
Numerous species of bacteria have been isolated from wetwood. Many are naturally found in the soil and enter the tree primarily through root injury which can be “natural” such as drought or freeze damage, or unnatural through physical injury. The bacteria thrive in the anaerobic conditions of the wetwood and as they digest the wood fiber, they release various gases including methane mixed with odoriferous sulfur compounds.
The gasses build pressure to a point where fluids are forced out of openings in the tree. The colorless fluid seeps from openings in the trunk and has a foul odor. The openings may be fissures that split the bark, or the fluids may flow from a pruning cut, broken branch, or some form of deep stem damage like a lightning strike.
These are the same openings from which spring sap flow can issue forth which presents the first challenge with separating slime flux from spring sap flow. The second challenge is that the fluid can become colored (yellow, red, etc.) primarily by yeast fungi that colonize the seeping fluid. Finally, both conditions can lead to bark staining.
Of course, the overall impacts of the two conditions is very different. Spring sap flow is a "surface" condition. Nothing is happening to the wood stem beyond the point of sap flowing from a wound. Slim flux indicates that a more serious condition is occurring within the stem.
Frothy Flux: This is by far the most serious condition associated with fluids leaking from woody stems. Although the exact cause(s) is even less understood than the conditions associated with slime flux, the appearance of frothy flux does not bode well for the long-term health of the affected tree.
The frothy white fluid has a fermentative odor because it is commonly colonized by microorganisms that convert the sugar-rich sap into alcohol. The carbon dioxide bubbling through the liquid during fermentation produces the "froth." Combined, this gives the condition the alternative name of alcoholic frothy flux.
The flux is highly attractive to a number of insects in search of an adult beverage. These include flies as well as various wasps such as yellowjackets, paper wasps, and bald-faced hornets. There is nothing worse than a gag of drunk, belligerent hornets cruising around in their black jackets!
The fluid issues forth from small canker-like cracks in the bark; most often on the main stems of young trees. Exactly how or why the cracks form is unknown. However, the wounds remain evident after the fluids dry up.
Alcoholic flux is most commonly associated with severe stress; most commonly from water-related issues. I first came across the condition with newly planted lacebark elms (Ulmus parvifolia) located in parking lot tree islands. The islands were under almost constant irrigation over a two week period while the irrigation system was being adjusted. Ultimately, around 75% of the trees had to be replaced.
White frothy fluid may also issue forth from bark beetle wounds. The beetles release sap as they penetrate the bark and tunnel through the sugar-rich phloem. Of course, bark beetles are also attracted to stressed trees. Various vascular wilt fungi may also induce alcoholic flux.