What causes something is running down my trunk — sap, or dark wet streaks?
Bacteria enter through a wound, crack, or root injury and colonize the wet interior wood, producing gas as they ferment sap until pressure finds an exit — which is why it flows from cracks and old cuts, and flows hardest in spring when sap pressure peaks. Elm, maple, oak, poplar, mulberry, and willow are the common hosts. Foamy flux is yeast fermenting sap beneath the bark of a heat- or drought-stressed tree. Insect ooze is the tree's own resin or sap pushing out around a boring tunnel — peachtree and dogwood borers, and gummosis around entry holes in cherry, plum, and peach. Bleeding cankers come from fungi and water molds — Phytophthora at the root collar across many species, Botryosphaeria on drought-stressed trees — infecting bark and cambium so the killed tissue leaks. Most of these start with a wound: mower and trimmer strikes at the base, poor pruning cuts, and mulch piled against the trunk.
What you'd see
What is coming out tells you most of it. A wet, dark, slimy streak running down from a crack, an old pruning wound, or a branch crotch, drying gray or tan, sometimes smelling sour or fermented, with flies and wasps working it. White or tan foam bubbling out of the bark in hot weather. Clear sticky sap or amber blobs, especially with sawdust-like frass or a hole at the center. Dark rusty, oily-looking streaks running down from a sunken area of bark, where cutting the bark shows reddish-brown discolored tissue underneath. Note the location too: from the root flare, from a wound, or from apparently nowhere.
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