Gummosis

A symptom, not one organism

also called Gumming

Amber beads and runs of gum on a peach, cherry or plum. Gum is the tree responding to injury — the question is always what the injury was.

Bacterium Serious Rain splashSpread by insectsSpread on tools & handsAirborne spores Spreads to neighbors

What you'll see

Clear to amber gum beading out of the bark, on the trunk, a branch or around a fruit.
Gum at the base of the trunk mixed with sawdust-like frass, which points at a borer rather than a disease.
Sunken, dark, sour-smelling bark around the gum, which points at bacterial canker.
Gum on fruit at a puncture point, which points at an insect or hail.
Whole branches dying beyond a heavily gumming lesion.

Why it happens

Gum is what a stone fruit produces whenever its tissue is injured — the tree walling off a wound. The gum itself is not the disease. Behind it there is always something else: bacterial canker, a fungal canker, a peachtree borer, winter injury, sunscald, a mower wound or waterlogged roots. Reading which one from the surrounding tissue is the whole of the diagnosis.

Stone fruit on heavy or wet soil, trees stressed by drought or a hard winter, and trees with trunk damage. Wet springs favor the bacterial and fungal causes; hot dry summers favor the borers.

Treating it

Find the cause before treating anything: scrape the bark beside the gum and look at the tissue underneath.
Sour, brown, sunken bark with a definite margin means bacterial canker — cut the branch out well below it in dry summer weather.
Gum with sawdust frass at the soil line means peachtree borer — cut the grub out with a knife and clear the weeds and mulch from the trunk base.
Gum with no rot behind it usually means mechanical or winter injury; leave it, keep the tree watered, and it will wall the wound off itself.
Improve the drainage if the tree is standing in wet ground, which is behind a great deal of chronic gumming.

Keeping it away

Plant stone fruit on a slight mound in free-draining soil and never in a wet corner.
Prune in dry summer weather rather than in winter wet, which is when the canker bacteria are active.
Keep mowers and strimmers away from the trunk and keep the base weed-free and dry.
Paint or wrap young trunks against winter sunscald and rodent damage.
Water deeply in drought and mulch, but keep the mulch clear of the bark.

Who gets it

Peaches, plums, cherries, apricots and almonds — the stone fruits — and citrus.