Cytospora canker of spruce and other trees: what the extension services record
What The Morton Arboretum, Colorado State University Extension, the Missouri Botanical Garden and UC IPM say about Cytospora canker: the fungi, the spruce and broadleaf hosts, the resin-coated cankers, the link to stressed and wounded trees and how the spores spread.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the four pages read lists Cytospora canker as regulated or notifiable. Each section names its source, all read on October 9, 2026. The full list of diseases is on the tree diseases page. The fungus is named differently by source, and spruce needle cast, a separate disease, is not covered here.
Cause and names
Colorado State University Extension attributes Cytospora canker to a number of Cytospora fungus species, Cytospora kunzei (also called Leucostoma kunzei) among them. The Morton Arboretum names the pathogen Cytospora kunzei, also written Leucocytospora kunzei. The Missouri Botanical Garden names Leucostoma kunzei, formerly Cytospora kunzei, and Valsa kunzei for the perfect stage. Colorado adds that some Cytospora species are host-specific while others infect several kinds of tree, and gives willow, cottonwoods and aspen as susceptible to one species.
Hosts as the sources name them
| Source | Hosts named |
|---|---|
| The Morton Arboretum | Colorado blue spruce is the common and destructive case; Norway and white spruce are somewhat resistant but not immune; occasionally Douglas-fir, hemlock and larch. |
| Missouri Botanical Garden | Many spruces, especially Colorado blue and Norway spruce, from the Midwest to New England; branches and trunks of black, Engelmann, Norway, red and white spruce; also Douglas fir, hemlock, larch and balsam fir. |
| Colorado State University Extension | In Colorado: aspen, cottonwood, poplars (Lombardy and others), fruit trees (apple, cherry, peach, plum), birch, willow, honeylocust, mountain ash, silver maple, Siberian elm and spruce. |
| UC IPM | Describes cankers on bark, twigs, branches and trunk and shows one on a prune branch; no host list beyond that. |
Signs as the sources describe them
On spruce, the Arboretum says the disease often starts on the lowest branches and climbs over several years, though it can strike random branches; infected needles turn yellow, then brown and drop, leaving dry brittle twigs. The Missouri Botanical Garden also names dying of the lower branches as the first symptom, brown needles that drop after a few months, and white or light blue patches of resin on dead bark. The Arboretum says purplish resin oozes from the canker and dries to a thin white layer that can flow several feet down the trunk, and that resin flow can also come from other injuries. Colorado says that on spruce the disease shows as sunken resinous areas surrounded by swollen callus with a gall-like look, and that resin flow without such sunken areas may mean other stresses, diseases or insects.
On broadleaf trees, Colorado describes elongated, slightly sunken, discolored areas in the bark, bark that splits along the canker edge, diseased inner bark turning reddish-brown to black and watery, and wood below stained brown. It adds that the fungus can grow so fast on stressed trees that no sunken canker shows. UC IPM describes irregular brown sunken areas, amber sticky gum at the margins, orange threadlike structures exuding from cankers, and leaves that shrivel and die. Colorado and the Arboretum both mention black pinhead-sized fruiting bodies in the cankered area, and Colorado says orange coiled spore tendrils may ooze from them in moist conditions.
Disease cycle and conditions
The Arboretum says the fungus survives as mycelium and spores in diseased stems, enters through wounds, infects the cambium and leaves dead bark, and that infections occur in cool, wet weather. Spores spread by splashing rain, wind, sprinklers, pruning tools and possibly insects and birds. Colorado says the spores infect freshly wounded tissue, are released when fruiting bodies take up water in rain, and that fruiting bodies are often not formed because cankered tissue dries too quickly in the dry western climate. It says the fungus can attack bark in fall, winter and spring when temperatures are warm but the tree is dormant and cannot defend itself. The Missouri Botanical Garden says spores spread by rain, wind, insects, birds and people.
All three longer sources tie the disease to stressed trees. The Arboretum says trees under stress or growing outside their natural range are more prone, that the disease most commonly affects trees 15 years old or older, and that symptoms become more common one or two years after a severe drought. Colorado lists drought, late spring frost, defoliation by insects or fungi, sunscald, herbicides, mechanical injury and root damage from transplanting or construction, and says drought and flooded soil are the two most common predisposing stresses. It names oystershell scale as an insect that stresses the tree and predisposes it to infection; see oystershell scale. The Missouri Botanical Garden says the disease is more common on trees over 15 years old and on trees of low vigour, shallow roots or unfavourable sites.
Harm to the tree
The Arboretum says infected spruce are substantially weakened and damaged but not necessarily killed, and are often damaged to the point of no longer suiting landscape use. The Missouri Botanical Garden says the disease destroys the symmetry of spruce and in time may kill them. Colorado says the fungi eventually girdle the plant and kill the parts above the canker, and that dead bark may stay on the tree for several years.
Status and control wording
None of the pages names a quarantine or a report line. All four describe stress reduction and removal of infected branches as the main approach. This page gives no treatment steps, product names, rates or timings. For who may apply pesticides see pesticide licences.