Flattened, ribbon-like stems (fasciation): what extension services say

This page describes what the named sources say. It does not diagnose a tree, say whether any tree or growth is safe or unsafe, or tell anyone to cut, keep or treat one. Each cause and claim belongs to the source that gives it.

What the sources say it looks like

UC IPM calls fasciation a malformation of plant stems that commonly shows as enlargement and flattening, as if several stems were fused. It lists swollen, flattened and twisted stems, with small stems or leaves that can be abnormally abundant and undersized, and terminal growth that can be dense and tuftlike, which it says is also called a witches' broom. Washington State University's HortSense describes a ribbed stem that looks as if several stems were joined side by side, often twisting, with leaves that are usually normal in shape but may be undersized. South Dakota State University Extension adds that it can show up in almost any plant part, most noticeably stems and flowers, and that stems can look flatter and wider than normal, sometimes called ribboning. The Royal Horticultural Society says the flattened shoots can carry an abnormal number of flowers and that normal branches may arise from a fasciated stem.

Causes the sources name

Source	Causes it names
UC IPM	The bacterium Rhodococcus fascians for at least some infectious cases, and certain viruses for some. Some cases are noninfectious, possibly a genetic disorder. The cause of many has not been identified.
Royal Horticultural Society	Random genetic mutation or disruption, the bacterium, viral infection, and damage from frost, animals including insects, chemical or mechanical injury. It says hoeing or forking around a plant has been implicated.
Washington State University (HortSense)	Changes in the genetics of the growing point. On maple it calls the cause a mutation in the growing point of the stem and says it is not a disease, not contagious, but may sometimes be spread by seeds or through propagation of affected parts.
Montana State University Extension	Abnormal activity in the growing tip, from hormonal or genetic factors, pathogens (bacterial, fungal or viral), insect or animal damage, or chemical exposure, mechanical injury and frost. It says trees in the Prunus and Salix genera and the rose family are examples that show it most often.
South Dakota State University Extension	A mutation during cell division, with the exact reason some plants mutate and others do not unknown. It says injury, cold, growth hormones or a pathogen can trigger the change, and shows a sweet cherry with fasciation after pruning injury.

Whether the sources call it harmful

UC IPM says fasciation rarely threatens the survival of established woody plants and that most cases are aesthetic damage and uncommon, but that plant parts fasciated by the Rhodococcus fascians bacterium may die prematurely. South Dakota State says it is generally not a cause for concern, because the condition that formed the mutation will not likely be repeated. The Royal Horticultural Society says it is unpredictable, usually limited to a single stem, and seldom recurs on herbaceous plants the following year, although it names forsythia as a plant that can repeat. Washington State notes that in some cases it may be a desirable horticultural attribute, and Montana State and the Royal Horticultural Society both say some plants are kept on purpose for their fasciated form.

What the sources list for management

Washington State and Montana State list pruning the affected part out. The Royal Horticultural Society says affected parts of shrubs and trees can be pruned out if desired. UC IPM lists sanitation, pathogen-free planting stock and avoiding wounding, and says to avoid propagating symptomatic plants. Washington State recommends no chemical treatment, and Montana State says there is no treatment for fasciation. For who may apply pesticides, see pesticide licences.

Look-alikes the sources name

South Dakota State says a gardener may first take the distortion for herbicide damage, which it describes as generally cupping, twisting and mild to severe tissue damage, and that a witches' broom may be mistaken for fasciation. The herbicide page is herbicide injury to trees and the broom page is witches' brooms.

Other growth forms are on the odd tree growth page.