Armillaria root rot (honey fungus): what the sources record

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 three pages read lists Armillaria root rot 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, and the fungus also appears as a cause of death in older ash on ash dieback.

Names

The University of California Statewide IPM Program, called UC IPM, says the disease is caused by fungi in the genus Armillaria, also known as oak root fungus, though it says the fungus has no specificity for oaks. Missouri Botanical Garden gives Armillaria mellea as the fungus and also says it is often called oak root rot fungus, because it is commonly found on oaks or where oak trees once grew, such as cleared forest land. UC IPM says Armillaria mellea was once thought to be the only North American species, that more than a dozen are known, and that only two are widespread in California, A. gallica and A. mellea, the second far more virulent. It names the mushrooms honey mushrooms because of their colour and not their taste. Other Armillaria species in forests cause heart rot and act as saprophytes, living on dead wood, according to UC IPM.

Hosts

Missouri Botanical Garden says the fungus occurs sporadically in its region and has been reported to infect over 25 species of ornamental trees and shrubs, also fruit and nut trees, conifers, ornamentals and herbaceous plantings. UC IPM says it has a broad host range, potentially thousands of ornamental and crop plants, and names peach and Peruvian pepper as highly susceptible. The University of California wood decay notes (Pest Notes 74109, revised August 2019) list many conifers and broadleaved woody species and add fig. UC IPM says no cultivars or varieties are completely immune. Missouri Botanical Garden lists plants reported resistant, among them black cherry, bald cypress, ginkgo, hackberry, holly, maple and sweet gum.

Three signs the sources name

The first is mushrooms. UC IPM describes clusters of several to dozens of tan to honey-coloured mushrooms with a ring on the stem, at the base of infected trees or shrubs, sometimes from a buried root away from the tree or around an old stump. It warns that mushrooms may not appear and some trees die without ever making them. Missouri Botanical Garden calls them the most positive sign, with stalks 4 to 6 inches high and caps 2 to 4 inches wide. The University of California notes say mushrooms can form after fall and winter rains. For the mushrooms as a symptom, see mushrooms at the base of a tree.

The second is white to cream, fan-shaped sheets of fungal tissue, called mycelial fans, between bark and wood at the root collar and on roots. UC IPM says that where the fungus has killed the cambium a flat lesion can appear on the bark. The third is black root-like strands called rhizomorphs. Missouri Botanical Garden says they are visible attached to larger roots once soil is removed from the trunk base, and UC IPM says those of A. mellea are 1/32 to 1/8 inch across, generally darker and smoother than fine roots and without a woody centre.

Symptoms above ground

Missouri Botanical Garden says symptoms resemble other root disorders: reduced height growth, sparse foliage, and stunted yellow leaves, sometimes with an abnormal flow of sap at the root collar. UC IPM says the first and most obvious symptom may be sudden death with leaves that turn brown but do not fall, or a slower decline with dieback of shoot tips over months or years. It also lists flat cankers near the ground, flat surfaces on an otherwise round stem, cracking bark and bleeding sap, and says that by the time canopy symptoms appear the fans are very likely well developed on the lower trunk or large roots.

How it spreads and persists

Missouri Botanical Garden says it spreads mainly by root-to-root contact or by root-like fungal strands in the soil, and that successfully attacked trees die only when infections girdle the base. It says on healthy, vigorous trees the fungus is not lethal and may begin to grow when the tree dies of other causes, that young trees are more likely to be killed, and that trees 15 to 20 years old are more tolerant. UC IPM describes a dual nature, a pathogen of living tissue and a decay organism on dead wood, so that infective tissue can persist for decades underground in partly decayed roots after the host is gone. It says growth below ground produces a widening circle of dead trees, a disease center, and that spores from the mushrooms are thought seldom to infect new hosts directly. It says most deaths of infected trees in California landscapes can be attributed to excessive irrigation or other stresses.

Related decay fungi and reporting

The University of California notes give Armillaria mellea a row in a table of wood decay fungi. Another root and butt decay fungus is covered on Heterobasidion. None of the sources names a regulation or report line. This page gives no treatment steps, product names or rates.