Winter Tree Care: Identifying Common Tree Diseases in Winter, Part 2

This is the second part of a series on winter tree diseases and disorders. This article examines Armillaria root rot and annosum root rot.


During the winter months, many of the fungal pathogens that affect trees enter dormancy. The pathogens overwinter on their hosts or in the soil, awaiting spring’s arrival. Despite the frigid temperatures, trees suffering from fungal diseases may still exhibit infection symptoms in winter, especially if the disease has advanced into its later stages. The following describes some of the most common diseases to overwinter on trees, and how they may be detected.

Armillaria Root Rot

Armillaria root rot, also called fungal root rot, or Armillaria root disease, is a plant disease caused by several members of the genus Armillaria. The fungus most often identified as the causal agent is Armillaria mellea. Several closely related species may also infect susceptible plants. Armillaria fungi act as parasites, infecting trees, and causing the inner wood tissue to decay. Infected trees often decline, with many eventually succumbing to the disease.


Armillaria root rot infects hundreds of species of trees, shrubs, vines, and forbs growing in forests, along roadsides, and in landscape settings. Larch and birch are the only trees that exhibit a resistance to the disease.

Symptoms of Infection

Armillaria fungi typically inhabit roots. Mushrooms may be found growing in clusters around the base of infected trees and stumps. Mushrooms are produced sporadically from late summer to fall. They are most abundant during moist periods. Mushrooms have yellow or brown stalks around two inches long. Some mushrooms form a ring around the stalk just below the gills. Stalks have yellow caps that are two to five inches in width. The upper side of the cap is laden with dark brown scales, and may be slightly sticky. The underside of the cap features light-colored gills, which produce millions of light yellow to white spores.

Crown symptoms on conifers and broad-leaved trees vary. Generally, the foliage thins and becomes discolored, turning yellow before finally browning. Shoot and branch die back may occur, with foliar growth significantly reduced. On large, vigorous trees, crown symptoms develop gradually over several years. Broad-leaved trees may form sunken cankers on infected limbs and branches. These cankers are covered with loose bark, or bark infiltrated with resin. Conifers, particularly Douglas-fir and western larch, frequently produce an abnormally large crop of cones, referred to as stress cones, as they decline. On most conifers, the infected portions of the lower stem will enlarge, and exude resin. The infected portions of the roots often become encrusted with resin, soil, and fungal tissue. On stressed or weakened trees, crown symptoms develop rapidly, with the tree often succumbing to the disease within a year.

Removing the bark on infected trees will reveal white mycelial mats on the rhizomorphs that form between the wood and the bark. Rhizomorphs also grow through the soil, becoming entwined with healthy roots. Rhizomorphs are black or reddish brown, with a compact outer layer of dark mycelium, and an inner core of white mycelium. The thick mycelial mats on the rhizomorphs decompose, leaving impressions on the inner bark.

Stripping the bark away may also reveal a white rot of infected wood. When the decay process first begins, infected wood turns light brown, and looks faintly water soaked. As the decay advances, the infected wood turns light yellow or white, and may be marked by numerous black lines. In hardwoods and conifers, the texture of decayed wood varies. Decayed wood is spongy in hardwoods, but stringy in conifers.

Trees that are suffering from drought stress, or have incurred injuries through mechanical wounding, insects, or other fungi may produce symptoms associated with those caused by Armillaria fungi.


  • Armillaria root rot cannot be completely eradicated. Management should be directed towards limiting the buildup and spread of the disease.
  • Chemical fumigants, including chloropicrin, methyl bromide, and carbon disulfide may be applied to reduce infection levels. Fumigants should be applied in and around the base of infected stems, or in perforations created in the soil after trees have been removed.
  • When planting, select a mixture of species that exhibit a resistance to Armillaria fungi.
  • Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during dry periods. Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and retain soil moisture.
  • Uproot infected or susceptible root systems and stumps to reduce the number of food sources available to Armillaria fungi.

Annosum Root Rot

Annosum root rot, also referred to as annosus root rot, is a fungal disease of pine, spruce, and fir trees. The causal agent of the disease is the fungus, Heterobasidion annosum. Heterobasidion annosum is capable of infecting a wide range of host species in forested and landscape settings. It is most problematic in areas that feature an abundance of pines. Annosum root rot is considered one of the most destructive diseases of conifers in the northern hemisphere.


Annosum root rot generally targets pines, but it may also affect spruce and fir trees. It is particularly common on white fir. Infections occur on subalpine fir, Douglas-fir, blue spruce, and Engelmann spruce, albeit with less frequency.

Symptoms of Infection

The first symptoms of infection are often most apparent beneath the soil. Once the fungus has infiltrated a tree’s root system, it causes abnormalities to develop in the root structure. These abnormalities inhibit the tree’s vascular system, preventing it from absorbing water and nutrients. More than half of the tree may be killed before infection symptoms become visible. As the infection progresses, the tree’s bark turns pale yellow. Eventually, the bark coloration deepens to a light brown before transitioning to white, with a speckling of irregular black dots. Resin flow may be evident at the root collar, an indication of the tree’s attempts to fend off the pathogen. Infected trees will gradually wither, before finally succumbing to the disease. In landscape settings, a ring of dead trees will gradually develop, with the oldest tree rooted in the center.

In pines, the disease is most active in the sapwood, where it eradicates healthy tissue. While decay may occur in pines, the rapid tissue death often precludes the decay process from becoming too extensive. In spruce and fir trees, the fungus grows in the root collar, often resulting in butt rot. Decay in any host may be preceded by a pink staining of the wood. Later, this decay is defined by pockets, or pits, that form in the wood. Small black flecks can often be observed in the more enlarged pockets. The pockets tend to be lost as the decay process renders the wood spongy, or stringy.

Conks are frequently found in moist sites around white fir. The conks may measure up a foot in diameter. The conks produced by annosum root rot may have a prominent brown cap or bracket. They may also be completely flat on the substrate. The pore surface of the conks is white, with small pores. The flesh is tan. Developing conks often emit a fresh mushroom aroma.


  • When planting, select trees that are resistant to annosum root rot.
  • A prophylactic stump treatment can be applied immediately after infection to stifle disease progression. Prophylactic stump treatments increase the pH level of the stump to the point where the fungus is unable to survive.
  • Several benign fungi are aggressive stump colonizers, and can be administered as stump top treatments to prevent the annosum root rot pathogen from becoming established in susceptible wood.

Photo courtesy of Jason Hollinger CC-by-2.0