Heterobasidion Root Disease (Heterobasidion occidentale and Heterobasidion irregulare)
Trees Affected (Heterobasidion occidentale)
Common: Sitka spruce, western & mountain hemlock (usually in trees 120 years and older), and true firs
Occasional: Incense cedar, junipers, pines, Douglas fir, coast redwood (stumps/resprouting stumps only), western red cedar, giant sequoia
Trees Affected (Heterobasidion irregulare)
Common: Pines, western juniper, incense cedar, manzanita, madrone
Occasional: true firs, reported on many other conifers and hardwoods
Heterobasidion root disease, also known as Annosus or Annosum root disease, is primarily caused by one of two fungi in the genus Heterobasidion, both of which can have aggressive pathogenic abilities and affect a wide variety of hosts. It may establish an infection through spore dispersal onto wounded areas of a tree (most commonly through stumps), but can also spread to other trees through mycelial growth through roots and soil near infected trees. This disease can be hard to identify because of the wide range of symptoms associated with it, many of which are generic to most root diseases (i.e., crown thinning and discoloration, trees in various stages of decline, and fallen trees laying at random angles with evidence of rotted, weakened roots and bases). For this reason, it is important to consider the land management context surrounding a diseased stand, as well as to search for more specific signs and symptoms.
Heterobasidion often does not fruit until after its host has died, so it is helpful to be able to identify the rot this fungus produces. In particular, this fungus produces a yellow to white, spongy, pitted, white rot, sometimes with small black flecks in it (Fig. 1). When dry, this rot becomes stringy or laminated (Fig 2). An additional sign of this fungus is the presence of groups of small (<1 mm), white pinhead-like asexual reproductive structures (conidiophores) on rotted areas or roots. When this pathogen does produce fruiting bodies, they vary in form, from small, leathery conks (Fig 3), to larger, woody shelf conks, with a dark brown surface and an orange-tinted, cream underbelly with small, round pores (Fig. 4 and 5). Additionally, fruiting bodies are not always obvious, and may form only the light, spore-producing underside (hymenophore) in areas where a full fruiting body would be infeasible.
Although there are some morphological differences between H. occidentale and H. irregulare, it is often possible to make a reasonable assumption of the species of Heterobasidion based on which tree species appear to be affected (see “Trees Affected” for details).
Both species appear widespread throughout the North Coast region, roughly following the distribution patterns of their primary host species. However, this disease is especially prevalent in single-species stands (e.g., on christmas tree farms; see Fig 9) and in areas with many cut stumps. See page 19 of this powerpoint for a map from a general survey of Heterobasidion root disease in California.
For spruce and hemlock, the current best practice is to try and limit stand ages to 120 years or younger. Heterobasidion generally is found in older stands, so limiting the age of the stand limits opportunities for this disease. Limiting the number of harvests and avoiding damage to trees and roots of trees that are not being logged is also helpful since Heterobasidion spores can gain entry via cut stumps and open wounds and subsequently move into the root system. Borate treatments can prevent infections on cut stumps if applied soon after harvest, but are not effective if spores have already germinated on stumps. If harvesting in a mixed stand, another option is to cut the diseased tree species, destroy their roots and stumps, and replant with species more resistant to the local species of Heterobasidion (i.e. occidentale or irregulare).
Pests and Pathogens with Similar Symptoms
Other root diseases, including Black Stain Root Disease and Armillaria Root Disease: Most root diseases share a similar suite of generalized symptoms including crown thinning and discoloration, trees in various stages of decline within a stand, and fallen trees laying at random angles (creating a “pick-up-sticks” look) throughout a stand, with evidence of rotted, weakened roots and bases. Heterobasidion can be distinguished from black stain root disease by inspecting rotted wood. If the sapwood is streaked with long, vertical, black stains instead of with white pocket rot flecked with black spots, this is a good sign that the tree likely has black stain root disease. Armillaria can be distinguished from Heterobasidion root disease by its distinctive black rhizomorphs underneath the bark, as well as by the annual production of clusters of fleshy, tan to honey-colored, gilled mushrooms, as opposed to Heterobasidion’s tough, flat to bracket-shaped fruiting bodies with porous undersides.
Red-belted Conk (Fomitopsis pinicola): Although this fungus does not cause root disease, it is a common wood decayer across the North Coast whose fruiting bodies can be mistaken for those of Heterobasidion. While the red-belted conk has a brown top with a reddish band near a rounded edge with a white, smooth-looking underside, Heterobasidion fruiting bodies have a brown top with a sharper edge and a cream-colored to slightly orangish underside with small but mostly visible pores. Additionally, this fungus creates a crumbly, brown rot instead of Heterobasidion’s stringy, white pocket rot and does not inhabit the living parts of trees.
Ganoderma species: Species in this genus of fungus are common wood decayers on conifers and hardwoods across the North Coast. Similar to the red-belted conk, these fungi do not inhabit and kill living trees, but may have similar fruiting bodies to Heterobasidion fruiting bodies. The easiest way to distinguish these fruiting bodies is to scratch the whitish underside of a mature fruiting body. If the underside turns brown when scratched, then it is a Ganoderma species, not Heterobasidion.
A summary of Heterobasidion occidentale and irregulare with photos.