Heterobasidion Root Disease

Heterobasidion occidentale Otrosina & Garbel

80 host plants · Fungal

Last updated

Heterobasidion root rot affects conifers in cool, moist forests, particularly spruce, fir, hemlock, and lodgepole pine. Infected trees show gradual decline as the fungus colonizes the root system and lower trunk, slowly girdling the tree and cutting off water and nutrient movement. Infection spreads through root contact between trees and through spores that land on freshly cut stump surfaces, making harvest timing and site conditions critical factors in disease development.

This disease takes years to kill a tree, making early detection difficult in ornamental settings. The practical impact concentrates in forestry and dense conifer plantings where tree-to-tree contact creates highways for fungal spread. Minimizing fresh wounds, avoiding harvest during spore release seasons, and spacing trees to prevent root grafting reduce risk; once infection occurs in a stand, management options are limited.

Quick Reference

Agent Type
fungal
Causal Agent
Heterobasidion occidentale Otrosina & Garbel
Host Plants
80
Spread
wind
Favorable Conditions
Basidiospore production and dispersal: peak June-October in PNW; highest on w...

Management

Vulnerability Window

Fresh stumps: highest infection risk June-October (peak spore season), with greatest vulnerability in first 2-4 weeks after cutting before competing organisms colonize the surface. Root-to-root spread: continuous but most active October-May during cool, moist soil conditions. Living trees: vulnerable whenever roots contact infected wood; no seasonal resistance period. Once infection is established in roots, disease progression is year-round but slow. Visible crown symptoms: 3-10 years post-infection. Windthrow risk: elevated year-round once significant root decay is present, but highest during fall/winter storm events.

What Triggers Infection

Basidiospore production and dispersal: peak June-October in PNW; highest on warm (>50°F / 10°C), humid days with moderate air movement. Spore concentrations drop dramatically below 40°F. Stump infection: requires freshly cut wood surface with adequate moisture; stump surfaces remain receptive for weeks to months depending on drying rate and competing fungi. Root-to-root spread: most active October-May when soil is cool (40-60°F / 4-15°C) and consistently moist. Mycelial growth rate increases with soil moisture. The fungus is adapted to maritime PNW conditions; mild wet winters provide extended mycelial activity periods. (Source: PNW Plant Disease Management Handbook; USFS research; BC Forest Practices)

Cultural Controls

  • Schedule tree removal and thinning for January through March USFS Forest Pathology; UC ANR Heterobasidion Root Disease
  • Remove stumps and large root fragments from cleared land before replanting conifers PNW Plant Disease Management Handbook
  • Favor resistant species in replanting on infested sites USFS R6 DecAID; BC Forest Practices
  • Leave sites fallow 1-2 years before replanting susceptible conifers PNW Plant Disease Management Handbook
  • Do not move infected wood or chips to clean sites UC IPM
  • Monitor for conks during fall tree risk assessments ISA
Regional Notes

H. occidentale is widespread in managed and natural conifer stands throughout the Puget Sound lowlands. Most relevant to arborists when assessing tree risk on formerly logged residential lots where old conifer stumps remain below grade. Western hemlock and grand fir planted over or near infected stumps are at highest risk. Windthrow hazard increases significantly with root decay; conks are most visible at stump bases in fall. In landscape settings, the disease is less common than in forest plantations because stump density is lower and mixed hardwood/conifer plantings disrupt root-to-root spread.

Host Plants (80)

Abies alba European Silver Fir, Common Silver Fir Abies amabilis Pacific Silver Fir Abies balsamea Balsam Fir Abies concolor White Fir, Colorado Fir Abies delavayi Delavay's Fir Abies grandis Grand Fir Abies koreana Korean Fir Abies lasiocarpa Subalpine Fir Rocky Mountain Fir, Alpine Fir Abies magnifica California Red Fir, Red Fir, Slivertip Fir Abies nordmanniana Nordmann Fir, Caucasian Fir Abies numidica Algerian Fir Abies pindrow West Himalayan Fir, Pindrow Fir Abies pinsapo Spanish Fir, Spanish Pin Fir Abies pinsapo subsp. marocana Moroccan Fir Abies procera Noble Fir Calocedrus decurrens Incense Cedar Picea abies Norway Spruce Picea abies var. pendula Weeping Norway Spruce, Pendulous Norway Spruce Picea asperata Dragon Spruce Picea breweriana Brewer Spruce, Brewer's Weeping Spruce Picea engelmannii Engelmann Spruce Picea gemmata Szechwan Spruce Picea glauca White Spruce, Canadian Spruce, Skunk Spruce Picea koyamae Koyama's Spruce, Yatsugatake-tohi [Japanese] Picea mariana Black Spruce, Bog Spruce, Swamp Spruce Picea omorika Serbian Spruce Picea orientalis Caucasian Spruce, Oriental Spruce Picea pungens Colorado Spruce Picea pungens var. glauca Colorado Blue Spruce Picea rubens Red Spruce Picea sitchensis Sitka Spruce Picea wilsonii Wilson's Spruce Pinus albicaulis Whitebark Pine, Scrub Pine, White Pine Pinus aristata Bristlecone Pine, Rocky Mountain Bristlecone Interior Bristlecone Pinus attenuata Knobcone Pine Pinus banksiana Jack Pine, Scrub Pine, Banksian Pine, Hudson Bay Pine Pinus bungeana Lacebark Pine, Whitebark Pine, Baipi Song Pinus canariensis Canary Island Pine Pinus cembra Swiss Stone Pine Pinus contorta Lodgepole Pine Pinus contorta var. contorta Shore Pine Pinus contorta var. latifolia Rocky Mountain Lodgepole Pine Pinus coulteri Coulter Pine, Bigcone Pine Pinus densiflora Japanese Red Pine Pinus edulis Pinyon Pine, Piñón Pine, Two-needle Pinyon Colorado Pinyon Pinus elliottii Slash Pine, Swamp Pine Pinus flexilis Limber Pine, Rocky Mountain White Pine Pinus halepensis Aleppo Pine, Jerusalem Pine Pinus heldreichii Bosnian Pine, Snakeskin Pine Pinus jeffreyi Jeffrey Pine, Western Yellow Pine, Bull Pine Pinus kwangtungensis Kwangtung Pine, Guangdong Pine Pinus lambertiana Sugar Pine Pinus leucodermis Bosnian Pine Pinus monophylla Singleleaf Pinon Pine, One-leaved Pine Pinus monticola Western White Pine Pinus mugo Mugo Pine Pinus mugo var. pumilio Mugo pine, Dwarf Mugo pine Pinus nigra Austrian Pine Pinus parviflora Japanese White Pine Pinus parvifola Pinus parvifola Pinus pinaster Maritime Pine, Cluster Pine, Turpentine Pine Pinus pinea Italian Stone Pine, Umbrella Pine Pinus ponderosa Ponderosa Pine, Western Yellow Pine Pinus pumila Japanese Stone Pine, Dwarf Siberian Pine Pinus radiata Monterey Pine Pinus resinosa Red Pine Pinus sabiniana Gray Pine, Foothill Pine Pinus strobiformis Southwestern White Pine, Mexican White Pine, Chihuahua White Pine Pinus strobus Eastern White Pine Pinus sylvestris Scots Pine, Scotch Pine Pinus thunbergii Japanese Black Pine Pinus virginiana Virginia Pine, Scrub Pine, Jersey Pine Pinus wallichiana Himalayan Pine, Bhutan Pine Pinus yunnanensis Yunnan Pine Taxus canadensis Canadian Yew, American Yew, Ground Hemlock Tsuga canadensis Canadian Hemlock, Eastern Hemlock Tsuga canadensis var. pendula Weeping Hemlock Tsuga diversifolia Northern Japanese Hemlock Tsuga heterophylla Western Hemlock Tsuga mertensiana Mountain Hemlock

Sources & References

Primary: PNW Plant Disease Management Handbook

Data Maturity
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