Laminated Root Rot

Formerly considered a single disease, it is now recognized that in North America (NA) there are two pathogens, mostly on different hosts. Sometimes these diseases are distinguished by name as ‘laminated root rot’ and ‘cedar laminated root and butt rot’ ​[9]​.

Hosts

The most important host in NA, at least in terms of forestry impacts, is Pseudotsuga menziesii (Douglas-fir). The non-cedar form of the disease is also commonly found on Abies amabilis, A. concolor, and A. grandis (Pacific silver, white, and grand firs) and Tsuga mertensiana (mountain hemlock) ​[15]​. In the Inland Northwest, A. lasiocarpa (subalpine fir) is also considered highly susceptible ​[2]​. Less commonly infected, unless they grow with common hosts, are other Abies spp. (true firs), Picea spp. (spruces), T. heterophylla (western hemlock), and Larix occidentalis (western larch).

The cedar form of the disease occurs primarily on members of the family Cupressaceae. Thuja plicata (western red cedar) is a common host, but Cupressus nootkatensis (Alaska yellow cedar) is also infected where it grows in the pathogen’s range. In Asia, Juniperus (including Sabina) spp. (junipers) are infected.

Pathogen

For many years only one pathogen was recognized, Poria weirii (pronounced by Americans as ‘weary-eye’). Over the years it has been placed in Fomitiporia, Phellinus, Inonotus, and Phellinidium. It was known for some time that the pathogen on Thuja plicata was different from the one on other hosts. It was found that they were intersterile ​[1, 12]​. The pathogen on western redcedar was the one first described as Poria weirii, so it retains that specific epithet. The one from Douglas-fir and other hosts was determined to be a species described from Siberia, Phellinus sulphurascens ​[12]​.

More recently, both species were placed in a new genus, Coniferiporia ​[20]​.  So currently we have Coniferiporia weirii on western redcedar and other members of Cupressaceae, and C. sulphurascens on the other hosts. They do cross hosts sometimes, but for management purposes that can be ignored.

We should note that the decision to identify the American non-cedar species with P. sulphurascens was based largely on sexual compatibility between the two ​[12]​. However, it is not uncommon for sister species to remain sexually compatible when they are allopatric (with non-overlapping distributions) ​[10]​. So it should not be unexpected if the American species someday is recognized as a different species from its Siberian relative, and needs a new name. Just sayin’ 😉 (The day after writing this, I found the study that shows they are indeed different ​[13]​. Guess what comes next?)

A third species, Coniferiporia qilianensis, has been described in China on Juniperus spp. ​[5]​. It was first identified as C. weirii ​[6]​. Although they can be separated in a molecular phylogeny, the molecular and morphological differences are small, and an intermediate specimen exists ​[20]​. One can consider describing that intermediate form as another species, or consider the whole weirii-qilianensis clade to represent a single species.

Environment

In the US Inland Northwest, the disease is associated with moist sites ​[2]​.

Host vigor or stress appears to have little or no effect on the likelihood of infection and severity of disease.

Symptoms and signs

Non-cedar hosts

In non-cedar hosts, crown symptoms are typical of a root disease. They include crown thinning and chlorosis (yellowing of foliage), reduced shoot and radial growth, and a stress cone crop (numerous small cones as the tree pours its last resources into reproduction).

However, in some cases roots are so weakened mechanically that trees fall before crown symptoms fully develop. This is likely on good sites, where the tree can maintain its crown with fewer functional roots than on poor sites, and of course on small trees. Fallen trees with roots mostly decayed and leaving short stubs on the root crown are useful in diagnosis. They often fall in random directions rather than in one direction, which typically is the case where wind is a primary cause of failure.

An important feature is the clustering of disease into expanding root disease centers. These may contain fallen trees as just described, dead standing trees, and at the periphery, symptomatic trees. However, the disease often occurs spread diffusely through the forest, not only in discrete centers ​[17]​.

Decay begins in the roots as a red to brown stain in the heartwood or sapwood. In vertical sections it appears as streaks; in cross section the streaks may appear circular, crescent-shaped following annual rings, or irregular. The decay can be characterized as a pitted, laminated white rot, but the color tends to light yellow-brown. As decay proceeds, the annual rings become separable, the namesake of the disease. The separated annual rings have small pits on both sides. Brown mycelium develops in the decayed wood with setal hyphae (thick-walled, dark brown, pointed hyphae) that can be seen with a hand lens.

Decay may initially appear in heartwood, but, at least in the roots, sapwood and whole roots are also killed and decayed. Roots with advanced infections often have ectotrophic mycelium growing on the surface of the bark. It looks like the mycelium in the wood, but may also be yellow or cream-colored ​[2, 17]​.

Cedar hosts

Coniferiporia weirii in Thuja plicata generally decays the inner wood of the roots and butt ​[2]​. This usually causes no crown symptoms and little effect on growth. Trees rarely die standing. They normally do not fail by uprooting and leaving a root ball, as is common in the non-cedar disease. More often, failure occurs by snapping at the butt ​[12]​.

Fruiting

Coniferiporia species produce brown, unremarkable conks that are resupinate (flat on substrate, all pore surface with no upper cap), but they are not commonly found. Coniferiporia sulphurascens is annual, but C. weirii is often biennial or perennial. When patches of new-year growth appear over the previous year’s tube layer, it can give the conks a roughly mottled appearance.

Disease Cycle

This is another disease, like Armillaria root disease, where the spores don’t seem to play much of a role. Presumably they occasionally or rarely infect trees, but all the inoculum of management significance comes from infected stumps and roots. These fungi can’t grow through soil like Armillaria, but root contacts and grafts are sufficient to keep it going.

Distribution and Damage

Coniferiporia sulphurascens is very widespread in Asia and eastern Europe, where it mostly infects Picea, Abies, and Larix spp.:

• It was described from Irkutsk, Siberia on Larix sibirica. There it causes laminated root rot around Lake Baikal, just north of Mongolia.
• Elsewhere in Russia, it has been found in:
  • Nizhny Novgorod, about 400 km east of Moscow, well west of the Ural Mountains, on Picea sp. ​[20]​. The Ural Mountains are considered to separate Europe and Asia geographically.
  • the southern Ural Mountains, on Picea sp. ​[11]​.
  • Primorsky, on the far southeastern seacoast of Siberia ​[13]​.
• Across the Sea of Japan from Primorsky, it occurs on the island of Hokkaido, Japan ​[13]​.
• In China, it has been found:
  • on the other side of Primorsky, in neighboring Jilin Province, northeastern China, on Picea sp. ​[7, 20]​.
  • in Yunnan Province, southern China, on Pinus sp. ​[20]​.
• It was found in Tibet, on Abies sp. ​[20]​.
• Also just outside Europe, it was found in southern Türkiye ​[8]​. Here it was uncharacteristically infecting Juniperus spp. (Cupressaceae).

In Asia, C. weirii sensu stricto was reported from the Qilian Mountains, northeastern Qinghai Province, China ​[6]​ (blue dot in Asia map). The fungus was causing laminated root rot of Sabina (Juniperus) przewalskii in distinct disease centers up to a hectare in size. “Symptomatic trees showed slow growth, thin crowns, and chlorotic foliage . . . . cambial necrosis and wood decay were found, and the trees apparently died when the cambial necrosis girdled the base of the trees” ​[6]​. The host is in Cupressaceae, as are the hosts in North America, but the disease as described from Qinghai was more aggressive.

Further study led to description of the Qilian Mountains fungus as a new species, now called C. qilianensis ​[5]​. It is quite similar to C. weirii, and an intermediate form was collected in Turkmenistan, also on Juniperus ​[20]​.

In North America, the fungus we call C. sulphurascens ranges in the Pacific Northwest and Inland Empire, from central British Columbia down to the very north of California, and in the east down to northern Idaho and northwestern Montana ​[13, 14, 19]​. The range of C. weirii overlaps substantially, but extends less to the south and farther north into southeastern Alaska.

Annual losses due to laminated root rot have been estimated at 4.4 million m³ of timber throughout the range of C. sulphurascens ​[15]​. Loss occurs through mortality, growth loss, and cull of the butt log.

Management

Laminated root rot is a major management problem in some of the most productive forests in North America. Various disease guides available online have extensive information on management ​[2, 15–18]​. Rather than repeat that here, I will just summarize the main approaches.

Resistance. There are frequently discrete infection centers with this disease. Infection centers can be regenerated to resistant species. If a stand is diffusely infested, this can be applied stand-wide. There is a range of resistance. Pines, cedars and hardwoods are most resistant (to C. sulphurascens). They should be favored where disease occurs.

Inoculum Reduction. This may involve stump removal or fumigation. Stumping is limited by terrain for equipment and is of course expensive. Fumigation is more experimental.

Host removal. There is one other approach that can be tried in sapling stands. Cut all trees with symptoms and adjacent nonsymptomatic trees that are susceptible. The idea is to remove host material from the developing infection centers so the fungus dies out. It won’t invade dead roots. However, the pathogen can survive for many years in old stumps.

Early harvest. In older stands, salvage symptomatic trees where possible and consider final harvest earlier than usual. Thinning is not good if the disease is dispersed because there will be asymptomatic, infected trees left as crop trees, and windthrow will be more likely.

Management of this disease may be expensive, but it is a lot better than doing nothing in severely infested stands on average or good sites.

Other Issues

Although C. sulphurascens has been found on the outskirts of Europe geographically, neither pathogen has been found in the European Union. They would like to keep it that way. After a pest categorization study ​[9]​, both pathogens were designated as A1 quarantine pests in 2019 ​[3, 4]​.