Laminated root rot is a typical root and butt rot. It leads to expanding centers of mortality. Small trees die standing, and bigger ones fall over. It is restricted in distribution to the Pacific Northwest and Inland Empire area, but it occurs on a variety of important conifers there and causes very significant losses (60 million cu. ft./yr. in WA+OR). Advanced decay is laminated and has white mycelium with brownish hairs in it.
There are two closely related species in North America that cause laminated root rot on different hosts. They were first in the genus Poria, then Phellinus, briefly in Phellinidium, and most recently were segregated to a new genus, Coniferiporia. For many years they were considered a single species, Coniferiporia weirii (pronounced by Americans as weary-I).
Coniferiporia weirii. Polypore, produces brown, unremarkable conks that are resupinate (flat on substrate, all pore surface with no upper cap), but they are not commonly found.
It has become increasingly obvious that there are two forms of the pathogen, one on western redcedar and the other on Douglas-fir, true fir and hemlock. More and more information has come to light, and recently the two forms were formally recognized as distinct species. Unfortunately, the traditional name, P. weirii, went with the less important one (on redcedar). The more important one on Douglas-fir is called P. sulphurascens. They do cross hosts sometimes but for management purposes that can be ignored.
Disease Cycle, Epidemiology
This is another one, like Armillaria, where the spores don’t seem to play much of a role. All the inoculum of significance comes from infected stumps and roots. It can’t grow through soil like Armillaria, but root contacts and grafts are sufficient to get it around. Stress is not an issue.
Resistance. There are usually discrete infection centers with this disease. As described above, infection centers can be regenerated to resistant species. There is a range of resistance. Pines, cedars and hardwoods are most resistant (to the Doug-fir form). They should be favored around infection centers.
Inoculum Reduction. As described above, this may involve stump removal or fumigation. Stumping is limited by terrain for equipment. 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.