Big White Pocket Rot

This is one of those great disease names that tell you the most noteworthy feature of the disease.   If you’re used to seeing white pocket rots caused by Porodaedalea pini or Onnia tomentosa, you can tell this one apart immediately by the size of pockets as seen in longitudinal section.  Not what you might call shockingly large, but noticeably so.

Host

Many conifers can be hosts.  In the northwestern United States, Pseudotsuga menziesii, Larix occidentalis, Thuja plicata, and Tsuga spp. are hosts ​[2–4]​.  In northern Idaho, it is common in Abies grandis ​[5]​.  In many areas, the disease and fruiting are most common on Picea spp.  Picea engelmannii is a favored host in the Rocky Mountains ​[1]​.  In Fennoscandia and Europe, fruiting is common on logs of Picea abies ​[8, 13]​.

Pathogen

Phellopilus nigrolimitatus is the somewhat mysterious cause of this disease.  It generally fruits long after trees have fallen ​​[3]​​.  Basidiomata are located mainly along the underside of old logs or in otherwise protected micro-sites.  They are perennial, resupinate to effused-reflexed, and can grow to be large, up to 30 cm long.  The pore surface is cinnamon-colored but darkens with age and the pores are quite small.  The upper context is unusually soft for the perennial members of this family (Hymenochaetaceae); the context is brown and typically has one or more black lines (thus the specific epithet).

Environment

 The fungus is generally associated with boreal forests, often Picea-Abies ecosystems.  In western North America, big white pocket rot is often associated with large, old trees.

In Fennoscandia, fruiting of Phellopilus nigrolimitatus is associated with old-growth forests, highly productive sites, and Picea abies logs larger than 30 cm diameter with advanced decay ​[13]​.  It is thought to be threatened in commercial forests there with an estimated 95% decline compared to forests that had been unharvested for several tree generations, because of the lack of large logs that it thrives on.  However, in České Švýcarsko National Park, Czech Republic, it was found commonly at low elevations on smaller spruce logs, in somewhat disturbed areas, and even in plantations ​[8]​.

In Abies grandis in northern Idaho, a significant association was found between occurrence of big white pocket rot and attack by bark beetles, especially Scolytus ventralis ​[5]​.

Symptoms

There are usually no external indicators of infection.  Observations suggest that the disease can lead to mechanical failure, but it is not known if crown symptoms can be present before that point.

Decay is a distinct white pocket rot with large pockets.  The elliptical pockets can be up to 1 x 2.5 cm (up to 5 cm long according to Boyce ​[2]​). They may contain white cellulose fibers and are surrounded by pale brown, firm wood.

Distribution and Damage

Phellopilus nigrolimitatus occurs throughout western North America and into southeastern Alaska.  It occurs in central and northern Europe north of the Mediterranean countries, and in boreal Asia.   In Eurasia it occurs generally in Picea-Abies ecosystems.  In Europe it seems to function more often as a saprotroph, colonizing fallen logs.  In North America also, it apparently can colonize downed logs as well as causing butt rot ​[3]​.

In western North America, root and butt rot is associated with loss of wood volume in the butt log, as it grows far up the stem.  Losses also occur as mortality through mechanical failure and likely by growth loss.

In Colorado, big white pocket rot was the most important butt rot in terms of cull volume in both Picea engelmannii ​[6]​ and Abies lasiocarpa ​[7]​.   In P. engelmannii, decay columns extended an average of 8.3 m up the stem and caused an average cull of 0.42 m³.  In A. lasiocarpa, average cull of infected trees was 0.17 m³.

Management

There has been little to no study of the disease.  Infection courts are unknown.  It is not known if the disease can spread by fungal growth through root contacts or grafts, as can other root diseases.  There are no management guidelines specific to this disease, but since it tends to occur in large, old trees, harvesting trees before they become overmature would reduce losses from it.  In developed sites where tree hazard is a concern, the disease is challenging because it is very difficult to detect.

Other Issues

In Fennoscandia, the fungus is associated with old-growth forest stands and populations are said to be impacted by forest fragmentation ​​[10, 13]​​.  In many northern European countries, it is on a red list as “near threatened”.

In Fennoscandia, it typically fruits on old conifer logs and is apparently not regarded as a pathogen by mycologists who work on it.  The population structure appears to be consistent with that assessment.  Somatic incompatibility groups were studied locally in Norway ​[9]​.  These groups likely represent genetic individuals (genets).  Six logs were sampled intensively.  All logs had at least 6 and at most 12 genets.  The only case of a genet shared between logs was a point where two logs were in contact.  This is not a population structure one would expect of a pathogen causing root and butt rot.  Sometimes genets appeared to be discontinuous, with samples of one genet interrupted by samples of another.  One interesting result was that the size of basidiomata, specifically the area of the pore surface, was correlated with the size of the genet that produced it.

Based on surveys of basidiomata, it was reported that P. nigrolimitatus is a late colonizer of big logs in a late stage of decay ​[13]​. However, it was detected in recently fallen logs by extracting DNA  from wood ​[11, 12]​.  This suggests that fruiting of P. nigrolimitatus  occurs long after its establishment.