Root diseases caused by Heterobasidion spp. occur across the northern hemisphere in temperate regions. The diseases are very different in pines vs. non-pines. Pines tend to be killed outright; the fungus grows in the cambium and girdles the root collar. Non-pines (true fir, spruce, hemlock) tend to get butt rot and hang on quite a while before death. You usually get disease centers with it, but they tend to peter out before they get very big. We don’t know why.
Most conifers can be attacked.
There are more than a dozen species of Heterobasidion. Many appear to be nonpathogenic. The most notorious pathogens are member of the H. annosum complex. For many years they were lumped into the one species, but with more intensive study, molecular genetics, and mating studies, it has become clear there are at least five species. The newly recognized species are more or less intersterile where they are sympatric.
These species form annual or perennial conks with a white pore surface, cream-colored inner tissues, and an irregular, dirty brown upper surface. In dry climates, they tend to be hard to find, growing under duff or inside hollow stumps and root channels. They can be on living or dead trees. There is an asexual stage, but it is not very important in epidemiology. There are no rhizomorphs.
Colonization of a freshly cut stump top is less selective than that of living trees. Thus, host specialization is less evident when sampling stumps. Even in living trees, host specialization is not absolute; crossovers happen.
In eastern North America west to the Rocky Mountains, only one pathogen occurs, H. irregulare. From the Rocky Mountains west, both H. irregulare and H. occidentale occur, and both cause serious disease. What controlled the distribution of the types? What would happen if H. occidentale were introduced to the east coast? I don’t know!
|Disease name||Pathogen||Common Hosts||Distribution||Notes|
|annosus root disease||H. annosum sensu stricto||Pinus spp.||Western to eastern Europe and into west-central Asia|
|H. parviporum||Picea abies||Across Asia to eastern Europe; Japan to Norway and France|
|H. abietinum||Abies alba||Central to southeastern Europe|
|pine pitch root disease||H. irregulare||Pinus and less common on Juniperus spp. and Calocedrus decurrens||Temperate and subtropical North America, introduced into west-central Italy|
|western root disease||H. occidentale||Abies, Picea, Tsuga spp., and in some areas Pseudotsuga menziesii and Sequoiadendron giganteum||Western North America from Mexico to southeastern Alaska|
Some abiotic factors are somewhat important for at least some of the diseases.
The disease cycle of H. annosum differs from Armillaria in two respects: 1) spores play a much bigger role in the disease cycle, and; 2) no rhizomorphs – requires root contacts or grafts for secondary spread. The greater role of spores is the most important difference. Stress is not really an issue.
Spores commonly infect stumps after a thinning. In this way it can get into a completely new stand that doesn’t yet have it. It grows down the roots of the stump, crosses over to contacting roots of living trees, and attacks. Stumps are susceptible for several weeks after cutting.
Spores may also infect wounds, but only in non-pines. Thus logging scars can be an infection court for establishment of new centers.
Spores can directly infect roots in the soil, but we don’t know how common this is or how big a role it plays in disease cycle. Probably it can be ignored for practical matters.
Longevity of inoculum in stumps is an issue here. Big stumps in the West may sustain the fungus for well over 50 yrs. Major threat from one rotation to the next. In the Southeast, pine plantations, because of the heat and humidity the fungus consumes a stump in less than 10 years. So when a stand is cut, the fungus starts, seedlings are planted. But the seedlings take time to contact the roots of the old stumps. By that time, the fungus is a goner. So H. annosum is not a threat between rotations, though it is within a rotation after thinning.
The difference between hosts in tissues attacked is apparently due to host differences more than pathogen differences.
Distribution and Damage
- Reduce wounding in non-pines.
- Protect stumps with chemicals (borax) or biocontrol (Phlebia gigantea). This would only be worthwhile in stands where: a) the fungus is not already well established, and; b) there is a significant hazard of its establishment (e.g., spore inoculum produced in area, high-hazard site, stand type known to have problems with annosum root rot when it gets in). Borax is strongly recommended in the SE. Biocontrol is sometimes used instead of borax when the pathogen is already established on the site. The good fungus may be able to replace the bad. Other ways of reducing stump infection:
- Avoid or delay thinning. One way is to decrease planting density.
- Thin during times when infection risk minimal. This is strongly recommended in the SE, May to August.
- Thin when stumps are small. Below a certain size, they are unlikely to carry the fungus to nearby trees.
- In hemlock, annosum increases with stand age, so pathological rotation becomes a consideration.
- Manage for resistant species and mixed-species stands. This is much more of an option now that we know about host specialization. The pathogen got a little too sophisticated for its own britches – we can use that against it. Not as easy as it sounds though – the alternate species are not always suited for the site.
- Hazard rating of sites. In SE, sandy dry soils have more annosum root rot. In Europe, the disease is more severe on alkaline soils. Knowing this can help plan for management approaches
Protection of Stump Tops
Fresh stump tops can be protected from basidiospores germinating, establishing a mycelium, and colonizing the stump. Protection can be chemical or biological. This does not eradicate the pathogen from stumps already colonized.
The most common chemical agents are boric acid and its salts. In the USA, these have included the products Tim-bor (disodium octaborate tetrahydrate, or borax), Sporax (sodium tetraborate decahydrate), and Cellu-Treat (same as Tim-bor). All these agents convert to mostly boric acid in water. Currently, only Cellu-Treat is registered for use as a stump protectant for these diseases. Its properties, usage, and health and environmental characteristics were extensively documented in a report for the US Forest Service. The label requires that it be sprayed on as a solution, unlike Tim-bor (the same chemical), which could be sprinkled on as a powder. This likely increases its effectiveness, and also reduces the exposure of applicators to boron.
However, borax, used sensibly, is quite benign. Those of us of a certain age in the USA may remember the television western Death Valley Days, sponsored exclusively by 20 Mule Team Borax. Borax was mined in Death Valley, California and transported in wagons pulled by teams of 20 mules. 20 Mule Team Borax was widely used for laundry detergent, working well even in hard water. It has other household uses and is also used as the “Boraxo” brand hand soap. It is still available commercially.
Biological protection of stumps using the fungus Phlebiopsis gigantea is also practiced in some areas. Rotstop C is a product used in North America. It is a wettable powder that is applied as an aqueous mixture within 24 hours of cutting, according to the producer. It is registered for use in Canada and in the following US states: Alabama, Florida, Georgia, Michigan, North Carolina, South Carolina, Virginia, and Wisconsin. Additional state registrations are pending.
Phlebiopsis gigantea has an advantage over borax in that the saprobe colonizes the root system, preventing access by the pathogen not just at the stump top. On the other hand, studies showed that it allowed colonization by Heterobasidion sp. of 10% or more stumps.