Root diseases caused by Heterobasidion spp. occur across the temperate northern hemisphere. The diseases are very different in pines vs. non-pines. Pines tend to be killed outright; the fungus grows in the vital tissues and girdles the root collar. Non-pines (true fir, spruce, hemlock) tend to get butt rot and hang on quite a while before death. Expanding disease centers form through mycelial growth between trees at root contacts, but they tend to peter out before they get very big. We don’t know why.
Hartig (1878) wrote with passion, “Just as this parasite certainly must be considered the most dangerous enemy of spruce in the plant kingdom, the same can be said of its relationship to pine.”
Most conifers can be attacked. See table below for common hosts of each disease.
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 naturally 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 stump 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, maladie du rond||H. annosum sensu stricto||Pinus spp., but can occur on Larix decidua, Picea abies||Western through eastern Europe and into west-central Asia|
|H. parviporum||Picea abies, but can occur on Abies sibirica, Larix decidua||From Norway and France across Europe and Asia to Japan and China|
|H. abietinum||Abies alba||Southcentral to southeastern Europe|
|maladie du rond||H. irregulare||Pinus and Juniperus spp., less common on Calocedrus decurrens||Temperate and subtropical North America||Introduced to west coast of Italy|
|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. Forests on abandoned farmland, pastures, and otherwise fertile soils tend to have the greatest hazard for disease (Neger).
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 may be due to host differences more than pathogen differences. H. annosum and H. irregulare advance in and kill the sapwood, cambium, and phloem of pines (Hartig). Mortality occurs primarily by girdling of the root collar or the stem just above it.
Distribution and Damage
- Reduce wounding in non-pines (wounds may be infection courts).
- Avoid or delay thinning (avoid creation of stumps, a major infection court). One way is to decrease planting density.
- Thin when stumps are small. Below a certain size, they are unlikely to carry the fungus to nearby trees.
- Thin during times when risk of stump-top colonization is minimal. This is strongly recommended in the SE, May to August.
- 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.