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 internal butt rot and survive quite a while. Expanding disease centers form through mycelial growth between trees at root contacts, but they often stop before they get very big. We don’t know why.
Robert Hartig, the Father of Forest Pathology, 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 at least 11 species of Heterobasidion [3, 14]. Many appear to be nonpathogenic. The most pathogenic are members of the H. annosum complex. For many years they were lumped into that 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 eastern North America? 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 . Also, in general, disease is more severe in alkaline than acidic soils.
In the southeastern United States, soil properties strongly influence disease risk. Well-drained, sandy soils have greatest disease risk, while clay soils and those with high water table for part of the year have lower risk .
The disease cycle of H. annosum differs from Armillaria in two respects: 1) basidiospores 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 the pathogen 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. Stump tops are susceptible for several weeks after cutting, until the top dries or competitive saprobes colonize it.
Spores may also infect wounds, but primarily 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, so the disease can be a major threat from one generation 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 has burned out the stump. So H. irregulare generally 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. Heterobasidion annosum and H. irregulare advance in and kill the sapwood, cambium, and phloem of pines . Mortality occurs primarily by girdling of the root collar or the stem just above it.
Distribution and Damage
See the table above for general geographic ranges of the species.
In North America, H. irregulare (which attacks pines) occurs across the continent, from southern Quebec and Ontario south to Florida, and from Washington south to Central Mexico. It was recently found in southcentral British Columbia .
Heterobasidion occidentale (non-pines) is restricted to the West [3, 8], from southeastern Alaska and British Columbia to southern Mexico, and east to the Rocky Mountains.
Looking closer at the distribution of H. irregulare, it is interesting that it has not been found in the extensive pine forests of Colorado, nor in the Black Hills of South Dakota, the most productive pine forest of the Rocky Mountains. But it is present in Nebraska, where pines do not occur naturally and the disease was found in plantations after thinning . Is it just a matter of time until the pathogen becomes established in the Black Hills by long-range spore dispersal? Its occurrence in Nebraska could be explained by such dispersal, but it could also be native there on Juniperus spp, other hosts of H. irregulare.
It has been suggested that the distribution of H. irregulare has expanded in Michigan, Wisconsin, and Quebec in recent decades [2, 13]. The disease was first detected in Wisconsin in 1993, and has since been found in 28 counties. It was first found in the lower peninsula of Michigan in 2011, and Minnesota in 2015 .
- 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 Southeast, May to August.
- In hemlock, the disease 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.
- Risk rating of sites. In the Southeast, sandy dry soils have high risk. In Europe, alkaline soils have high risk. 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 these midwestern and southeastern 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.
Spore dispersal distances
In management, it is sometimes helpful to consider how far spores can disperse. Is there some distance we can get from inoculum that will make stumps safe? Rishbeth  trapped viable spores on islands up to 320 km from any source. He washed them onto fresh spruce disks to germinate them and identifying them by conidial structures. Kallio used fresh spruce disks and agar plates to trap viable spores in Finland directly from the air . Spores were trapped all day long through much of the year (excluding winter). Spores were trapped over the open sea up to 500 km from likely inoculum sources. These studies suggests that inoculum is ubiquitous.
However, recent authors suggest that infection over such long dispersal distance is vanishingly rare, and that effective dispersal distances may be as short as 100 m to 1.25 km . If so, we may assume that inoculum is generally local, and that from more distant forests can be ignored.
Spore dispersal and infection by season
In the southeastern United States, summer temperatures limit spore production and viability of spores and mycelium on stump surfaces. Thus, there is a period of relative safety when thinning can occur without stump treatment.
In northern areas, it has been noted that trapping of airborne spores decreases dramatically during very cold weather or when snow is deep . It is often recommended that this is a safe period for thinning without stump treatment. However, this ignores viable spores that are resident on bark surfaces, and can be carried into the wood while cutting. By rinsing bark and inoculating wood disks or selective media, it is easy to demonstrate that such spores are often abundant [4, 12, 16]. Thus, such recommendations should be based on actual cutting trials and isolations rather than number of spores in the air.
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