Click on any
image for a more
detailed view

Disease Management

Although Armillaria root disease is among the most studied diseases of trees in the world, and there are many potential approaches to disease management, the fact remains that, in many cases, we do not have an effective, practical means of reducing the disease. Further, we do not understand and cannot confidently predict the impact of some management activities, such as silvicultural thinning, on the development of the disease. However, in some areas and in some host systems, some among the following approaches may be effective at reducing disease incidence.

In cases where the disease is determined to be secondary, following some other stress factor, management may be best directed at alleviating the stress rather than directly controlling Armillaria root disease.

Tree species selection

Although many tree species are hosts of Armillaria root disease, there is usually variation in susceptibility among local species. In some areas, disease severity is associated with a shift of species composition. For instance, in many forests of the western United States, shade-intolerant pines occur early in successional sequences, but are maintained by fire, which tends to kill the shade-tolerant Douglas-fir and true firs that grow among the pines. Reduction in fire and selective harvesting of pines since European settlement have led to a shift away from the resistant pines to more shade-tolerant, susceptible Douglas-fir and true firs. This unnatural situation is responsible for high levels of root disease in some areas.

When opportunities arise to influence species composition, relatively resistant species can be favored. This can occur during planting, regeneration cuts, thinning, and partial harvests. Local seed sources should be used to maximize resistance in the local environment.

In horticultural and amenity plantings, any flexibility in species can be exercised in favor of resistant species. Within many horticultural species, varieties are known that vary in resistance. Such varieties are often used as rootstocks.

Inoculum removal

Robert Hartig, in 1874, first recommended removal of infested root systems from soil as a means of reducing future infections. Stump removal, at least in part for disease control, has been a common practice in high-value orchard crops for many years. More recently it has been found to be effective in forests. Although it is practiced in some forests, its practicality is limited to high-value sites, amenable slope and soil type, and certain silvicultural systems. It is most effective when combined with root raking to remove mid-sized roots that are not extracted with the stump.

Robert Hartig (1839-1901) is known as the Father of Forest Pathology. His father, Theodor Hartig, contributed much to forestry and botany and is best remembered for his studies of phloem cells and mycorrhizae; the Hartig net is named after him. Although Robert Hartig idolized his father, they disagreed on the origin of diseases, particularly Armillaria root disease. Theodor, following the doctrine of spontaneous generation prevalent at that time, believed that the disease in conifers was caused by a failure of sap flow, resulting in resin exudation or Harzüberfülle, a German name for the disease. Armillaria was thought to result from a transformation of diseased tissues. Robert Hartig, through careful inoculations and observations, proved that the fungus caused the disease and that his beloved father was wrong. What an interesting relationship they must have had! Robert Hartig's meticulous observations are impressive by today's standards. They are shown in his best-known work, Wichtige Krankheiten der Waldbäume (1874), translated and published as Phytopathological Classic No. 12, Important Diseases of Forest Trees (1975).

Avoidance

When there is a choice among sites for silvicultural or horticultural investment, the presence, species, and abundance of Armillaria should be determined in the candidate sites. Based on local knowledge of the behavior of Armillaria species, sites can then be chosen to minimize future disease. In addition to amount and species of Armillaria, other factors, such as soil or habitat type, may be known locally to be associated with hazard of Armillaria root disease and should be similarly considered.

An example of avoidance is establishment of new campgrounds in forests. Where Armillaria root disease is a potential concern, candidate sites should be compared in terms of tree species composition and the presence and abundance of Armillaria species. Considering this information in choosing a site would help in maintaining a desirable forest cover in the campground and avoiding problems with diseased trees that are hazardous to campers.

Thinning

Thinning is potentially a double-edged sword as it affects Armillaria root disease. Where tree density is causing stress, and stress is increasing disease, thinning should reduce disease. Similarly, thinning can be used to adjust species composition, to favor more resistant species in the stand. However, thinning may also stimulate disease by supplying stump resources to the fungus and increasing inoculum potential. We do not fully understand what determines which of these effects is dominant, so predicting the effects of thinning is difficult.

Biological Control

There is great promise and potential for use of biological control of Armillaria root disease. Most concepts target stumps, using antagonistic fungi to preemptively colonize or to eliminate Armillaria species in the wood. However, more research, much of it tailored to local hosts, conditions and fungal communities, is needed before it can become operational.

Soil barrier

In some situations, particularly in orchards, creating a barrier in the soil to root and and rhizomorph growth may be a practical way to limit the pathogen's spread. Often called trenching, this is done by digging a trench down to 1 m (about 3 ft), lining it with plastic, and backfilling. To be effective, of course, the pathogen must be absent from the side of the trench to be protected.

Chemical treatment

Fumigants, such as chloropicrin, carbon disulphide, and methyl bromide are sometimes used in orchard crops to help eradicate inoculum from soil, usually after stump removal and before planting. Fumigants have also been used successfully to eradicate the fungus from stumps, but they are not used in practice. Soil treatments around existing trees, to prevent infections or cure existing infections, generally have been unsuccessful or inconsistent.