14.4 Armillaria (Basidiomycota)

Several species of Armillaria cause ‘Honey Fungus’ (or ‘Bootlace Fungus’) diseases of trees and shrubs. It’s an extremely aggressive pathogen. One individual clone of A. ostoyae has been found in the mixed-conifer forest in the Blue Mountains of northeast Oregon in the USA which is estimated to be 400-1000 years old, and has killed 30% of the Ponderosa pines in its area.

This is currently the world’s largest individual organism, and covers 965 ha (2384 acres) of forest (to put that into perspective; the University of Manchester campus covers about 300 acres, so this one fungus is 8 times bigger than the University). The maximum distance between isolates from this 965 ha individual was approximately 3.8 km, and use of three estimates of the rate of A. ostoyae spread in conifer forests resulted in age estimates for the fungus ranging from a minimum of 1900 to a maximum of 8650 years (Ferguson et al., 2003).

Some years earlier a clone of A. bulbosa had been identified as the largest and oldest living organism (Smith, Bruhn & Anderson, 1992) in a northern Michigan hardwood forest; this individual was ‘only’ 15 ha (37 acres) in size and 1500 years old. Large individuals of Armillaria are not limited to the USA; in the Swiss National Park in the Central European Alps, A. ostoyae individuals averaged 6.8 ha in size, the largest so far found being approximately 37 ha (Bendel, Kienast & Rigling, 2006). Basidiomycete mycelia are ubiquitous in forest soils and several studies show that mycelia of many ectomycorrhizal, saprotrophic and pathogenic basidiomycetes can spread vegetatively for considerable distances through forest soil (Cairney, 2005; Bendel et al., 2006).

What enables Armillaria, in particular, to spread rapidly through a stand of trees and bushes, especially in cold and relatively dry conditions, is the combination of its aggressive pathogenicity with its rhizomorphs; the multi-hyphal highly organised, highly protected, structures that allow water and nutrients to be transported across the forest floor from the existing food base to support the exploratory, foraging rhizomorphs as they search for more food bases (Fig. 3) (see also the section entitled Linear structures: strands, cords, rhizomorphs and stipes in Chapter 9; CLICK HERE to view the page). Clearly, then the rhizomorphs contribute enormously to Armillaria pathogenicity.

Honey fungus or Bootlace fungus, Armillaria mellea, fruit bodies and rhizomorphs
Fig. 3. Honey fungus or Bootlace fungus, Armillaria mellea. A, fruit bodies and rhizomorphs (photograph by David Moore). B, rhizomorphs visibly emerging from beneath the bark of a felled log. C, close-up of the rhizomorphs shown in B (photographs B and C by Elizabeth Moore).


Resources Box

More information about plant pathogens

More information about plant pathogens can be found in the British Society of Plant Pathology’s Pathogen profiles, which are a regular feature in the journal Molecular Plant Pathology and BSPPWeb providing brief overviews of the latest research on particular pathogens.

Profile summaries are available at http://www.bspp.org.uk/ [go to <Publications> and then click on <Molecular Pl. Pathology>]
Full articles can be downloaded from the Blackwell Molecular Plant Pathology web site [http://www.blackwellpublishing.com/journal.asp?ref=1464-6722&site=1]

We would also strongly recommend a visit to the teaching materials on the website maintained by the Department of Plant Pathology of the University of Wisconsin-Madison, USA at this URL:


Updated December 17, 2016