Secrets of the subterranean pathosystem of Armillaria

Abstract

Armillaria root disease affects fruit and nut crops, timber trees and ornamentals in boreal, temperate and tropical regions of the world. The causal pathogens are members of the genus Armillaria (Basidiomycota, Physalacriaceae). This review summarizes the state of knowledge and highlights recent advances in Armillaria research. Taxonomy:Armillaria includes more than 40 morphological species. However, the identification and delineation of species on the basis of morphological characters are problematic, resulting in many species being undetected. Implementation of the biological species' concept and DNA sequence comparisons in the contemporary taxonomy of Armillaria have led to the discovery of a number of new species that are not linked to described morphological species.Host range: Armillaria exhibits a range of symbioses with both plants and fungi. As plant pathogens, Armillaria species have broad host ranges, infecting mostly woody species. Armillaria can also colonize orchids Galeola and Gastrodia but, in this case, the fungus is the host and the plant is the parasite. Similar to its contrasting relationships with plants, Armillaria acts as either host or parasite in its interactions with other fungi.Disease control: Recent research on post-infection controls has revealed promising alternatives to the former pre-plant eradication attempts with soil fumigants, which are now being regulated more heavily or banned outright because of their negative effects on the environment. New study tools for genetic manipulation of the pathogen and characterization of the molecular basis of the host response will greatly advance the development of resistant rootstocks in a new stage of research. The depth of the research, regardless of whether traditional or genomic approaches are used, will depend on a clear understanding of where the different propagules of Armillaria attack a root system, which of the pathogen's diverse biolymer-degrading enzymes and secondary metabolites facilitate infection, and how the course of infection differs between resistant and susceptible hosts.

Genome expansion and lineage-specific genetic innovations in the forest pathogenic fungi Armillaria

“…Individuals reach this immense size via growing rhizomorphs, dark mycelial strings 1-4 mm wide that allow the fungus to bridge gaps between food sources or host plants 1,6 (hence the name shoestring root rot). Rhizomorphs develop through the aggregation and coordinated parallel growth of hyphae, similar to some fruiting body tissues 7,8 .…”

“…Rhizomorphs develop through the aggregation and coordinated parallel growth of hyphae, similar to some fruiting body tissues 7,8 . As migratory and exploratory organs, rhizomorphs can grow approximately 1 m yr −1 and cross several metres underground in search for new hosts, although roles in uptake and longrange translocation of nutrients have also been proposed 1,9,10 . Root contact by rhizomorphs is the main mode of infection by the fungus, which makes the prevention of recurrent infection in Armillariacontaminated areas particularly difficult 1 .…”

“…As migratory and exploratory organs, rhizomorphs can grow approximately 1 m yr −1 and cross several metres underground in search for new hosts, although roles in uptake and longrange translocation of nutrients have also been proposed 1,9,10 . Root contact by rhizomorphs is the main mode of infection by the fungus, which makes the prevention of recurrent infection in Armillariacontaminated areas particularly difficult 1 . Despite their huge impact on forestry, horticulture and agriculture, the genetics of the pathogenicity of Armillaria species is poorly understood.…”

Diversity of Armillaria ostoyae in Scots pine plantations in Poland

“…Considering that they grow towards rather than away from the soil surface (Morrison 1976), they may help mycelium to respire in the low oxygen and high carbon dioxide environment beneath the root bark (Baumgartner et al 2011). Rhizomorphs may affect the activity of other soil organisms, including mycorrhizal fungi (Leake et al 2004).…”

A Fivefold Parallelized Biosynthetic Process Secures Chlorination of Armillaria mellea (Honey Mushroom) Toxins

“…he basidiomycete genus Armillaria includes numerous species that are known as notorious butt and root rot agents (1). They are globally distributed as hardwood or conifer pathogens (2) in managed and unmanaged forests and also damage fruit trees and grapes.…”