NAME OF THE ORGANISM: Armillaria mellea (ARMIME)

Not candidate

Armillaria mellea, also known as honey fungus or oak root fungus, has a wide host range (CABI 2021; Raabe, 1962). The pathogen can infect Vaccinium corymbosum (Caruso, 2017; Prodorutti et al., 2009).
The pathogen occurs in landscapes and urban soils as well as a natural pathogen in forests and on lands converted to farming (Downer & Faber, 2019). The presence of infected roots (of various plant species) is an important and likely source of inoculum for highbush blueberry (Prodorutti et al., 2017). Armillaria mellea can spread though soil using rhizomorphs, a multicellular structure formed through the aggregation and interweaving of many hyphae (shoe-string-like surviving structures). Their appearance is similar to the roots of higher plants. The growth and distribution of A. mellea rhizomorphs are mainly located between 2.5 and 20 cm below the soil surface and are rare below 30 cm. Rhizomorphs grow slowly through the soil and searching for nutrients; disease foci can grow 0.2-2.5 m per year. When Armillaria mycelium/rhizomorph encounter the root, they start to grow epiphytically over the host root surface and produce branches of hyphae that grow into the tissues. Woody roots are more susceptible to infection by Armillaria spp. than the fine roots (Termorshuizen, 2000; Devkota & Hammerschmidt, 2020).
Once trees are infected, Armillaria spp. can also spread by root-to-root contact. This root-to-root infection results in the outward expansion of clusters of diseased trees as neighboring trees subsequently become infected (Termorshuizen, 2000).
New sites of infection also can become established in non-infested areas of the orchard by moving infected root pieces through cultivation, erosion gullies, and careless tree removal practices (Cox, 2004); or when plants that are infected, or contain soil infested with A. mellea are introduced (Termorshuizen, 2000).
Besides its parasitic behavior, A. mellea can also persist as a saprophyte in the form of a mycelium, colonizing the dead roots and wood in the soil of e.g. orchards and timber plantations. The colonized and infected plant tissue or woody debris in soil serve as a long-term source of inoculum, colonizing and infecting the roots of new-planted trees through physical contact, which also increases the risk of mortality in the next rotation of trees. The saprophytic behavior enables the fungal inoculum residing inside the roots and wood to persist for many years in a forest stand (Kedves et al., 2021). In particular, Armillaria spp. have been found on coniferous bark used as mulch in highbush blueberry plantings (Prodorutti et al., 2007).
Armillaria mellea can also spread by basidiospores, although the spores are speculated to have low epidemiological importance relative to the infection by fungal rhizomorphs. Nevertheless, they are regarded as an important source of genetic variation within species and a means of long-distance dispersal. Difficulties in tracking the dispersal and establishment of windborne spores may have made it difficult to understand their absolute role in infection (Devkota & Hammerschmidt, 2020).
Armillaria spp. could be isolated from infected bark which was used to mulch plants (Palmieri et al., 2006).
Although A. mellea can spread by infected propagation material, the major issue is planting orchards on land previously used for forestry (A. mellea is a common disease in forests).
[In responses to the questionnaire, NL commented that the pest was 'airborne and widespread in nature'. NL and SI did not consider plants for planting as the main pathway.]
The Fruit SEWG agreed that plants for planting should not be considered as a significant pathway compared to other pathways: it only infects weak plants and the role of nursery plants in infections may not be significant. Although Armillaria mellea can spread by infected propagation material, the main risk is when planting orchards on land previously used for forestry (Armillaria mellea is a common disease in forests). Nevertheless, the Fruit SEWG highlighted the lack of measures available when the pest becomes present in an orchard. When Vaccinium fruits are produced indoor, sterilized soil/new substrate is used and plants for planting is neither considered as a significant pathway.