Armillaria collection methods: Tips, tricks, and lore
| Authors: | John W. Hanna, Aaron L. Smith, Mee-Sook Kim, Patrick I. Bennett, Ned B. Klopfenstein |
| Year: | 2025 |
| Type: | Paper |
| Station: | Rocky Mountain Research Station |
| Source: | Corvallis, OR: Oregon State University, Engineering Resources and Management, Swiss Needle Cast Cooperative |
Abstract
This report is aimed toward improving general awareness of various types of Armillaria samples and recommended survey practices to obtain collections for isolation/culture, DNA-based identification, and/or future studies. High-quality samples yield a higher success rate for DNA-based identification and allow isolation in the laboratory to obtain pure, living Armillaria cultures that can be archived for long-term storage for use in future research projects. In root disease surveys, samples of pathogenic Armillaria are frequently obtained from trees showing symptoms of disease, such as yellowing needles (Fig. 1A), thinning crown, basal resinosis (Fig. 1B), upturned root balls/wads (Fig. 1C), or recently “red dead” needles (Fig. 1D). However, above-ground symptoms likely only represent the “tip of the iceberg” as it relates to Armillaria in both a pathogenic and non-pathogenic state. In most cases, Armillaria can cause reduced growth and root disease in trees that show no obvious above-ground symptoms (Cruickshank 2011). Alternatively, Armillaria exhibiting little, or no signs of pathogenicity is also of interest for surveys and research studies. Armillaria is most often present as rhizomorphs on trees that appear to be perfectly healthy. For example, rhizomorph-producing A. altimontana that exhibited little/no sign of pathogenicity has been associated with improved health of western white pine (Pinus monticola) in northern Idaho, USA (Warwell 2019). In contrast, studies and empirical observation suggest that various disturbances, such as stress from other pathogens, insects, fire, soil compaction, tree thinning/harvesting, fertilization, extreme weather, climate change, etc., may “trigger” Armillaria root disease (McDonald 1987a, Woods 1994, Lundquist 2000, Kim et al. 2010, Glaeser and Smith 2013, Kubiak et al. 2017). This interaction can be evident with insect attack, where stress due to insect damage may induce pathogenicity from pre-existing Armillaria rhizomorphs on the roots. A reciprocal relationship is also common in which bark beetles may be more attracted to stressed trees predisposed by Armillaria root disease (Tkacz and Schmitz 1986, Lalande et al. 2020, Sierota and Gradzki 2020). Studies in northern Idaho, USA found that 92-96% of trees attacked by bark beetles also had root disease (Hertert et al.1975, Kulhavy et al 1984).