Armillaria root rot in Christmas tree plantations
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EmailArmillaria root rot is a silent killer of woody trees. Proper identification and management can lessen future tree loss.
Armillaria root rot is an important and increasingly concerning disease for Christmas tree producers. It is caused by several pathogenic species of Armillaria, a genus of fungi that infects over 500 woody plant species. These fungi are natural inhabitants of forest ecosystems, where they act both as pathogens and saprophytes. They infect and colonize the root system, disrupting the uptake of water and nutrients and often cause girdling at the root crown and lower stem below the soil line. This can result in rapid tree decline and eventual death.
Traditionally, Armillaria root rot has been associated with weakened or stressed trees growing on poor sites. However, healthy, vigorous trees can become infected. Once introduced, the fungus can persist in dead roots and woody debris for decades, making it difficult to prevent future infections.
Increasing reports of tree mortality, particularly in Christmas tree plantations, highlight the growing impact of Armillaria root rot on both young seedlings and mature trees.
Symptoms of Armillaria root rot infection
Armillaria root rot symptoms can closely resemble those caused by drought stress or Phytophthora root rot, which makes accurate diagnosis challenging. If you suspect Phytophthora, refer to the Michigan State University Extension article, “Managing Phytophthora root rot in Christmas tree plantations.”
Trees affected by Armillaria root rot may exhibit a range of symptoms. In some cases, trees die suddenly without prolonged signs of decline. More commonly, trees appear stressed for one to two years before dying, showing progressive needle discoloration ranging from mild fading to intense browning (see Photo 1).
These aboveground symptoms are nonspecific, so confirming Armillaria root rot requires inspecting the roots and lower stem for physical signs of fungal infection.
Signs of Armillaria root rot infection
To confirm Armillaria root rot, look for the following distinct signs of fungal presence on or near the roots and lower trunk.
Rhizomorphs: One of the most distinctive signs of Armillaria root rot infection is the presence of dark, string-like fungal structures that resemblance shoelaces. These are often found on the surface of roots or in the adjacent soil but spotting them isn’t easy. They frequently mimic the appearance of fine roots, making close inspection essential (see Photo 2). These cord-like structures are composed of an aggregated mass of fungal hyphae and serve as the nutrient conducting structures, allowing the fungus to spread from infected to healthy trees. They are one of the primary ways Armillaria invades new host trees, even in the absence of direct root-to-root contact.
Mycelial fans: If Armillaria has infected your tree, it often leaves behind a distinctive sign: a white, fan-shaped mycelial mat growing just beneath the bark of the root or root collar (see Photo 3). These fungal growths may look delicate, but they are indicators of Armillaria root rot infection, especially in recently infected or recently dead trees.
To find them, you’ll need to get your hands dirty. Carefully excavate the soil around the base of the tree, and using a pocketknife, gently peel back the bark near the root crown. If nothing appears at first, don’t give up you may need to inspect multiple roots before locating mycelium, especially if the infection is in its early stages.
Spotting mycelial fans is one of the most reliable ways to confirm active Armillaria infection. However, in trees that died many years ago, these fans may no longer be visible. In such cases, you’ll often find white, stringy, decomposed stump tissue—evidence that the fungus has already consumed much of the host material (see Photo 4). When fans are absent, rhizomorphs may still be present and can help confirm past infection.
Armillaria mushrooms: Another visible sign of Armillaria root rot is the appearance of honey-colored mushrooms. These mushrooms often emerge in clusters at the base of infected trees, growing on wood, buried roots or around the lower trunk especially during cool, moist periods in early summer or fall.
They can be quite eye-catching, but also extremely short-lived, sometimes lasting only a few days. Because of their brief appearance, it's important to note that the absence of mushrooms does not mean your trees are Armillaria-free. In fact, many infected trees never produce visible fruiting bodies at all. Still, when present, these mushrooms are a useful field clue and a sign that the fungus has been active for some time.
Disease cycle: Armillaria spreads primarily through root-to-root contact, where fungal mycelium moves directly between grafted roots of adjacent trees, sometimes without the involvement of rhizomorphs. These rhizomorphs also facilitate infection by extending through the soil at a rate of up to 3 to 5 feet per year, allowing the fungus to colonize neighboring healthy trees. In addition to this underground spread, long-distance dispersal occurs via airborne basidiospores, which can infect new hosts far from the original source. This combination of localized root invasion and sporadic spore dispersal enables Armillaria to persist and expand in forest and plantation environments over many years.
Disease management
Planting disease-tolerant species offers a sustainable approach to managing Armillaria root rot, but the susceptibility of many Christmas tree species remains unknown. We have developed a rapid in-vitro screening method to screen woody species to Armillaria root rot. This may be used in future research efforts to evaluate the resistance of Christmas tree species to Armillaria root rot.
Currently, no effective chemical treatments exist for controlling Armillaria root rot. Michigan State University Extension advises against planting new trees near stumps infected with Armillaria root rot (see Photo 4) and recommends removing infected stumps from fields.
Related articles
- Devkota, P., & Hammerschmidt, R. (2025). Responses of woody hosts to Armillaria species infections: current insights and knowledge gaps. Physiological and Molecular Plant Pathology, 102686.
- Devkota, P., Iezzoni, A., Gasic, K., Reighard, G., & Hammerschmidt, R. (2023). Rapid In Vitro Screening of Prunus Genotypes for Resistance to Armillaria Root Rot Using Roots of Young Rootstocks. HortScience, 58(10), 1163-1169.
- Devkota, P., & Hammerschmidt, R. (2020). The infection process of Armillaria mellea and Armillaria solidipes. Physiological and Molecular Plant Pathology, 112, 101543.
