I learned that mushrooms are normally classified as either decomposers or symbiotes. I would like to know the differences between the two types and whether they are detrimental or beneficial to their host, often a tree.
It probably makes sense to classify mushrooms (fungi) in two separate ways:
symbiotic vs. free-living: symbiotic organisms are those that live in close association with a host (etymologically sym="with", biosis="life"), free-living organisms don't. So a mycorrhizal fungus that lives within or on the surface of a plant root is a symbiote; a fungus that lives in the soil is free-living.
mutualistic (benefiting from and benefiting a host) vs. parasitic (benefiting from but harming a host) vs. saprotrophic (consuming already-dead material; these are your "decomposers"). (Commensalism [benefiting from, but neither hurting nor harming, a host] is a fourth possibility.) Mycorrhizal fungi are generally mutualistic. Fungi like chestnut blight or the genus of phythophthora, which includes the organism that causes potato blight, are pathogens; so are the myco-heterotrophs. Fungi such as wood-decay fungi are saprophytic.
To answer your question, species of fungi that are associated with trees may be either mutualistic (e.g. mycorrhizal fungi) or parasitic (e.g. chestnut blight).
- Parasitic fungi might also be called pathogenic (i.e., disease-causing)
- In popular speech, "symbiotic" is synonymous with "mutualistic" (i.e., organisms that benefit each other); in biology, however, it may refer to either a parasitic or a mutualistic organism. An individual symbiotic organism is often called a "symbiont".
- In general, saprotrophs would be free-living (since they don't get their resources from a host, it doesn't really make sense for them to live on a host)
- Individuals of a some fungal species can switch lifestyles/categories depending on their environment: these are called facultatively mutualistic/commensal/parasitic, e.g. see Redman et al. 2001.
Redman, Regina S., David D. Dunigan, and Rusty J. Rodriguez. “Fungal Symbiosis from Mutualism to Parasitism: Who Controls the Outcome, Host or Invader?” New Phytologist 151, no. 3 (2001): 705–16. https://doi.org/10.1046/j.0028-646x.2001.00210.x.