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Dianjunus rex (D. rex) and Armillaria are “giant” in fundamentally different ways. D. rex produces an exceptionally large individual fruiting body—up to 50 centimetres tall and 2.2 kilograms—whereas an Armillaria genet can spread through soil and roots across hundreds of hectares and weigh hundreds of tons. Those measurements describe different biological units, so there is no single honest winner without first defining what “bigger” means.
D. rex is a newly described ascomycete in its own genus and family, Dianjunaceae, within Xylariales. Armillaria is an established basidiomycete genus containing root pathogens and wood decomposers. Scientists compare them with field observations, microscopy, cultures, genome sequencing, phylogenetics, molecular clocks and, for Armillaria, clonal genotyping.
A quick answer to the questions readers ask
Which is bigger, D. rex or the humongous fungus? D. rex is larger as one measured fruiting body. An Armillaria genet is vastly larger as a connected organism spread through soil and roots. The two figures cannot be placed on one scale unless the biological unit is specified.
How do scientists know a fungus is one organism? They combine physical continuity with genetic evidence. In Armillaria, matching genetic markers from separated roots and soil samples, together with compatible rhizomorph connections, support the interpretation that samples belong to one genet. A visually continuous patch alone is not enough.
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Is Dianjunus rex related to Armillaria? Both are fungi, but they belong to different major lineages. D. rex is an ascomycete in Xylariales; Armillaria is a basidiomycete genus in Agaricales. The closest reported relative of D. rex is Graphostroma platystoma, not Armillaria.
What does “giant” mean for a fungus? It can mean a tall or heavy fruiting body, a large genetically defined area, a large biomass estimate or an extensive ecological footprint. Good comparisons state which meaning they use.
What “giant” means for a fungus
Fungi do not have one obvious body plan. The mushroom or other fruiting structure that people see is often a temporary reproductive organ. Most of the organism may be a network of microscopic hyphae inside wood, soil or living roots. A meaningful size comparison therefore has to identify the unit being measured.
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- Fruiting-body size: height, diameter or mass of one visible reproductive structure.
- Genet extent: the area occupied by one genetically matching individual, including its connected mycelium and rhizomorphs.
- Biomass: the mass of all fungal tissue assigned to that individual, when it can be estimated.
- Ecological reach: the number of hosts, roots or habitat patches influenced by the organism.
D. rex is remarkable on the first measure. The famous Armillaria examples are remarkable on the second and third. Comparing 2.2 kilograms of one D. rex stroma directly with 600 tons of an Armillaria genet would mix unlike measurements, just as comparing one tree’s fruit with the mass of its entire root system would.
There is also a time component. A stroma can be a seasonal reproductive structure, while an Armillaria genet may persist and expand through roots and soil over many years. A record based on one season’s visible structure and a record based on a long-lived underground network answer different biological questions.
D. rex: a record-sized visible structure
Identity and discovery
Dianjunus rex is a newly described ascomycete from Yunnan, China. It belongs to Dianjunaceae, a new family within the order Xylariales. Field teams found whitish stromata emerging from woody roots, collected voucher specimens for the Kunming Institute of Botany, cultured spores on laboratory media and then examined the fungus at macroscopic, microscopic and genomic levels.
The species was reported in a Frontiers science-news article dated October 13, 2026, covering the underlying work in Frontiers in Fungal Biology. Associate professor Indunil C. Senanayake described the species as a new family that offers a rare view of how extreme fungal body forms evolve.
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How large is it?
Individual D. rex fruiting bodies reach up to 50 centimetres in height and weigh up to 2.2 kilograms. The report identifies this as the largest known fruiting body within Ascomycota. “Largest” here means the dimensions of the visible stroma, not the total underground extent or biomass of the fungus that produced it.
| Measurement | D. rex value | What the number represents |
|---|---|---|
| Height | Up to 50 cm | One fruiting body (stroma) |
| Mass | Up to 2.2 kg | One fruiting body, not the entire organism |
| Habitat reported | Woody roots in Yunnan, China | Field location where the structures were observed |
| Taxonomic placement | Ascomycota; Xylariales; Dianjunaceae | Evolutionary classification |
The phrase “up to” is important. It identifies the largest value reported among the documented specimens, not a guaranteed size for every D. rex individual. Fruiting-body dimensions can vary with developmental stage, available substrate and local conditions, so the record should remain tied to the measured specimens and the report that described them.
Why a tall stroma might evolve
The authors propose that a tall fruiting body could lift spores above dense forest litter and vegetation, improving wind dispersal. Building such a structure would require a substantial energy investment. This is an evolutionary hypothesis reported by the authors, not a demonstrated advantage measured in a field experiment.
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A molecular-clock analysis places the origin of Dianjunaceae at approximately 65 million years ago, about one million years after the Cretaceous–Paleogene mass extinction. The authors suggest that the post-extinction appearance of large quantities of dead plant material may have opened opportunities for wood-associated decomposers. That explanation is also a hypothesis rather than an established causal sequence.
The date is an estimate of the family’s evolutionary origin, not the age of an individual D. rex stroma. Molecular clocks depend on the DNA data, the inferred tree and the calibration information used to translate genetic differences into elapsed time.
Armillaria: a giant connected individual underground
What the genus does
Armillaria is a basidiomycete genus that includes forest and woody-plant pathogens. Species can kill root cambium and later continue as decomposers of dead wood. Their visible mushrooms are only seasonal portions of a much larger mycelial system.
This dual lifestyle explains why Armillaria can be both a disease organism and a decomposer. A genet may first exploit living roots, then continue growing through the resulting dead wood. Its ecological reach is therefore measured in host roots and connected substrate as well as in the mushrooms that appear above ground.
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Armillaria produces dark rhizomorphs—cord-like bundles of mycelium about 1–4 millimetres wide. These structures bridge gaps between food sources and host plants. A comparative-genomics study reports rhizomorph growth of approximately 1 metre per year, allowing a genet to occupy new roots and patches of soil over long periods.
Rhizomorphs are physical evidence of connection, but they do not by themselves prove that every cord in a landscape belongs to one individual. Separate genets can occupy the same general habitat. That is why researchers pair mapping and field observations with genetic markers.
The “humongous fungus” measurement
A 2017 comparative-genomics paper reports a so-called humongous Armillaria individual reaching up to 965 hectares and 600 tons. Those are reported maximum dimensions for one connected genetic individual, not the dimensions of one mushroom. The area is a landscape-scale footprint; the mass is an estimate of the associated fungal biomass.
A separate 2019 study identified an Armillaria gallica genet extending across at least 37 hectares in Michigan’s Upper Peninsula and encompassing hundreds of tree-root systems. Matching genetic markers across samples from roots and soil supported the conclusion that the samples belonged to one individual rather than many unrelated colonies.
| Measurement | Armillaria example | What the number represents |
|---|---|---|
| Connected area | Up to 965 ha | Reported footprint of one genet |
| Estimated biomass | Up to 600 tons | Mass attributed to that connected individual |
| Rhizomorph width | About 1–4 mm | Typical cord diameter |
| Rhizomorph growth | Approximately 1 m per year | Reported rate under the study’s conditions |
| A. gallica genet | At least 37 ha | One genetically identified individual in Michigan |
The 965-hectare and 600-ton figures should be read as reported maxima, not as a standard size for the genus. The 37-hectare Michigan example is a different species, site and study design. Both illustrate the same principle: “one fungus” can mean a genetically coherent system distributed among hundreds of roots.
Are D. rex and Armillaria related?
They are fungi, but D. rex is not an Armillaria species and the two are not close relatives in the same genus. D. rex is an ascomycete in Xylariales; Armillaria is a basidiomycete genus. Their nearest reported relationship is not a recent genus-level connection.
The closest known relative reported for D. rex is Graphostroma platystoma. Molecular-clock analysis places the origin of Dianjunaceae at roughly 65 million years ago. Armillaria’s evolutionary placement is studied within the mushroom-forming order Agaricales and in relation to close taxa such as Guyanagaster. These are separate phylogenetic questions, resolved with DNA comparisons rather than by similarity in size or habitat.
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Species boundaries in Armillaria can be difficult, which is one reason multilocus and genomic studies are important. A shared ecological role, a similar mushroom or a large size does not establish common ancestry. Classification follows the total pattern of morphological and molecular evidence.
How scientists decide whether a fungus is one organism
Start with field context and morphology
For D. rex, researchers began with repeated observations of large whitish stromata on woody roots. Shape, tissues, spore-bearing surfaces and microscopic characters were compared with known fungi. Armillaria investigations examine mushrooms, rhizomorphs, infected or decayed roots, host associations and disease symptoms. Morphology identifies candidates, but it cannot by itself prove that distant samples are one individual.
Field notes also establish where a specimen grew and what substrate it used. Without that context, a measurement such as height or area cannot be interpreted reliably: a stroma on a root and a rhizomorph crossing soil represent different structures and functions.
Preserve vouchers and grow cultures
Voucher specimens create a permanent reference for the name and description. D. rex vouchers were deposited at the Kunming Institute of Botany, and spores were grown on media. Cultures make it possible to observe growth, obtain uncontaminated DNA and compare characters under controlled conditions.
A voucher also allows later researchers to revisit the original diagnosis if new relatives are discovered. Cultures provide a bridge between field appearance and laboratory measurements, although growth in a dish does not reproduce every condition of a forest root.
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Genomic data reveal relationships and biological capabilities that a mushroom’s appearance cannot. The Armillaria comparative study sequenced A. ostoyae, A. cepistipes, A. gallica and A. solidipes. Their genome assemblies ranged from 58 to 85 megabases and contained predicted totals of 20,811 to 25,704 genes.
Those values are assembly and annotation results for four named species, not a universal genome size for every Armillaria isolate. Differences can reflect genuine biology as well as sequencing, assembly and gene-prediction methods.
Genome sequencing also helps separate a new lineage from a familiar species that happens to look unusual. In D. rex, the sequence data supported recognition of a distinct genus and family rather than simply expanding the size range of an existing Xylariales genus.
Use phylogenomics for placement
To test Armillaria’s position among mushroom-forming fungi, researchers compared 835 conserved single-copy genes across Agaricales. Broader multilocus and genomic analyses help resolve Armillaria species and its relationship with Guyanagaster. Using many independent genes reduces the chance that one misleading gene history determines the whole classification.
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Phylogenomics answers ancestry questions, not size questions. A large tree of genes can show that two fungi are distant relatives, but it does not tell researchers how many tons of tissue a genet contains. Conversely, a mapped footprint does not reveal whether two fungi share a recent ancestor.
Estimate lineage ages with molecular clocks
Molecular clocks compare DNA changes among lineages whose evolutionary relationships are known and calibrate those changes against fossils or other dated events. The D. rex analysis compared its sequence with hundreds of fungi of known evolutionary age and estimated the Dianjunaceae origin at approximately 65 million years. Armillaria studies likewise use phylogenomic divergence estimates. A clock output is an estimate with assumptions, not a direct observation of an ancient organism.
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Test clonality with genetic markers
For a suspected giant Armillaria, the crucial question is whether samples from separated roots and soil share the same genetic identity. Researchers genotype samples at multiple markers and look for matching profiles. When samples across a large area match and are connected by compatible mycelial structures, the evidence supports one genet. The Michigan A. gallica example used this approach to link hundreds of tree-root systems across at least 37 hectares.
Genotyping is therefore a requirement for a strong “one organism” claim. Sampling only one mushroom cannot establish the boundaries of a genet, and finding several mushrooms in one forest cannot show that they are clones. The spatial pattern of samples and the markers chosen both affect the strength of the conclusion.
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The comparison, axis by axis
| Axis | D. rex | Armillaria | How to interpret it |
|---|---|---|---|
| Visible structure | Stroma up to 50 cm and 2.2 kg | Mushrooms are small portions of a larger network | D. rex leads for a single reported fruiting body |
| Whole-organism footprint | Not established in the report | Up to 965 ha for a reported genet; another measured at least 37 ha | Armillaria leads on connected area |
| Biomass | 2.2 kg refers to one stroma | Up to 600 tons for a reported genet | Not comparable unless the biological unit is specified |
| Taxonomy | New species, genus and family | Established genus containing multiple species | D. rex is taxonomically novel; Armillaria is a diverse lineage |
| Ecological role | Reported from woody roots and discussed as a wood-associated decomposer | Root pathogen that can shift to dead-wood decomposition | Both use woody resources, but their documented roles differ |
| Primary evidence for “giant” status | Direct fruiting-body measurements | Genetic identity, connected rhizomorphs, mapped area and biomass estimates | The evidence measures different levels of organization |
Why the two size records cannot be merged into one ranking
Calling D. rex “larger” because its stroma is 50 centimetres tall would ignore Armillaria’s underground body. Calling Armillaria “larger” because one genet covers 965 hectares would ignore that D. rex’s claim concerns a single visible reproductive structure. The correct result depends on the question:
- Largest known ascomycete fruiting body in the cited report: D. rex, at up to 50 cm and 2.2 kg.
- Largest connected fungal individual in the cited Armillaria example: Armillaria, at up to 965 hectares and 600 tons.
- Largest by confirmed D. rex whole-organism footprint: not stated in the report.
- Closest relative of D. rex: Graphostroma platystoma, not Armillaria.
These statements answer different questions and should not be collapsed into a single “world’s biggest fungus” claim. A headline that omits the unit can make two accurate measurements appear contradictory when they are actually describing different levels of organization.
What each fungus teaches scientists
D. rex and extreme fruiting-body form
D. rex gives evolutionary biologists a case study in how a lineage can produce an unusually tall and heavy reproductive structure. Its placement in a new family means the discovery changes classification as well as the size record. The combination of field morphology, cultured spores and genome data allows researchers to connect the conspicuous structure to a distinct lineage rather than treating it as an unusual form of a familiar genus.
Armillaria and long-lived clonal growth
Armillaria demonstrates that fungal individuality can persist across decades or longer while spreading through roots and soil. Rhizomorphs provide the physical bridges; genotyping provides the evidence that separated samples belong to one genet. Its ecological effects can therefore operate at forest scale even though the active cords are only millimetres wide.
Different ecological strategies
D. rex is reported in association with woody roots and discussed as a wood-associated decomposer. Armillaria species attack living roots, can kill root cambium, and then exploit dead wood. Both are associated with woody substrates, but one headline concerns a giant reproductive structure and the other concerns a persistent, spreading network that interacts with many hosts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read future “giant fungus” claims
- Identify the unit. Ask whether the number describes a mushroom, stroma, mycelial patch, genet, colony or estimated biomass.
- Check whether genetic identity was tested. A continuous-looking patch is not automatically one organism; matching markers across samples are stronger evidence.
- Separate measured values from estimates. Height and mass can be measured directly, while total underground biomass is often inferred from sampling and models.
- Check the taxonomic name. A dramatic size record does not make unrelated fungi close relatives.
- Read the geographic and temporal qualifier. “Up to” values are maxima reported for particular sites, specimens or studies, not measurements of every member of the species.
- Distinguish hypothesis from demonstration. Proposed benefits such as improved wind dispersal or post-extinction ecological opportunity require separate testing.
- Look for the evidence chain. Strong claims connect field observations, preserved material, cultures, DNA data and a clearly stated measurement method.
Frequently asked questions
Does D. rex weigh more than an Armillaria mushroom?
The cited D. rex stroma can weigh up to 2.2 kilograms, but the comparison is not a reported record for an individual Armillaria mushroom. Armillaria’s headline figures concern an entire genet, not one mushroom, so the available measurements do not support a direct mushroom-to-stroma ranking.
Could D. rex also have a large underground mycelium?
It may have tissue beyond the visible stroma, but the cited report does not establish a whole-organism footprint or biomass for D. rex. The defensible claim is therefore limited to the measured fruiting body.
Are all Armillaria mushrooms parts of the 965-hectare individual?
No. The 965-hectare figure describes a reported connected genetic individual. Armillaria includes multiple species and many genets, and mushrooms found elsewhere—or even nearby—are not automatically part of that individual without matching genetic evidence.
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Why use both morphology and DNA?
Morphology records what the organism looks like and how it develops; DNA tests its evolutionary placement and, in clonality studies, whether separated samples share an identity. Either type of evidence alone can be ambiguous, especially when unrelated fungi converge on similar forms.
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What does 835 conserved single-copy genes add?
Comparing 835 genes gives a broad phylogenomic basis for placing Armillaria among Agaricales. It reduces dependence on one gene that might have an unusual history, but it still addresses evolutionary relationships rather than the size of a genet.
How certain is the 65-million-year date?
It is a molecular-clock estimate for the origin of Dianjunaceae. The result depends on the sampled DNA, the phylogenetic model and calibration assumptions, so it should be reported as an approximate age rather than an exact timestamp.
Why are Armillaria species boundaries described as difficult?
Several Armillaria species share ecological settings and can be similar in visible features. Multilocus and genome-scale comparisons help distinguish lineages and clarify relationships, including the connection with Guyanagaster.
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Leaving the unit unstated. A fruiting-body height, a genet’s mapped area and an estimated biomass are not interchangeable. The headline should identify the structure measured, the location or study context and whether the value is direct or inferred.
The defensible verdict
D. rex wins the narrow contest for the largest reported individual fruiting body in Ascomycota: up to 50 centimetres tall and 2.2 kilograms. Armillaria wins the landscape-scale contest represented by the cited genet measurements: up to 965 hectares and 600 tons, with rhizomorphs about 1–4 millimetres wide and growth of approximately 1 metre per year. They are not competing species in one lineage, and their size records cannot be compared without naming the biological unit and the evidence used to measure it.
Frequently Asked Questions
Which fungus is bigger overall, D. rex or Armillaria?
There is no single overall winner. D. rex is bigger as a documented fruiting body, reaching 50 cm and 2.2 kg. Armillaria is bigger as a connected genet, with a reported example spanning up to 965 hectares and weighing an estimated 600 tons.
Is D. rex an Armillaria species?
No. D. rex is an ascomycete in the new family Dianjunaceae within Xylariales. Armillaria is a basidiomycete genus in Agaricales.
How do researchers prove that an Armillaria colony is one organism?
They compare genetic markers from samples collected across roots and soil, then assess whether matching profiles and compatible rhizomorph connections support one genet. Physical continuity alone is insufficient.
What is the closest known relative of D. rex?
The closest reported relative is Graphostroma platystoma, not Armillaria.
What does the 600-ton Armillaria figure measure?
It is an estimate of biomass assigned to a reported connected genetic individual. It is not the weight of one mushroom.
How fast can Armillaria rhizomorphs grow?
The 2017 comparative-genomics study reports approximately 1 metre per year under the study’s conditions. Rhizomorphs are dark cords about 1–4 millimetres wide.
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A molecular-clock analysis estimates that Dianjunaceae originated approximately 65 million years ago, about one million years after the Cretaceous–Paleogene mass extinction.
Why can’t the two fungi be ranked by one size number?
D. rex’s figure measures one visible stroma, while Armillaria’s figures measure a genetically defined underground individual. They represent different biological units and evidence types.
The Bottom Line
Bottom line: D. rex is the standout for visible fruiting-body size—up to 50 cm and 2.2 kg—while Armillaria is the standout for connected genet area and biomass—up to 965 hectares and 600 tons in the cited example. The fair comparison is not “which species is biggest?” but “which fungal structure or individual is being measured, and how was it verified?”
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