Diaporthe is no obscure corner of the fungal tree of life. With more than 1,000 described taxa, it ranks among the most speciose and ecologically versatile genera in the Ascomycota — the vast fungal phylum that includes yeasts, morels, and many plant pathogens. Members of the genus live a double life: some quietly inhabit plant tissues as harmless endophytes, others recycle dead matter as saprobes, and still others are notorious pathogens capable of devastating economically important woody plants. Leaf spots, fruit rots, branch cankers, and dieback caused by Diaporthe species have been documented on crops ranging from citrus and blueberry to kiwifruit across multiple continents. Yet despite two decades of intensive taxonomic and phylogenetic work, drawing species boundaries within this genus has proven stubbornly difficult, because the fungi exhibit extraordinary morphological plasticity — meaning that physical traits alone often fail to separate one species from another.

The research team, led by mycologists surveying forests in Yangshan County in Guangdong Province and Changjiang Li Autonomous County in Hainan Province, collected symptomatic leaves from nine different Fagaceae host species between 2019 and 2020, including Castanopsis chunii, C. hystrix, Cyclobalanopsis austrocochinchinensis, Cy. blakei, Cy. glauca, Cy. kerrii, Lithocarpus amygdalifolius, L. levis, and L. naiadarum. Back in the laboratory, they employed a classic isolation technique: leaf segments cut from the margin between diseased and healthy tissue were surface-sterilized in 75% ethanol, plated onto potato dextrose agar, and incubated at 25 degrees Celsius until fungal mycelium emerged. Pure cultures were then established from hyphal tips, and holotype specimens were deposited in the Herbarium of the Chinese Academy of Forestry, with living ex-type cultures preserved in the China Forestry Culture Collection Center.

What sets this study apart is its integrative taxonomic framework — a multi-pronged approach that combines DNA-based phylogenetics with careful morphological characterization and statistical species-delimitation modeling. The researchers extracted genomic DNA from 20-day-old cultures and amplified five partial gene regions: the internal transcribed spacer (ITS) region, calmodulin (cal), histone H3 (his3), translation elongation factor 1-alpha (tef1), and beta-tubulin (tub2). These five loci were concatenated into a single dataset totaling roughly 2,100 to 2,600 characters per species complex, and phylogenetic trees were constructed using both Maximum Likelihood and Bayesian Inference methods. The analyses were organized around four species complexes within Diaporthe — the D. arecae, D. gardeniae, D. siamensis, and D. virgiliae complexes — each resolved with its own dedicated dataset and rooted with appropriate outgroup taxa.

The results were striking. Within the D. arecae species complex, two isolates from Hainan formed a fully supported clade that the researchers described as Diaporthe liangxii, named in honor of Professor Liang Xi for his contributions to Chinese forestry research. Two additional isolates from the same province resolved as D. cyclobalanopsidicola, a nod to their host genus Cyclobalanopsis. In the D. gardeniae complex, two isolates from Lithocarpus naiadarum constituted the new species D. lithocarpicola, while in the D. siamensis complex, two isolates from Cyclobalanopsis kerrii were described as D. changjiangensis, named after the county where they were collected. Finally, in the D. virgiliae complex, two isolates from Castanopsis chunii in Guangdong formed the new species D. castanopsidicola. Together, these five novel species — along with three known lineages (D. apiculata, D. arecae, and D. siamensis) — bring the total number of distinct Diaporthe lineages recovered from this single survey to eight.

But phylogenetic trees alone were not enough. The researchers deployed two coalescence-based species-delimitation methods — the Poisson Tree Processes (PTP) model and its multi-rate variant (mPTP) — to statistically evaluate whether the observed genetic clusters correspond to independently evolving lineages. The results were revealing, and sometimes contradictory. In several cases, the mPTP analysis proved overly conservative, lumping together species that the multi-locus phylogeny clearly separated. For instance, mPTP merged D. castanopsidicola with D. linzhiensis, D. virgiliae, D. alnicola, and D. heterophyllae into a single molecular entity, despite the fact that D. virgiliae and D. heterophyllae are well-established biological species. Similarly, mPTP collapsed D. lithocarpicola and D. sennicola into one lineage, and D. changjiangensis with D. yunnanensis.

To break these ties, the team turned to the Pairwise Homoplasy Index (PHI) test, a statistical tool that detects evidence of genetic recombination between putative species. If two lineages are truly distinct species, there should be no recent gene flow between them — and that is exactly what the PHI results showed. All five pairwise comparisons between the novel species and their closest relatives yielded p-values well above the 0.05 threshold (ranging from 0.447 to 1.0), confirming the absence of significant recombination. Combined with substantial nucleotide divergence in protein-coding genes — for example, D. lithocarpicola differs from D. sennicola by 13 base pairs in cal and 14 base pairs in tef1, while D. liangxii differs from D. zhaoqingensis by 45 base pairs in cal and 34 base pairs in tub2 — the evidence for species status became irrefutable. Morphological differences reinforced the picture: D. castanopsidicola produces conidiomata measuring 800–2000 micrometers in diameter, roughly four times larger than those of D. linzhiensis, and D. changjiangensis has distinctly wider alpha conidia than its sister species D. yunnanensis.

The study also delivered some sobering news about host range. While the five novel species were isolated exclusively from Fagaceae hosts — suggesting a degree of host preference — two known species, D. apiculata and D. siamensis, were recovered from Fagaceae leaf spots for the first time. D. apiculata had previously been documented as an endophyte on Camellia sinensis (tea) and as a canker pathogen on Rhus chinensis, while D. siamensis had been recorded on Citrus and Dasymaschalon. Their appearance on Fagaceae raises the possibility that these fungi are generalists capable of jumping between forest trees and economically important crops — a phenomenon with real implications for disease management in mixed-plantation forestry and agricultural landscapes alike.

Perhaps the most important takeaway is methodological. The researchers argue that automated species-delimitation algorithms, while powerful, should never be treated as gospel. Their results demonstrate that coalescence-based models can over-lump distinct species when datasets contain uneven sampling or reflect recent rapid radiations, and that algorithmic outputs must be cross-validated against genealogical exclusivity tests, genetic distance data, and morphological evidence. In an era when fungal diversity is being catalogued at unprecedented speed, this study serves as both a discovery and a cautionary tale: the hidden species are out there, but finding them — and naming them correctly — demands rigor at every step.

With a 62.5% novelty rate among the Diaporthe lineages recovered from a geographically limited survey of Fagaceae forests, the findings suggest that subtropical Chinese forests harbor a substantially underestimated reservoir of fungal diversity. As the authors note, the hyper-diversity of Diaporthe appears to be driven largely by its associations with diverse woody hosts in natural ecosystems — a reminder that even the most familiar-looking plant diseases can conceal evolutionary stories waiting to be told.

Subject of Research: Cryptic diversity of Diaporthe fungi associated with leaf spot diseases on Fagaceae trees in China

Article Title: Unveiling cryptic diversity of Diaporthe associated with leaf spots of Fagaceae in China using an integrative taxonomic approach

Article References: Zhong, X., Voglmayr, H., Xue, H., Li, Y., & Jiang, N. (2026). Unveiling cryptic diversity of Diaporthe associated with leaf spots of Fagaceae in China using an integrative taxonomic approach. IMA Fungus, 17, Article e186438. https://doi.org/10.3897/imafungus.17.186438

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.186438

Keywords: Diaporthe, Fagaceae, cryptic species, integrative taxonomy, phylogenetics, leaf spot disease, fungal diversity, China, forest pathology, species delimitation, Unveiling, cryptic