Colletotrichum is no ordinary fungal genus. First introduced by Corda in 1831, it is currently recognized as the sole genus in the family Glomerellaceae, order Glomerellales, class Sordariomycetes, phylum Ascomycota. Its species are distributed worldwide and have been reported as plant pathogens, saprophytes, endophytes, human pathogens, and even insect pathogens. With an extensive host range spanning numerous plant families, the genus causes severe pre- and post-harvest yield losses, quality degradation, and economic damage valued at billions of dollars annually. The taxonomy of the genus has shifted dramatically over the decades, from morphology-based identification to a robust polyphasic approach integrating multilocus phylogenetics, phylogenomics, and morphological, geographical, and ecological data. A recent taxonomic study and the EPPO-Q-BANK database suggest that approximately 334 accepted species currently exist, distributed across 15 species complexes and a number of singletons.

The Colletotrichum gigasporum species complex, first described by Liu and colleagues in 2014, initially included six species. Subsequent studies have added species from Brazil, China, Vietnam, Thailand, and the Philippines, each discovered in remarkably different niches: an endophyte from the root of an endangered orchid, healthy roots and stems of Nothapodytes pittosporoides, healthy leaves of an unknown plant, leaf spots of Xanthophyllum sylvestre in Vietnam, rotting durian fruit in Thailand, and a wilted cacao cherelle in the Philippines. Despite this wide geographic distribution, with species recorded across Asia, Africa, Central and South America, and Oceania, the complex remains the least commonly reported in the entire genus. The two new species from Hainan therefore represent a significant addition to a poorly documented lineage.

The discovery began with systematic fieldwork. From 2023 to 2024, the researchers collected large numbers of plant leaves exhibiting distinct signs of fungal necrosis or typical spot symptoms from gardens, streets, or the wild in or near the Hainan Tropical Botanical Garden and Wuzhishan National Nature Reserve. Pure colonies were obtained using tissue isolation techniques: small fragments cut from the edges of diseased tissues were surface-sterilized in 75% ethanol and 10% sodium hypochlorite, rinsed in sterile water, blotted dry, and placed on potato dextrose agar. After incubation at 25 °C for three days, hyphae were picked from the periphery of the colonies and inoculated onto new plates to obtain pure cultures. The strains were preserved in 10% sterilized glycerol and stored at 4 °C, with dried cultures deposited in the Herbarium of the Department of Plant Pathology at Shandong Agricultural University and living cultures deposited in the Shandong Agricultural University Culture Collection and the China General Microbiological Culture Collection Center.

Morphological characterization was carried out with stereomicroscopes and compound microscopes equipped with digital cameras, and structural measurements were performed using Digimizer software with a minimum of 20 measurements taken for each structure, such as conidiophores, conidiogenous cells, and conidia. Cultural characteristics were observed and photographed after 7 and 14 days of incubation at 25 °C in darkness. Both new species produce acervular conidiomata, subglobose and superficial to semi-immersed, with hyaline cylindrical conidia containing granular contents. Neither species produced appressoria or setae on PDA, and no sexual morph was observed. Colletotrichum micheliae produces conidia measuring 21.5–32 × 10–14 µm with an L/W ratio of 2.1, while Colletotrichum myristicae produces longer conidia measuring 30–38.5 × 9.5–12.5 µm with an L/W ratio of 3.1. Colonies of both species grew at an average radial rate of 10–16.5 mm per day, covering a 9-cm Petri plate within seven days.

To confirm that these morphological differences reflected genuine species boundaries rather than environmental variation, the researchers extracted genomic DNA using a modified CTAB method and amplified and sequenced six loci: partial actin (ACT), chitin synthase 1 (CHS-1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), histone 3 (HIS3), the internal transcribed spacer (ITS), and beta-tubulin 2 (TUB2). The concatenated alignment comprised 2,466 characters and was analyzed using maximum likelihood and Bayesian inference with RAxML-NG and MrBayes respectively. The best evolutionary model for each partition was determined using PartitionFinder 2 with the AICc criterion. The maximum likelihood optimization likelihood was −10,158.175175, and the matrix exhibited 879 distinct alignment patterns. The Markov chain Monte Carlo analysis ran over 5,000,000 generations, yielding 10,002 trees, of which the first 2,500 were discarded during the burn-in phase. Colletotrichum gloeosporioides served as the outgroup in an analysis of 32 strains.

The phylogenetic results were unambiguous. The strains SAUCC 6771-5 and SAUCC 6771-6, isolated from Michelia × alba, formed a fully supported lineage with bootstrap support of 100% and Bayesian posterior probability of 1.0. Similarly, the strains SAUCC 8077-1 and SAUCC 8077-2, isolated from Myristica fragrans, also formed a fully supported lineage. Both new species sit sister to Colletotrichum jishouense, Colletotrichum durionigenum, and each other within the gigasporum complex. Pairwise DNA sequence dissimilarities between the two new species ranged from 1.90% in CHS-1 to 8.00% in GAPDH, with 5.15% in TUB2 and 2.24% in ITS. These molecular differences, combined with consistent morphological distinctions, satisfied the criteria of Genealogical Concordance Phylogenetic Species Recognition, the powerful tool that has proven essential for species delimitation within the genus.

The morphological evidence reinforces the molecular case. Colletotrichum micheliae differs from Colletotrichum jishouense in its larger conidiogenous cells, measuring 16.5–30.5 × 6–8 µm versus 4–11 × 2–3 µm, and larger conidia, measuring 21.5–32 × 10–14 µm versus 5–14 × 3–5 µm. It differs from Colletotrichum myristicae and Colletotrichum durionigenum in its shorter conidia, measuring 21.5–32 µm versus 30–38.5 µm and 29–40 µm respectively. Colletotrichum myristicae, in turn, differs from Colletotrichum durionigenum by producing shorter conidiogenous cells and slightly wider conidia, and Colletotrichum durionigenum uniquely produces conidia with pale purple pigments that have not been found in Colletotrichum myristicae. Species within the gigasporum complex produce larger conidia than most other Colletotrichum species, which serves as a valuable diagnostic feature at the species complex level, even though single-gene data such as ITS usually lack the resolving power needed for species-level identification across most complexes.

Hainan itself is one of the key global hotspots for biodiversity conservation and one of the regions in China with the richest biodiversity, characterized by a unique tropical rainforest ecosystem along with typical marine ecosystems such as coral reefs, mangroves, and seagrass beds. The annual mean temperature ranges from 22 to 27 °C and annual precipitation from 1,000 to 2,600 mm, conditions that support exceptionally high fungal diversity. Previous researchers have recorded 15 Colletotrichum species from areca palm, coffee, rubber, and tea-oil camellia in Hainan Province, and Colletotrichum magnoliae and Colletotrichum siamense were recorded from Michelia × alba in China as early as 1979. Colletotrichum gloeosporioides was recorded from Myristica fragrans in India and Indonesia. The two new species from Hainan therefore enhance understanding of the biodiversity of the genus in the province and contribute valuable data for future research on fungal systematics and biodiversity conservation.

One important question remains open. Although both new species were isolated from diseased plant leaves, further research is needed to verify through Koch’s postulates whether they are genuine pathogens of their hosts or merely opportunistic colonizers. Until pathogenicity tests are completed, the ecological role of Colletotrichum micheliae and Colletotrichum myristicae cannot be definitively assigned. What is certain is that the discovery underscores how much fungal diversity remains undocumented even in well-studied tropical regions, and how the combination of careful fieldwork, detailed microscopy, and multilocus phylogenetics continues to reveal new species hidden in plain sight on the leaves of familiar tropical trees.

Subject of Research: Two new Colletotrichum fungal species, C. micheliae and C. myristicae, discovered on diseased leaves of Michelia × alba and Myristica fragrans in Hainan Province, China.

Article Title: Two new Colletotrichum species from Michelia × alba and Myristica fragrans in Hainan Province, China

Article References: Dong, Z., Yuan, Y., Lv, Y., Zhang, Z., Zhang, X., & Xia, J. (2026). Two new Colletotrichum species from Michelia × alba and Myristica fragrans in Hainan Province, China. MycoKeys, 141, 95-109. https://doi.org/10.3897/mycokeys.141.203785

Image Credits: AI Generated

DOI: 10.3897/mycokeys.141.203785

Keywords: Colletotrichum, fungi, taxonomy, Hainan, anthracnose, Michelia × alba, Myristica fragrans, gigasporum species complex, multilocus phylogenetics, new species, plant pathology, biodiversity