Phellinus noxius: molecular diversity among isolates from Taiwan and its phylogenetic relationship with other species of Phellinus based on sequences of the ITS region

Pest categorisation of Pyrrhoderma noxium

Following the commodity risk assessment of bonsai plants (Pinus parviflora grafted on Pinus thunbergii) from China performed by EFSA, the EFSA Plant Health Panel performed a pest categorisation of Pyrrhoderma noxium, a clearly defined plant pathogenic basidiomycete fungus of the order Hymenochaetales and the family Hymenochaetaceae. The pathogen is considered as opportunistic and has been reported on a wide range of hosts, mainly broad-leaved and coniferous woody plants, causing root rots. In addition, the fungus was reported to live saprophytically on woody substrates and was isolated as an endophyte from a few plant species. This pest categorisation focuses on the hosts that are relevant for the EU (e.g. Citrus, Ficus, Pinus, Prunus, Pyrus, Quercus and Vitis vinifera). Pyrrhoderma noxium is present in Africa, Central and South America, Asia and Oceania. It has not been reported in the EU. Pyrrhoderma noxium is not included in Commission Implementing Regulation (EU) 2019/2072. Plants for planting (excluding seeds), bark and wood of host plants as well as soil and other growing media associated with plant debris are the main pathways for the entry of the pathogen into the EU. Host availability and climate suitability factors occurring in parts of the EU are favourable for the establishment and spread of the pathogen. The introduction and spread of the pathogen into the EU are expected to have an economic and environmental impact in parts of the territory where hosts are present. Phytosanitary measures are available to prevent the introduction and spread of the pathogen into the EU. Pyrrhoderma noxium satisfies all the criteria that are within the remit of EFSA to assess for this species to be regarded as potential Union quarantine pest.

Investigation of Asymptomatic Infection of Phellinus noxius in Herbaceous Plants

The white-rot fungus Phellinus noxius is known to cause brown root rot disease (BRRD) in woody trees and shrubs. To understand the pathogenicity of P. noxius in herbaceous plants, we investigated 23 herbaceous weed and turfgrass species in 32 BRRD-infested sites in Taiwan and/or tested them by artificial inoculation. In the field survey, P. noxius was isolated from seven symptomless herbaceous species (i.e., Typhonium blumei, Paspalum conjugatum, Paspalum distichum, Oplismenus compositus, Bidens pilosa, Digitaria ciliaris, and Zoysia matrella). Potted plant inoculation assays suggested that P. noxius is able to infect Artemisia princeps, O. compositus, and Z. matrella but not Axonopus compressus, Eremochloa ophiuroides, Ophiopogon japonicus, or Cynodon dactylon. A. princeps plants wilted within 2 weeks postinoculation, but inoculated O. compositus and Z. matrella were asymptomatic, and P. noxius could be isolated from only inoculated sites. The colonization of P. noxius in the cortex and vascular cylinder of roots was visualized by paraffin sectioning and trypan blue staining of juvenile seedlings grown on water agar. To evaluate the effect of replantation for the remediation of BRRD-infested sites, P. noxius-inoculated wood strips were buried in soil with or without vegetation. After 4 weeks, P. noxius could be detected only in the bare soil group. For the control of BRRD, the herbaceous hosts should be removed around the diseased trees/stumps and non-host turfgrasses (e.g., A. compressus, E. ophiuroides, O. japonicus, or C. dactylon) planted to accelerate the degradation of P. noxius.

Brown Root Rot Disease of Phyllanthus myrtifolius: The Causal Agent and Two Potential Biological Control Agents

Brown root rot (BRR), caused by Phellinus noxius (Corner) G. Cunningham, occurs on over 200 species of plants, especially woody trees and shrubs. Ceylon myrtle (Phyllanthus myrtifolius [Wight] Müll.Arg.), a common hedge plant, was recently observed to be infected with BRR. Disease diagnosis was performed by completing Koch's postulates, and Ceylon myrtle was confirmed to be a new host of P. noxius. Typical symptoms of BRR were observed, including reduction in leaf size, dieback of branches, and suspended growth of young leaves. A disease severity index was used to quantify BRR in this study. Compared with Malabar chestnut, Ceylon myrtle was relatively resistant to BRR. Surprisingly, phylogenetic analysis of the ITS and 28S sequences revealed that isolates identified as P. noxius from Taiwan and many other countries were clustered in the same clade but separate from the clade comprising isolates from China, which were designated Pyrrhoderma noxium based on P. noxius. Therefore, to temporarily distinguish these pathogens, the former clade was designated GPN (global P. noxius), whereas the latter clade was designated CPN (China Py. noxium). In biocontrol assays, Streptomyces padanus and Bacillus sp. were selected for BRR control of Ceylon myrtle. Disease severity was reduced from 0.51 to 0.37 by S. padanus and to 0.14 by Bacillus sp. in greenhouse trials. In addition, the two biocontrol agents, especially S. padanus, exhibited good growth-promoting effects on cuttings of Ceylon myrtle. With these double advantages, S. padanus and Bacillus sp. have great potential to control BRR in practical applications.

Soil Is Not a Reservoir for Phellinus noxius

Phellinus noxius causes brown root rot (BRR) of diverse trees. Basidiospores and diseased host tissues have been recognized as important sources of P. noxius inoculum. This study aimed to understand whether P. noxius could occur or survive in soil without host tissues in the natural environment. Soil was sampled before and after the removal of diseased trees at eight BRR infection sites (total of 44 samples). No P. noxius colonies were recovered in soil plating assays, suggesting that no or little viable P. noxius resided in the soil. To know whether P. noxius could disseminate from decayed roots to the surrounding soil, rhizosphere and non-rhizosphere soils were sampled from another two infection sites. Although P. noxius DNA was detectable with specific primers, no P. noxius could be isolated, even from the rhizosphere soils around decayed roots covered with P. noxius mycelial mats. The association between viable P. noxius and the presence of its DNA was also investigated using field soil mixed with P. noxius arthrospores. After P. noxius was exterminated by flooding or fumigation treatment, its DNA remained detectable for a few weeks. The potential of onsite soil as an inoculum was tested using the highly susceptible loquat (Eriobotrya japonica). Loquats replanted in an infection site that had been cleaned up by simply removing the diseased stump and visible residual roots remained healthy for a year. Taken together, P. noxius is not a soilborne pathogen, and diseased host tissues should be the focus of field sanitation and detection for BRR.

The Genome of the Medicinal Macrofungus Sanghuang Provides Insights Into the Synthesis of Diverse Secondary Metabolites

The mushroom, Sanghuang is widely used in Asian countries. This medicinal fungus produces diverse bioactive compounds and possesses a potent ability to degrade the wood of the mulberry tree. However, the genes, pathways, and mechanisms that are involved in the biosynthesis of the active compounds and wood degradation by Sanghuang mushroom are still unknown. Here, we report a 34.5 Mb genome—encoding 11,310 predicted genes—of this mushroom. About 16.88% (1909) of the predicted genes have been successfully classified as EuKaryotic Orthologous Groups, and approximately 27.23% (665) of these genes are involved in metabolism. Additionally, a total of 334 genes encoding CAZymes—and their characteristics—were compared with those of the other fungi. Homologous genes involved in triterpenoid, polysaccharide, and flavonoid biosynthesis were identified, and their expression was examined during four developmental stages, 10 and 20 days old mycelia, 1 year old and 3 years old fruiting bodies. Importantly, the lack of chalcone isomerase 1 in the flavonoid biosynthesis pathway suggested that different mechanisms were used in this mushroom to synthesize flavonoids than those used in plants. In addition, 343 transporters and 4 velvet family proteins, involved in regulation, uptake, and redistribution of secondary metabolites, were identified. Genomic analysis of this fungus provides insights into its diverse secondary metabolites, which would be beneficial for the investigation of the medical applications of these pharmacological compounds in the future.

Genome comparison and transcriptome analysis of the invasive brown root rot pathogen, Phellinus noxius, from different geographic regions reveals potential enzymes associated with degradation of different wood substrates

Phellinus noxius is a root-decay pathogen with a pan-tropical/subtropical distribution that attacks a wide range of tree hosts. For this study, genomic sequencing was conducted on P. noxius isolate P919-02W.7 from Federated States of Micronesia (Pohnpei), and its gene expression profile was analyzed using different host wood (Acer, Pinus, Prunus, and Salix) substrates. The assembled genome was 33.92 Mbp with 2954 contigs and 9389 predicted genes. Only small differences were observed in size and gene content in comparison with two other P. noxius genome assemblies (isolates OVT-YTM/97 from Hong Kong, China and FFPRI411160 from Japan, respectively). Genome analysis of P. noxius isolate P919-02W.7 revealed 488 genes encoding proteins related to carbohydrate and lignin metabolism, many of these enzymes are associated with degradation of plant cell wall components. Most of the transcripts expressed by P. noxius isolate P919-02W.7 were similar regardless of wood substrates. This study highlights the vast suite of decomposing enzymes produced by P. noxius, which suggests potential for degrading diverse wood substrates, even from temperate host trees. This information contributes to our understanding of pathogen ecology, mechanisms of wood decomposition, and pathogenic/saprophytic lifestyle.