Alien fungal species on asymptomatic live woody plant material imported into Canada

Abstract

Undescribed exotic and scientifically unknown fungal species with the potential to be pathogens are often difficult to detect on imported live plant material due to their inconspicuous nature and thus represent an important risk to Canadian forests. We have developed an early warning method based on a random sampling of asymptomatic woody live plant material imported into Canada to detect such potential alien fungal species. We received 150 asymptomatic sample lots collected by Canadian Food Inspection Agency (CFIA) inspectors. Samples were analysed by cloning and sequencing the PCR-amplified fungal nuclear ribosomal internal transcribed spacer (ITS) DNA from plant tissues to reveal fungal diversity. Out of 1845 fungal clones obtained, 267 fungal operational taxonomic units (OTUs) were identified. Using phylogenetic profiling methods, two fungal OTUs were categorized as having potential for a moderate impact on Canadian forests, 37 OTUs had a low impact and 18 OTUs could not be assessed given their low genetic similarity with other ITS sequences in GenBank. In all cases, the potential risk associated with these 57 fungal OTUs is based on (i) the fact that they can be considered unknown species to science; and (ii) they belong to orders, classes, genera and families in which pathogenic species are common. Fungal introductions with potential for a moderate impact on Canadian forests were observed at a very low frequency (0.2%) in the sampling units (clones). Only 1.3% of the CFIA samples had an OTU with a potential moderate impact on Canadian forests and 74% of the samples were free of fungal OTUs that could have any potential impact. This early warning method sheds light on the suite of exotic fungi that may enter Canada via plant material. Additionally, this method provides the tools to assess the potential risk that these fungi may post to Canadian trees and determines the magnitude of asymptomatic material that harbours fungal pathogens.Résumé: Les espèces fongiques exotiques non-décrites et inconnues de la science ayant le potentiel d'être pathogènes représentent un risque important pour les forêts canadiennes et sont souvent difficiles à détecter sur du matériel de plantes vivantes importé à cause de leur nature cryptique. Une méthode d'alerte précoce basée sur un échantillonnage aléatoire de matériel sur des plants ligneux vivants et asymptomatiques importés au Canada a été utilisée pour détecter des champignons précédemment non décrits et étrangers à potentiel pathogène. Nous avons traité 150 lots d'échantillons asymptomatiques collectés par des inspecteurs de l'Agence canadienne d'inspection des aliments (ACIA). Les échantillons ont été analysés par clonage et séquençage des régions génomiques encodant les espaceurs internes transcrits de l'ADN ribosomal fongique (régions ITS) présents dans les tissus des plants. À partir des 1845 clones de champignons ainsi obtenus, 267 unités taxonomiques opérationnelles (UTO) fongiques ont été définies. Par une analyse de profilage phylogénétique, nous avons catégorisé ...

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