Eight Novel Diagnostic Markers Differentiate Lineages of the Highly Invasive Myrtle Rust Pathogen Austropuccinia psidii

Austropuccinia psidii is the causal agent of myrtle rust in more than 480 species within the family Myrtaceae. Lineages of A. psidii are structured by their hosts in the native range, and some have success in infecting newly encountered hosts. For example, the pandemic biotype has spread beyond South America, and proliferation of other lineages is an additional risk to biodiversity and industries. Efforts to manage A. psidii incursions, including lineage differentiation, rely on variable microsatellite markers. Testing these markers is time-consuming and complex and requires reference material that is not always readily available. We designed a novel diagnostic approach targeting eight selectively chosen loci including the fungal mating-type HD (homeodomain) transcription factor locus. The HD locus (bW1/2-HD1 and bE1/2-HD2) is highly polymorphic, facilitating clear biological predictions about its inheritance from founding populations. To be considered as potentially derived from the same lineage, all four HD alleles must be identical. If all four HD alleles are identical, six additional markers can further differentiate lineage identity. Our lineage diagnostics relies on PCR amplification of eight loci in different genotypes of A. psidii followed by amplicon sequencing using Oxford Nanopore Technologies and comparative analysis. The lineage-specific assay was validated on four isolates with existing genomes, on uncharacterized isolates, and directly from infected leaf material. We reconstructed alleles from amplicons and confirmed their sequence identity relative to their reference. Genealogies of alleles confirmed the variations at the loci among lineages/isolates. Our study establishes a robust diagnostic tool for differentiating known lineages of A. psidii based on biological predictions and available nucleotide sequences. This tool is suited to detecting the origin of new pathogen incursions.