Plenodomus tracheiphilus, but not Dothiorella ulmi, causes wilt disease on elm trees in Alberta, Canada

Annual monitoring of wilt pathogens on elm trees in Alberta is part of a provincially-regulated prevention and control program for Dutch elm disease. Over the past eight years (2016–2023), twig samples with wilt symptoms from 200 elm trees across Alberta were tested for the presence of wilt pathogens. Plenodomus tracheiphilus , the causal agent of Mal secco disease of citrus trees, was isolated from 116 out of the 200 elm trees. The identification of this fungus was confirmed morphologically by comparison with the type culture, and sequencing the internal transcribed spacer region, the β-tubulin gene, the DNA-directed RNA polymerase II second largest subunit gene, the actin gene and eight protein-coding genes exclusively present in the P. tracheiphilus genome. The pathogenicity of P. tracheiphilus isolated from Alberta was tested by artificial inoculation on elm trees to fulfill Koch's postulates based on symptom observation and fungal re-isolation. Our data indicated that P. tracheiphilus is commonly present in Alberta’s elm trees especially in the Edmonton area and that the previously described Dothiorella elm wilt is actually caused by P. tracheiphilus and not Dothiorella ulmi .