Mycobiota associated with vascular wilt of poplar

In 2017, the 560-ha area of hybrid poplar plantation in northern Poland showed symptoms of tree decline. Leaves appeared smaller, yellow-brown, and were shed prematurely. Twigs and smaller branches died without distinct cankers. Trunks decayed from the base. Phloem and xylem showed brown necrosis. Ten per cent of trees died 1-2 months after the first appearance of symptoms. None of these symptoms was typical for known poplar diseases. Mycobiota was analysed with Illumina sequencing. A total of 69 467 and 70 218 OTUs were obtained from the soil and wood. Blastocladiomycota and Chytridiomycota occurred only in soil, with very low frequencies (0.005% and 0.008%). Two taxa of Glomeromycota with frequency 0.001% occurred in wood. In soil and wood, frequencies of Zygomycota were 3.631% and 0.006%, of Ascomycota 45.299% and 68.697%, and of Basidiomycota 4.119% and 2.076%. At least 400 taxa of fungi were present. Identifiable Zygomycota, Ascomycota, and Basidiomycota were represented by at least 18, 263 and 81 taxa, respectively. Many fungi were common to soil and wood, but 160 taxa occurred only in soil and 73 only in wood. Root pathogens included species of Oomycota. Vascular and parenchymal pathogens included species of Ascomycota and of Basidiomycota. The initial endophytic character of fungi is emphasized. Soil and possibly planting material may be the sources of pathogen inoculum, and climate warming is likely to be a predisposing factor. Water deficit may increase tree susceptibility