Cytospora friesii and Sydowia polyspora are associated with the sudden dieback of Abies concolor in Southern Europe

Abies concolor was introduced to Europe in the 19th century and commonly planted as an ornamental tree. In 2018, after several very warm and dry vegetation seasons, which has likely caused abiotic stress in the trees, a sudden dieback of A. concolor was observed in the city of Aranđelovac in Serbia. The external symptoms were the rapid discoloration of needles in a larger part of the crown and eventually the dieback of the trees. The aim of this study was to identify the possible agents of damage by analysing the fungal communities associated with symptomatic needles using high-throughput sequencing as no symptoms of damage were seen on the branches or stems. Symptomatic needles were collected from the lower branches of five standing trees. Half of the needles were surface sterilised and the remaining were left unsterilised. DNA was extracted, amplified using ITS2 rDNA as a marker and sequenced. Quality filtering showed the presence of 6 191 high quality reads, which, following a clustering analysis, was found to represent 79 non-singletons fungal taxa. The most common fungi in both surface sterilised and unsterilised needles were the pathogens Cytospora friesii (59.7%) and Sydowia polyspora (20.6%). The results demonstrated that C. friesii and S. polyspora can cause a rapid decline of A. concolor trees subjected to abiotic stress.