Phylogenetic and phenotypic diversity of Phoma isolates from Swedish forest nurseries

The production of conifer seedlings is an integrated part of forest regeneration in Sweden with over 400 million seedlings produced annually. During recent years, Phoma sp. has emerged as a significant disease in Swedish nurseries, leading to substantial loss of seedlings. Disease symptoms include tissue discolouration, defoliation, stem wounds and decay. The formation of pycnidia on infected tissues is also indicative of infection by Phoma species. The objective of this study was to phylogenetically and phenotypically characterise the species responsible for the disease in Swedish forest nurseries. In total, 181 isolates of Phoma were sampled from Picea abies, Pinus sylvestris and Pinus contorta seedlings from 11 nurseries across Sweden. Cultures were isolated from both shoot and root tissue. Multilocus phylogenetic analyses were performed using ITS, Bt2, CaM, EFA, and G3P markers. The phenotypic assessment included morphological characterisation and measurement of mycelial growth rate. Results showed that a single species, Phoma herbarum, is responsible for this emerging disease in Swedish forest nurseries. Phenotypic variation among the collected isolates was observed, along with genetic variation that revealed multiple clades in the phylogeny of the species. These findings provide valuable insights into the epidemiology of Phoma blight in Swedish forest nurseries.