Pathogenic ice‐nucleation active bacteria in willows for short rotation forestry

Extensive sampling of endophytically growing bacteria from shoots of diseased Salix viminalis clones were performed in trials at two experimental sites in central Sweden. Field, greenhouse and laboratory studies were conducted to identify and characterize the plant‐associated bacteria and determine their possible role in the severe dieback appearing in short rotation forestry (SRF) plantations in Sweden during the last few years. The most frequently isolated types were found to be non‐fluorescent Pseudomonas fluorescens (biotype A, C, G) and Sphingomonas spp. (23%). Erwinia spp., P. fluorescens, P. syringae and Xanthomonas spp. each constituted 11–14% of the bacterial isolates respectively. After biochemical characterization and screening for ice‐nucleation activity (INA) 78 of the originally 279 isolated bacterial strains were selected for pathogenicity tests and further analyses. Of the 78 selected isolates, 95% were found to be pathogenic, giving different degrees of necrotic reactions after inoculation to Salix plants. Half of these pathogens (46%) also showed strong INA (at temperatures between ?2.5 and ?5°C). With respect to both isolation frequency, clonal distribution in the field, ice‐nucleation ability and necrotic reactions of willow in laboratory tests, we consider the following types to be the most important pathogens in the field: Erwinia spp., P. syringae, Sphingomonas/P. fluorescens spp. (biotype A, C, G) and Xanthomonas spp. Reported damage and dieback in many SRF‐plantations are, according to our results, very likely to be a consequence of bacterial infections, particularly of pathogenic INA‐strains, which both initiate freezing stress and aggravate the frost damage of plants by subsequent invasion of plant tissues leading to complete death of plants under field conditions.