Life Cycle, Signs and Symptoms

This fungus is known only in its asexual phase (Dickenson 1980; Kelnarová et al., 2017) and its spread is through the asexual spores called conidia. These airborne conidia might mainly enter in the host tree through open wounds (Dickenson, 1980; Kelnarová et al., 2017) but seems also to have the ability to penetrate through intact bark (Bußkamp et al., 2024). Initially it infects the heartwood and subsequently the sapwood, cambium and phloem. The spread is mainly in the longitudinal direction of the trunk (Cech, 2018). Primarily it remains as an endophyte, saprophyte or latent invader and the tree remains asymptomatic (Kelnarová et al., 2017; Ogris et al., 2021; Brooks et al., 2023; Schlößer et al., 2023). When the host is weakening or the weather conditions are favorable for the fungus, it becomes an opportunistic pathogen which eventually kills the tree in a fast tempo. The aggressiveness and fast spread of this fungus prevails over other fungi as demonstrated in pathogenicity trials (Bußkamp et al., 2024). Ideal conditions for its pathogenic stage imply high summer temperatures and drought stress, being its optimal growth temperature of 25 °C (Dickenson, 1980). Therefore, increased number of SBD outbreaks are presumed to occur in accordance with the climate change predictions. As first signs of infection, branch dieback, wilting of leaves and epicormic shoots are typical and these early symptoms might have a slow progression (Gregory and Waller, 1951; Brooks et al., 2023; Tanney et al., 2024). Extensive damage might occur after warm summers (Abbey, 1978) as the fungus invades the cambium and the phloem of affected trees. In these advanced stages, the pathogen reaches the outer bark producing extensive necroses of subcortical tissues and developing subcortical stromata. The shedding of necrotic bark exposes the brown-black stromata (fruiting bodies) with conidiophores associated with masses of soot-like spores (Gibbs, 1997). These masses of conidia produced from the stromata under the bark gave rise to the name “sooty bark disease”. The peeled areas of dying branches or already dead trees remain covered with the black stroma even after dispersion of the wind-blown spores (Gregory and Waller, 1951; Koukol et al., 2015). After felling the tree, visual symptoms of the infection can be observed by the discoloration of the heartwood and sapwood (Koukol et al., 2015; Ogris et al., 2021). This discoloration is characterized by a greenish-brown, sharply edged stain (Gregory and Waller, 1951; Ogris et al., 2021) as a result of the secondary metabolism-defense of the tree against a broader range of fungal pathogens. However, this stain disappears after the tree dies (Gregory and Waller, 1951).