Sphaeropsis sapinea and Associated Endophytes in Scots Pine: Interactions and Effect on the Host Under Variable Water Content

Abstract

The ascomycete Sphaeropsis sapinea is the causal agent of the Diplodia Tip Blight disease on pines and other conifer species. This fungus has a symptomless endophytic life stage. Disease symptoms become visible when trees have been weakened by abiotic stress, usually related to warmer temperatures and drought. Currently, this disease is observed regularly in Scots pine (Pinus sylvestris) sites in parts of Europe, such as Germany, increasing dramatically in the last decade. Changes in climatic conditions will gradually increase the damage caused by this fungus, because it is favored by elevated temperature. Thus, host trees with reduced vitality due to climate change-related environmental stress are expected to be more susceptible to an outbreak of Diplodia Tip Blight disease. There is currently no established and effective method to control S. sapinea. This project aims to reveal the nature of the endophyte community of Scots pine. Utilizing the antagonistic core community of endophytes could serve as a novel tool for disease control. Results from this study provide a starting point for new solutions to improve forest health and counter S. sapinea disease outbreaks. We screened potential antagonistic endophytes against S. sapinea and infected Scots pine seedlings with the most common endophytes and S. sapinea alone and combination. The host was stressed by limiting access to water. The antagonism study revealed 13 possible fungi with the ability to inhibit the growth of S. sapinea in vitro, for example Sydowia polyspora. None of the tested co-infected fungi (Desmazierella acicola, Didymellaceae sp., Microsphaeropsis olivacea, Sydowia polyspora, and Truncatella conorum-piceae) showed strong necrosis development in vivo, even when host stress increased due to drought. However, the infection experiment demonstrated that drought conditions enhance the effect of the disease outbreak, triggering S. sapinea to cause more necrosis in the infected twigs.

The Diplodia Tip Blight Pathogen Sphaeropsis sapinea Is the Most Common Fungus in Scots Pines’ Mycobiome, Irrespective of Health Status—A Case Study from Germany

“…Cleary et al [27] suggested that this endophyte is an opportunistic pathogen, in that due to changes in climate, it could increase its pathogenicity. However, Blumenstein et al [13] showed that drought stress in the host did not increase pathogenicity of Sy. polyspora, indicating that it is a true endophyte of Scots pine.…”

“…It can live asymptomatically as an endophyte in its host tree [6,7], and can transform from a latent to an opportunistic pathogen [8,9] or/and saprotroph [10]. In combination with stress-inducing factors, such as drought, hail, extreme temperatures, or mechanical wounding [11,12], S. sapinea may rapidly become pathogenic, leading to sudden disease outbreaks [13][14][15]. Ghelardini et al [16] considered that cryptic and latent pathogens, such as S. sapinea, are the most important drivers of emerging fungal diseases in forests.…”

“…In Finland, S. sapinea has been found as a saprotroph on cones [26] and as an asymptomatic endophyte in shoots of healthy Scots pine [7]. The growth of S. sapinea is favored by a warmer climate [8], and it becomes more aggressive when the host is under drought stress [13]. Because of the endophytic stage of S. sapinea, the accumulation of this pathogen can proceed unnoticed before disease outbreaks [25].…”

“…Recently, S. sapinea has also been found as an endophyte in broadleaved trees such as Fagus sylvatica [31,32]. As local environments are changing, Scots pine trees in Germany have to face the loss of vitality due to expected increasing frequency of droughts, exacerbated by long periods of high temperatures and intense solar radiation [13]. Scots pines stressed by drought and/or hail have been found to be especially sensitive to disease outbreaks [9,13,33].…”

“…As local environments are changing, Scots pine trees in Germany have to face the loss of vitality due to expected increasing frequency of droughts, exacerbated by long periods of high temperatures and intense solar radiation [13]. Scots pines stressed by drought and/or hail have been found to be especially sensitive to disease outbreaks [9,13,33]. Typical disease symptoms of Diplodia tip blight are brown and short-needled, dead current-year shoots (Figure 1a) [34], resinous cankers on main stems and branches (branch and bole canker), dieback, and misshapen tops.…”