Diplodia tip blight pathogen's virulence empowered through host switch

Abstract

Increased drought combined with emerging pathogens poses an increased threat to forest health. This is attributable to the unpredictable behaviour of forest pathosystems, which can favour fungal pathogens over the host under persistent drought stress conditions. Diplodia sapinea (≡ Sphaeropsis sapinea) is one of the most severe pathogens in Scots pine (Pinus sylvestris) causing Diplodia tip blight (conifer blight) under certain environmental conditions. Recently, the fungus has also been isolated from non-conifer hosts, indicating that it has a broader host range than previously known. In this study we compared the impact of different levels of water availability on necrosis length caused by D. sapinea strains isolated as endophytes (eight strains isolated from asymptomatic Scots pine) and pathogens (five strains isolated from symptomatic Scots pine) and five strains isolated from symptomatic non-pine hosts. For all strains the decreased water availability increased the necrosis length in Scots pine shoots. The isolates from non-pine hosts caused the most severe reactions under all water availabilities. The results of the study indicate the likelihood that effects of climatic changes such as drought will drive D. sapinea damage in Scots pine-dominated forests and increase mortality rates in affected trees. Further, the higher necrosis in the Scots pines caused by strains that had performed a host switch are concerning with regard to future scenarios thus increasing infection pressure on Scots pine from unknown sources.

Keywords: Diplodia sapinea; Pinus sylvestris; Sphaeropsis sapinea; climate change; conifers; host-passage; symptomatic and asymptomatic infections; water-stress.

Figure 1

(A) Setup of Scots pines in the greenhouse; (B) symptoms due to Diplodia sapinea infections at the end of the experiment; (C) necrotic twig with bark; (D) necrotic twig with bark peeled off to measure the necrosis length.

Figure 2

Necrosis length in cm of the side shoots for the different water treatments (100% blue, 60% orange, 25% red) grouped according to the D. sapinea strain’s origin (isolated from symptomless material = endophytic, from symptomatic material = symptomatic, isolated from non-pine hosts = non-pine and for the control the mock-treatment with MYP-agar plugs = MYP). The letters indicate the statistical differences.

Figure 3

Boxplot of the height of the seedlings at the start of the experiment (A), at the end of the experiment (B) under different water treatments (25%, 60% and 100%). The height of the seedlings was not statistically different at the start of the experiment in the different water treatments (A). At the end of the experiment the height was statistically lower in the 25% treatment (B). (C) The growth decrease was statistically significant in the 25% water group compared to 60% and 100% treatments. The letters indicate the statistical differences.

Figure 4

Disease triangles; (A) The disease triangle describes the factors involved for a disease to appear: (1) The virulence, distribution and infection capacity of a pathogen; (2) Host immune system; (3) suitable environmental conditions that benefit the pathogen and cause stress to the host. (B) Disease triangle with a scenario leading to severe outbreaks of pathogens: (1) Pathogen settles to a new host (host-jump; putative activation of more pathogenicity genes) leading to a more aggressive pathogen; (2) More aggressive pathogen can now infect its typical host with higher virulence leading to more devastating symptoms; (3) Drought weakens the host tree and makes it even more susceptible to pathogens; (4) Environmental change can benefit emerging and invasive pathogens leading to new host species and evolution of more aggressive strains.