Diplodia tip blight affecting Scots pine. Factors determining infection and spread in Swedish forests

Diplodia sapinea (syn. Sphaeropsis sapinea) is a globally distributed pathogen of conifers. Symptoms of Diplodia tip blight can develop when its host is affected by stressors such as drought, heat or mechanical wounding. This thesis builds on the first detected outbreak of Diplodia tip blight on Scots pine (Pinus sylvestris L.) in Sweden, which marked the northernmost record of such an outbreak of this disease at that time. The aim of this thesis was to generate knowledge about the emergence of Diplodia tip blight. The work showed that the fungus has no apparent restrictions to cause damages on Scots pine in the Nordic climate. Climate, and variation in drought adaptation of pines were potential drivers of the first detected outbreak discovered in 2016. Further, the work showed that D. sapinea is not a recently introduced pathogen in Sweden and is genetically similar to the general European population. Large areas of Scots pine showing severe crown dieback were discovered after the drought of 2018 on Gotland. Diplodia sapinea was associated with the observed symptoms, though not the driver of the dieback. Mortality was highest in severely damaged pines, but pines could recover when the drought stress lessened. Diplodia sapinea was found to be present in healthy-looking stands, trees, and twigs. However, high abundances of D. sapinea were only found in symptomatic twigs of trees showing crown dieback. Endophytic fungal communities depended on health status and these differences were apparent across scales, from tissues to stands. Four species were associated with healthy tree tissue, indicating potential for antagonistic fungi in the endophytic community of Scots pine. A standardized in vitro sporulation method was developed to facilitate future research on the pathosystem D. sapinea – Scots pine. The findings in this thesis contribute knowledge about the range expansion of Diplodia tip blight to the North, and underline the importance of managing stress-related forest pathogens to maintain forest health in the ongoing climate change

Diplodia sapinea as a contributing factor in the crown dieback of Scots pine (Pinus sylvestris) after a severe drought

The frequency and impact of drought on global ecosystems have increased within the last century, while drought has affected tree health in many regions. Diplodia sapinea is a widespread, opportunistic pathogen infecting most conifers, causing Diplodia tip blight, thriving on hosts impacted by stress such as drought, heat, or mechanical wounding. In summer of 2018, a large-scale drought was recorded all over Europe. In late summer, pine trees all over Gotland showed crown dieback, where necrotic twigs and needles were found, especially in the upper part of the crowns. Symptoms were consistent with a potential outbreak of D. sapinea. Effects of the combination of drought and Diplodia tip blight on mortality or recovery of Scots pine in Nordic conditions are unknown. This study confirmed the presence and potential contribution of D. sapinea in the observed damages of Scots pine. Shoot blight and drought led to crown defoliation which was observed one year post-drought, while trees showed a clear recovery of newly grown shoots within the second year. Severely affected pines (>70% of the upper third of the crown with shoot blight) showed increased mortality. Recovery of the surviving trees was independent of previous dieback levels. Diplodia sapinea was most abundant in twigs with shoot blight of the symptomatic trees compared to healthy-looking twigs from the same trees and asymptomatic trees in affected and healthy pine stands. Sampling on affected and healthy sites showed possible endophytic infections with low abundance within healthy-looking twigs. Spore deposition of D. sapinea was monitored on healthy and affected sites for two consecutive years after crown damages occurred to confirm the presence of the opportunistic pathogen in the affected region. Spore deposition was observed during all seasons and correlated with high precipitation during sampling. Our observations provide insights into the emergence of Diplodia tip blight in the Northern countries and underline the potential impact of D. sapinea on tree health in the course of a changing climate

Diplodia Tip Blight on Its Way to the North: Drivers of Disease Emergence in Northern Europe

Disease emergence in northern and boreal forests has been mostly due to tree-pathogen encounters lacking a co-evolutionary past. However, outbreaks involving novel interactions of the host or the pathogen with the environment have been less well documented. Following an increase of records in Northern Europe, the first large outbreak of Diplodia sapinea on Pinus sylvestris was discovered in Sweden in 2016. By reconstructing the development of the epidemic, we found that the attacks started approx. 10 years back from several isolated trees in the stand and ended up affecting almost 90% of the trees in 2016. Limited damage was observed in other plantations in the surroundings of the affected stand, pointing to a new introduced pathogen as the cause of the outbreak. Nevertheless, no genetic differences based on SSR markers were found between isolates of the outbreak area and other Swedish isolates predating the outbreak or from other populations in Europe and Asia Minor. On a temporal scale, we saw that warm May and June temperatures were associated with higher damage and low tree growth, while cold and rainy conditions seemed to favor growth and deter disease. At a spatial scale, we saw that spread occurred predominantly in the SW aspect-area of the stand. Within that area and based on tree-ring and isotope (δ13C) analyses, we saw that disease occurred on trees that over the years had shown a lower water-use efficiency (WUE). Spore traps showed that highly infected trees were those producing the largest amount of inoculum. D. sapinea impaired latewood growth and reduced C reserves in needles and branches. D. sapinea attacks can cause serious economic damage by killing new shoots, disrupting the crown, and affecting the quality of stems. Our results show that D. sapinea has no limitations in becoming a serious pathogen in Northern Europe. Management should focus on reducing inoculum, especially since climate change may bring more favorable conditions for this pathogen. Seedlings for planting should be carefully inspected as D. sapinea may be present in a latent stage in asymptomatic tissues