Diplodia sapinea is a globally distributed pathogen, causing stem wounds and dead tree crowns on pines. In 2016 a whole Scots pine plantation infected with this pathogen was discovered in for the first time Sweden. Laura Brodde has investigated the establishment and spread of Diplodia tip blight in Sweden to increase our knowledge and opportunities to combat the disease.
Diplodia tip blight is a serious disease of conifers. It is caused by the fungus Diplodia sapinea and can be seen by the death of shoots in the upper part of the crown, the formation of stem canker and the wood turning blue. But the fungus can also be present in the tree without causing symptoms of disease.
The disease can then break out later if the host trees are affected by abiotic stress such as drought, heat or mechanical wounding. Scots pine (Pinus sylvestris) is susceptible to Diplodia tip blight. In Sweden, Scots pines account for 40% of the total forest area so there is some reason for concern and a need for more knowledge of Diplodia tip blight in Swedish conditions.
In her doctoral thesis, Laura Brodde has expanded our knowledge on Diplodia tip blight in Scots pine. She describes the first detected outbreak of Diplodia tip blight on Scots pine in Sweden, which marked the northernmost record of such an outbreak of this disease at that time. The purpose has been to gather new knowledge about how Diplodia sapinea has been established and is spreading in Sweden.
– The first outbreak in Sweden was discovered in 2016 between Odensala and Märsta. After the drought on Gotland in 2018, we also saw large pine areas with severe crown thinning. Here, we found Diplodia sapinea, contributing to the dieback of Scots pine, but the drought was likely the driver of the disease development, says Laura.
Warm temperatures are a contributing factor
A contributing factor to outbreaks of Diplodia tip blight in Sweden has been the climate. In an analysis of the ten years leading up to the first known large-scale outbreak in a Swedish pine stand, warm June and May temperatures favoured an accumulation of Diplodia tip blight and shoot dieback in the affected site.
– We also discovered that there were several different genetic individuals of Diplodia sapinea that caused the outbreak which were already present in Sweden a couple of years before the discovery of the large-scale diseased pine stand in 2016. They were genetically similar to other groups of D. sapinea in Europe, indicating that we did not find a new, more aggressive strain of the fungus in Sweden.
Laura followed the diseased pine trees on Gotland for two years after the initial outbreak in 2018. During this period, most trees recovered and formed new shoots, even trees heavily affected by D. sapinea. Drought in combination with the location and resilience of the individual pines were likely decisive for the outbreak of Diplodia tip blight.
– We used molecular methods to measure the growth of D. sapinea inside the pines. In visibly affected pines there were high levels of D. sapinea, but we also found very low levels of D. sapinea in pines that looked healthy.
Spores are spread with wind and rainfall
D. sapinea is spread via spores from fruiting bodies that can be found on infected cones, dead twigs and needles. By looking at spore dispersal patterns, Laura saw that the spores can be spread relatively short distances by the wind and by rainfall.
– I also investigated which fungal communities were present in healthy and sick pines. Four fungal species were found specifically in healthy pine tissue. This could mean that there are fungi that can protect the tree against the pathogen. Here, there may be an opportunity to develop biological control methods.
In another experiment, Laura and collaborators from Germany developed a quick and easy way of producing spores under laboratory conditions. This will facilitate future research on Diplodia sapinea.
– To protect the forest against stress-related forest diseases in connection with climate change, it is important that we increase of knowledge on Diplodia tip blight and other diseases that develop when trees are stressed, concludes Laura.
Read the doctoral thesis ”Diplodia tip blight affecting Scots pine. Factors determining infection and spread in Swedish forests”