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Severe pine twisting rust in Scots pine increases the probability of Diplodia tip blight
Published: 11 November 2024
In a new study, SLU researchers has investigated if the fungal disease pine twisting rust makes the pines more susceptible to Diplodia tip blight. ”The more the trees struggled with pine twisting rust, the more Diplodia we found in the shoots over the summer”, says Matilda Stein Åslund. There is likely a genetic component to this that can be used in pine breeding.
Diplodia tip blight is a relatively new threat in Swedish forests, primarily affecting various pine species. Globally, it’s a major concern that causes substantial losses as shoots in the tree canopy die, and wood quality deteriorates due to stem wounds and blue staining. The fungus causing Diplodia tip blight, Diplodia sapinea, thrives when trees are stressed by factors such as drought, heat, or mechanical injuries. It is believed that the fungus can remain latent in the tree, as an endophyte – a microbe that lives inside plant tissue without harming it – for long periods and then flare up and cause disease when the trees are weakened by unfavourable environmental conditions.
Does pine twisting rust make pines more susceptible to Diplodia tip blight?
Since the first major outbreak of Diplodia tip blight in Sweden, near Arlanda in 2016, there have been scattered reports from different parts of the country of young pine stands severely affected by both pine twisting rust (caused by the rust fungus Melampsora pinitorqua) and Diplodia tip blight. These reports raised the concern that the stress caused by the rust makes the pines more susceptible to Diplodia tip blight. Matilda Stein Åslund and her colleagues have sought to determine if this is the case by studying a pine stand planted after the large forest fire in Västmanland in 2014. The results of their work were published in the scientific journal Plant Cell & Environment recently.
– We chose to study this stand because it had experienced significant pine twisting rust and Diplodia tip blight over several years, and due to the abundance of aspen saplings in the stand, says Jan Stenlid, one of the study’s co-authors. The rust fungus alternates between aspen and pine, and with so much aspen thicket, we could be certain that all pines were exposed to pine twisting rust and that only those with resistance would remain healthy, he continues.
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Field assistants Maria Jonsson and David Schlindenbuch measuring the yearly height growth of the pines. Photo: Matilda Stein Åslund.
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Jan Stenlid inspecting a pine affected by both pine twisting rust and Diplodia tip blight. Photo: Matilda Stein Åslund.
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Pine shoots were collected and analysed
In the studied stand, there were trees with moderate pine twisting rust, trees with severe pine twisting rust and concurrent Diplodia infections, but also trees without symptoms of either disease, i.e., trees with so-called disease tolerance. Both tolerant trees and those with concurrent pine twisting rust and Diplodia infections were relatively rare in the stand, but the disease status of the trees remained consistent over years.
One of the questions the researchers wanted to investigate was whether severe pine twisting rust symptoms, or small lesions the disease causes on young shoots, could promote Diplodia tip blight. To examine this, the severity of disease symptoms was assessed several times over two seasons. In the spring and early summer of 2021, the researchers collected pine shoots with and without lesions from the rust, from trees with varying degrees of disease impact. The amount of pine twisting rust fungus (M. pinitorqua) and Diplodia fungus (D. sapinea) in the shoots was measured using quantitative PCR. Quantitative PCR is a method that can measure the amount of DNA from a specific fungus in a sample.
– We found no evidence that the rust lesions themselves favoured Diplodia tip blight, but rather that Diplodia tip blight was associated with the severity of pine twisting rust on the tree. The more the trees struggled with pine twisting rust during shoot growth, the more Diplodia we found in the shoots over the summer, regardless of whether they had lesions or not, says Matilda Stein Åslund, the lead author of the study.
A component that can be utilised in pine breeding
Another question the authors looked at was how pine twisting rust and Diplodia tip blight affected the trees’ defences by measuring the composition of amino acids and phenols in the young pine shoots, with or without rust lesions. The symptoms of pine twisting rust influenced the composition of amino acids and phenols more than the actual amount of pine twisting rust fungus in the shoots.
– We also observed that specific phenols were strongly associated with pine twisting rust symptoms and the presence of endophytic Diplodia in the shoots during the summer, Matilda explains. Our interpretation of the results is that severe symptoms from another disease on the tree may facilitate the establishment of Diplodia tip blight in young pine shoots by impacting the necessary defence mechanisms. But our results also show that, since the disease status of the trees remains consistent over the years, there is likely a genetic component to disease tolerance that can be utilised in pine breeding, Matilda concludes.
Written by Malin Elfstrand and translated by Matilda Stein Åslund
Links:
Link to the study: https://onlinelibrary.wiley.com/doi/full/10.1111/pce.15218