A new study led by Claudia Bashian-Victoroff and colleagues in the Journal of Fungi has unveiled the hidden impact of Beech Leaf Disease (BLD) on the critical relationship between trees and fungi. The disease, which causes disfigurement of leaves, canopy loss, and even death in beech trees, has been spreading across the midwestern and northeastern United States and southeastern Canada. This research is the first to investigate the effect of BLD on ectomycorrhizal fungi, the tree’s root symbionts.
Ectomycorrhizal fungi are essential for the health of trees as they form a symbiotic relationship with tree roots, exchanging nutrients and supporting growth. In turn, these fungi rely on the tree’s ability to photosynthesize, which is diminished by BLD. The researchers hypothesized that this might lead to less carbohydrate availability for the fungi, affecting their ability to colonize tree roots.
To test their hypothesis, the team sampled root fragments from cultivated Fagus grandifolia trees sourced from Michigan and Maine at two different timepoints – fall 2020 and spring 2021. The trees were part of a long-term beech bark disease resistance plantation at the Holden Arboretum. They compared fungal colonization of roots across three levels of BLD symptom severity and analyzed the fungal communities through high-throughput sequencing.
The results showed a significant reduction in ectomycorrhizal root tip abundance on trees with poor canopy conditions due to BLD, but only in the fall 2020 collection. There were more root tips from root fragments collected in fall 2020 than in spring 2021, suggesting a seasonal effect. Interestingly, the community composition of the fungi remained unaffected by the tree’s condition but varied between provenances.
The researchers identified significant species-level responses of ectomycorrhizal fungi to both tree condition and provenance. Two zOTUs (operational taxonomic units) showed a significantly lower abundance in high-symptomatology trees compared to low-symptomatology trees.
…we found evidence that the severity of BLD symptoms on F. grandifolia grown in a common garden experiment significantly reduced ECM colonization and had some specific effects on the occurrence of ECM taxa, although overall communities were unaffected by BLD. ECM provides a suite of ecosystem services that aid plant nutrition and resilience, without which tree and ecosystem health would suffer. The loss of ECM symbionts, and the loss of nutrients coincident with ECM reduction, could have knock-on effects on trees suffering from BLD where canopy loss from nematode feeding is accompanied by nutrient uptake reduction from loss of ECM fungi, further negatively affecting tree health. Furthermore, if the inoculum potential of ECM fungi is reduced in forest settings that are affected by BLD, reductions in the biodiversity of ECM fungi could cause reductions in beech regeneration in the years to come.Bashian-Victoroff et al. 2023
“Fungal mutualists are underappreciated and understudied, even among mycologists,” says Bashian-Victoroff in a press release. “The vast majority of research on fungi is on fungal pathogens, like the ones that disrupt agricultural systems. So focusing on fungi and what they do to help our ecosystems is extremely important.”
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Bashian-Victoroff, C., Brown, A., Loyd, A.L., Carrino-Kyker, S.R. and Burke, D.J. (2023) “Beech leaf disease severity affects ectomycorrhizal colonization and fungal taxa composition,” Journal of Fungi (Basel, Switzerland), 9(4), p. 497. Available at: https://doi.org/10.3390/jof9040497.