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News Article

Mystery beech disease a risk to US forests

Causal agent of lethal new beech disease unknown as spread is monitored

In an age of constant inter-continental trade and travel, horticultural and silvicultural practitioners everywhere are on constant watch for the invasion of foreign pests looking to gain a foothold in new countries. These pests can be imported in any form of plant material, from soil to wooden pallets making monitoring and identification very challenging. Notorious examples of exotic species wreaking havoc in foreign tree populations include ash dieback, sudden oak death and Dutch Elm disease. And now a new pathogen has been reported on American beech trees (Fagus grandifolia) and is spreading across state lines.

Beech leaf disease (BLD), as it has been termed, was first recorded in North Eastern Ohio in the USA in 2012. Since that date is has spread to over 30 Eastern USA states and been identified in ten counties in Ohio, eight in Pennsylvania and five in Ontario, Canada. This amounts to a spread of almost 1250 acres per year in Ohio for the period 2012-2016. So far symptoms have also been registered on European beech (F. sylvatica) and Oriental beech (F. orientalis) trees in Ohio.

The disease pathogenesis begins with dark green bands between veins on the lower branches of the beech tree. The leaves then fully darken, becoming shrunken, leathery and crinkled in the process. After this the symptoms spread to the buds, which become aborted and no new leaves develop, eventually leading to the death of the plant. Its lethal nature has meant this disease presents a great risk to beech forests in the US, with younger trees being even more vulnerable.

What is surprising about this new and lethal pathogen is that it is as yet uncharacterised. Though the causal agent has yet to be identified, the risk is poses to beech forests across the Northern Hemisphere is great. The estimated impact in Ohio if half of the region’s beech trees were to be lost and with it their ecosystem services such as water filtration, biodiversity and habitat provision and carbon capture, is estimated at $225. Due to the lack of chewing marks or boreholes, current thinking suspects a microbial agent rather than an insect. A theory which is being tested by sampling DNA and RNA from diseased trees. This still leaves a range of causative options including bacteria, fungi, virus or phytoplasma, necessitating further study into the disease if the spread is to be halted. With as yet no reports of resistance to the disease, it presents a worrying threat to North Eastern US forests.

Read the full paper here: Ewing, C.J., Hausman, C.E., Pogacnik, J., Slot, J. and Bonello, P., 2018. Beech leaf disease: An emerging forest epidemic. Forest Pathology, p.e12488.

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