The view along the Bovey Valley Woods is a dramatic scene that makes many passers-by stop and stare. Taking time to absorb this spectacle is one of the great pleasures of Dartmoor. It is one of those iconic panoramas of the eastern moor where the sweeping skyline of Lustleigh Cleave provides a bold and rocky backdrop for the diverse, soft patterns and colours of the tree cover that sweeps along the valley. Breath-taking at any time of year.

But recently, this familiar view was interrupted by an unusual patch of unseasonal colour. Looking across from the bulging spur of Trendlebere Down to the steep slopes of Hisley Wood, the pattern of undulating broadleaf canopy and stands of conifers was broken by an irregular patch of brown. One of the forestry team, familiar with the sights of the mixed woodland, had his eye drawn towards a cluster of dead trees. Knowing the valley so well, he decided to investigate. Following the tracks from Hisley Bridge to ‘The Cleave’ he managed to pinpoint the anomaly among the tree tops and, standing among the mighty Douglas fir, he began to wonder what had caused a group of six large trees to suffer such a decline.

For a woodland manager, the sudden death of a group of trees can be a concerning issue as the source of the problem is often a mystery. Sam Pyne, the forester contacted Dave Rickwood from the Woodland Trust, and they made plans. It was time to bring in some expertise from the Forestry Commission’s (FC) Tree Health Team. In recent years, we have heard a lot about the spread of tree diseases and in these circumstances, it is vital to act quickly.

One misty October morning, the Forestry Commission’s Mick Biddle made his way down the valley to meet Sam Pyne and his colleague Barry Green. They showed Mick the dying trees and explained their concerns about how they had noticed the tips of a few evergreen branches becoming yellow. A discussion began to cover many of the possibilities. Was it a fungal infection present in the soil or was it a pathogen moving through the canopy, passing from tree to tree?

It is a nervous time as an outbreak of Phytophthora ramorum in these Douglas firs would be serious, as it has been known to cause devastation in larch, one of the other conifers growing in the valley. Mick explained that, “It is less likely as this usually occurs in Douglas fir where a neighbouring stand of infected larch is present. That is not the case here.”

“Phytophthora ramorum is also not ‘sporulating’ in Douglas fir as it does in larch so there is less of a risk of transmission. It’s hopefully not that.”

He also spoke about Swiss Needle Cast, a disease that can also affect Douglas fir. “It seems to be under reported in the South West as it is often not fatal, though it can restrict growth and vigour of the trees as they lose their ability to photosynthesise.”

The group moved slowly around the bases of the six dead trees, searching for evidence, staring down at the soil and up to the sky. Barry pointed out something he had observed recently, “There are small patches of yellowing needles in the canopy, particularly at the tips of the branches.” In the case of Swiss needle cast, there are sometimes tiny visible spores on the underside of these yellow needles. Some samples would be needed to be sent to a laboratory for analysis.

Mick’s search continued as he spotted patches of honey fungus, the bronze clusters of fruiting bodies were quite abundant in some areas. Taking bio security precautions to prevent cross-contamination, he began to peel back patches of bark on the dead trees, saying, “I can’t see a mycelial mat but it could be at the early stages and may be in the roots. I need to collect a few samples for analysis.”

The next step was to collect samples of the yellowing foliage from high in the branches. The best way to do this is to fell a tree. After several minutes of deliberation, an adjacent tree was selected and felled. It was not yet dead but showed signs of reduced vigour in the canopy and was likely to be infected. These samples would again be sent to a Forestry Commission laboratory for analysis. So, at the end of the morning, no conclusion could be reached but the samples were collected and securely bagged.

The conversation continued for a while and it appeared that, though Phytophthora ramorum could not be ruled out, it may be that a combination of Swiss needle cast that can stress the trees which subsequently succumbed to attack from a damaging honey fungus, one of the Armillaria genus, has caused this group of trees to die. So, for now, only time and the laboratory results will tell and the next course of action will be taken to prevent further spread of this mystery disease – nipping it in the bud!

by Matt Parkins

To read more about tree disease in the NNR, you can read this blog from 2016: Pullabrook Larch – News from the Bovey Valley

You can also find out more about this fascinating and vital area of work by visiting the Observatree website – an award winning collaborative project to use citizen science to help spot new pest and disease threats to UK trees.