New Year, New Tree Disease – Phytophthora pluvialis

We are all too aware of diseases and infections that can spread around the globe and, whether these infect people or plants, their transmission may have an effect on us one way or another. In recent years there have been several plant pathogens that have caused concern, including the well-publicised ‘ash dieback disease’ that is presenting a number of problems, particularly where infected trees are considered a safety risk.

*A large diseased tree felled due to ash die back (infected wood is dark in colour). Diseased conifers in a Dartmoor woodland*

The latest tree disease to enter the UK has been found in the conifer plantations of the west coast of England and appears to be moving north, with cases appearing in Cumbria and Scotland.

Phytophthora pluvialis was detected in Cornwall in late 2021 and is described as a fungus-like pathogen that reduces that health and vigour of the host plants. When it was recently confirmed in Cornwall where it has been found to affect mature Douglas fir and western hemlock, infected trees exhibit lesions on the stem and suffer from needle cast (loss of foliage) and dieback of shoots.

*Bleeding from the trunk of an infected tree [photo: Forestry Commission guidance]*

The origins of the infection are interesting, if perhaps a little worrying. It was discovered first in Oregon in the Pacific northwest of USA where evergreen and mixed forests are managed for large scale timber production. The name ‘Pluvialis’ or ‘pluvial’ translates with the meaning relating to ‘abundant rain’ or ‘due to the action of rain’ and, as Dartmoor is wet and humid, this is one of the regions of the UK where it could become prevalent. Large areas of Oregon are also humid, western facing forest where the affected trees, including Douglas fir, western red cedar and western hemlock grow within their native range. Over the last two centuries, these trees have been grown here in the UK for their high quality timber and can be found in Fingle Woods and other coniferous woodlands around Dartmoor. The trees are suited to our climate, but so is the pathogen!

As yet, the extent and impact of this disease in this country are unknown so precautions are in place and the Forestry Commission is asking people and those connected to forestry work to be vigilant. They have produced a series of maps showing large areas of Cornwall and Devon are in this area where restrictions apply. Their advice states:

The Notices prohibit the movement of any wood, isolated bark and trees (including live trees, felled or fallen trees, fruit, seeds, leaves or foliage) of the genus Tsuga, Pseudotsuga, Pinus and Notholithocarpus, that has originated within the demarcated area.

and

Provision is made within the Notices to enable plant health inspectors to authorise movements and processing of material from the demarcated area where this can be achieved without risking the spread of Phytophthora pluvialis.

*Coniferous woodland and underplanting at Hall Farm, Dartmoor*

Hall Farm is a Woodland Trust site on south Dartmoor that is in the current Demarcated area and some conifer thinning work is currently going on. The (Douglas fir or Western hemlock) sawlogs are often too large to be transported by road over a narrow bridge so are being milled on site before being hauled away as smaller beams or boards. Recent conversations between the Woodland Trust’s manager Dave Rickwood and the Plant Health Regional Manager from the Forestry Commission have resulted in precautionary measures being employed to prevent the spread of the disease. The Plant Health Regional Manager said, “The demarcated area has expanded in the last few weeks so advice from the FC may change but, if timber is being moved off site, a movement authorisation is required, this involves a site inspection from Plant Health FC … that’s us”. Adding that “I have had further info this morning” explaining that, for milled timber, “you do not need to remove all of the sapwood, just square sawn, so that no bark or cambium is left on the products. I hope this helps you processing the timber off Hall Farm.”

For those who are enjoying the woods for walks and relaxation, there will be a few minor issues to consider when out and about. Forestry Commission are asking people to be aware of this disease and to report symptoms if you see them. For the most accurate updates, the gov.uk website will have all the information but, perhaps the most reasonable advice to those going for a walk in the woods is to read this poster.