Hidden Invaders – Fungal Foes in Our Trees. Ginette Alexander, Head of Tree Health and Training

If you go down to the woods today… you might just find an invisible battle being fought. Among the roots, branches, and leaves of our cherished trees, tiny fungal pathogens are at work—unseen, unstoppable (or so they think!), and utterly fascinating.

As part of UK Fungus Day 2025, I’m going to share how we’re tackling these tricky tree troubles at the Yorkshire Arboretum in North Yorkshire. Our 120-acre site is home to more than 9,000 trees from around the world and it’s a haven for biodiversity, education, and yes… fungi.

But not all fungi are friends. Let me introduce you to some of the villains of the arboretum:

The Ash Assassin: Hymenoscyphus fraxineus (Ash dieback)

This fungus arrived in the UK just over a decade ago and has changed our landscapes forever. Hymenoscyphus fraxineus causes ash dieback, attacking the tree’s vascular system and leading to dead branches, brittle stems, and often, eventual collapse. At the arboretum, we monitor our Fraxinus excelsior (common ash) specimens closely, collecting data to contribute to national research.

Look closely: leaf loss and diamond-shaped lesions on stems are classic signs. But the real trick? The fungus reproduces on the leaf litter below, where its tiny fruiting bodies fire off spores each summer.

   

Credit: Images taken from the Observatree Field Guide to Chalara – available from the Observatree website.

The Red Menace: Dothistroma septosporum (Red band needle blight)

Don’t be fooled by its dramatic splash of colour—Dothistroma septosporum is bad news for conifers, especially Pinus species including Corsican pine, lodgepole pine and Scots pine, along with some exotic pines in collections like ours.

It causes needles to turn reddish-brown with distinct transverse bands before falling off, leading to defoliation and reduced growth. Originating from overseas, it’s another example of why biosecurity matters. Wind-blown spores spread easily, so we practise strict hygiene when working around infected trees.

The Rusty Intruder: Gymnosporangium sabinae (Pear rust)

This one is a shapeshifter! A familiar sight on pear trees in late spring, pear rust is a vivid fungal disease that plays a double act. It needs both pear (Pyrus communis) and juniper (Juniperus spp.) to complete its life cycle.

It starts with orange spots on the upper leaf surface of the pear. These deepen and swell into spiky fruiting bodies underneath, releasing spores that drift off in search of a juniper. Once found, the fungus bides its time on the juniper until spring, erupting into jelly-like orange telial horns that release spores to reinfect pear.

The outcome for the pear is unsightly leaf spots, defoliation and reduced fruiting. It can be managed by removing affected leaves, avoiding planting susceptible junipers nearby, and choosing resistant pear cultivars where possible.

At the arboretum, we use this pathogen as a teaching tool—its bright colours and complex lifecycle make it perfect for storytelling. Look out for it on your garden pears!

A Rising Threat: Sirrococcus tsugae (Sirococcus shoot blight)

This lesser-known fungus is creeping into prominence. Sirococcus tsugae affects Tsuga species (hemlocks), causing tip dieback and shoot death in young trees. It can be tricky to spot, but early signs include the needles taking on a pink hue in spring, followed by browning and shrivelled shoots.

Our collection lost several trees in the last year due to favourable growing conditions allowing S. tsugae to spread rapidly. It’s a good reminder that even in our carefully curated collections, new fungal threats can emerge. That’s why we practise good tree hygiene or ‘Biosecurity’. We monitor symptoms year-round, and report any concerns to Plant Health and Seeds Inspectors at the Animal and Plant Health Agency or Forestry Commission Plant Health specialists who can take samples for diagnosis and advise on next steps.

Why It Matters

Fungal pathogens are nature’s great recyclers but when they overstep, they become agents of ecological change, even destruction. Climate change, global trade, and reduced tree genetic diversity all increase the risk of outbreaks by creating new pathways of introduction, a more hospitable climate for pathogens to survive and thrive and less diversity of tree species makes it easy for the villains to jump from one host to another and spread rapidly.  

At the Yorkshire Arboretum, we work to be part of the solution through good biosecurity practices, careful monitoring, education and training tomorrow’s tree health champions. Every leaf we examine, every sample we send for testing, every conversation we have about plant biosecurity helps protect our treescape.

Final Spores of Wisdom

Fungal pathogens may be microscopic, but their impact is massive. By learning to spot them, talk about them, and report them, we become guardians of the green.

Become a Tree Health Volunteer!

Live locally? Why not join our band of tree health volunteers on Wednesdays who survey all the trees in our collection for signs or symptoms of pests and diseases. Their valuable work gives us a vital early warning system of potential problems, allowing us to address them as soon as possible.

Join us on UK Fungi Day – Saturday October 4th

Let’s celebrate the weird and wonderful world of fungi this UK Fungus Day together… even the troublemakers. Come visit us at the Yorkshire Arboretum on UK Fungi Day to learn more from our displays and experts in our education room (more info on the event page) or you can join in:

Fungal Foray

Join expert mushroom forager Malcolm Greaves for a seasonal foray in search of the wide range of mushrooms commonly found around the collection in autumn.

20 people maximum, £20 per person

The Villain’s Symptom Trail

Join Plant Pathologist and tree health specialist Dr Charles lane at The Yorkshire Arboretum on UK Fungi day for a tour of the fungal plant pathogens on display in our collection, learn about how they got here and how you can help us to stop the spread.

20 people maximum, no charge.

Booking essential for these two activities please: email visit@yorkshirearbortum.org or phone 01653 648598

Citizen Science Fungi Trail

Take part in the BMS Find a Fungus iNaturalist survey at The Yorkshire Arbroetum. There will be a trail for you to follow with information about the fungal species we’re interested in an tips on where to find them, so you can log your sightings on the iNaturalist app in the ‘UK Fungus Day – Find-A-Fungus’ project .

Tree Health Centre opens at the Yorkshire Arboretum

We need healthy trees for a healthy future, but trees across the country and around the world are in trouble. Climate change and increasing numbers of pests and diseases are threatening the continued existence of many familiar species in our landscape, just at a time when we need to plant more trees than ever before to capture carbon.

Recognising this the Yorkshire Arboretum has established the country’s first dedicated Tree Health Centre. Launching in September 2021, with a formal opening event on 23 September, it aims to raise awareness of the threats to trees, the need for vigilance to avoid importing other pathogens, and to suggest ways of increasing the resilience of our treescape for the centuries ahead. We look to achieve this by offering classes, courses and public outreach events.

These events, led by specialists, will take place at the Yorkshire Arboretum, using the new, fully-equipped classroom and beautiful, botanically-rich surroundings for unique indoor/outdoor learning experiences.

From Friday 24 September to Sunday 3 October, the arboretum will host a range of activities to commemorate the launch of the Tree Health Centre. Families can get involved in a range of trails round the arboretum, interactive demonstrations, and a kid’s bug hunt and activity pack. There will also be tree health tours of the arboretum, and a talk hosted by guest speaker Prof Nicola Spence –Chief Plant Health Officer at Defra and an arboretum trustee.

Arboretum Director Dr John Grimshaw said: “The launch of the Tree Health Centre comes at a time when everyone is painfully aware of the consequences of climate change, and the need to plant trees to help combat it. We want to help people keep our trees healthy and thriving, and these events offer something for everyone to learn about how to do so.”

Funding included £286,000 from the York & North Yorkshire Local Enterprise Partnership’s Local Growth Fund allocation.

David Dickson, Chair of the York & North Yorkshire Local Enterprise Partnership Infrastructure and Joint Assets Board, said: “This specialist knowledge centre will build the capacity to protect tree health and lead the way for others. The opportunities for learning will make a real impact, and contribute towards the region’s ambitions to be greener, fairer and stronger.”

Tree Health Centre Blog: Cladding arrives from Shelmore Timber

Cladding for the Tree Health Centre has arrived, very kindly donated by Shelmore Timber, Norbury Park Estate. Shelmore Timber cladding is sourced from their own mature woodlands on the 1,500-acre Norbury Park and Ranton Estates.

Read about Shelmore Timber’s provenance and sustainability here.

#yorarb #yorTHC #treehealth #healthytreeshealthyfuture

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Hobson and Porter Begin Work on UK's First Dedicated Tree Health Centre

Yorkshire & Lincolnshire based construction firm Hobson & Porter Ltd have won the contract to build the UK’s first Tree Health Centre. The Yorkshire Arboretum, located near Castle Howard in North Yorkshire, has raised funds to build and operate the UK’s first purpose-built facility to expand public awareness about the threats to our trees posed by an increasing number of pests and diseases, and the need to keep our trees as healthy as possible. Capital funding for the project has come from the Peter Sowerby Foundation, £286,000 of the Government’s Local Growth Fund, secured by the York, North Yorkshire and East Riding Enterprise Partnership (YNYER LEP) and from private donations. Start up and operating funding has come from The Prince of Wales’s Charitable Fund.

Designed by SALT Architects of Beverley, East Yorkshire, the Tree Health Centre will offer training courses given by specialists and cater to all levels of expertise. Following successful pilot courses held in 2018, the centre will offer training on a commercial basis mixing classroom, lab and outdoor learning settings, as well as a public outreach programme.

Yorkshire Arboretum Chief Operating Officer Mathew Harrison says: “We are delighted to announce Hobson & Porter Ltd as building contractors for the Tree Health Centre. The company has strong links across Yorkshire with a proven track record of delivering third sector projects – they are keen to promote environmental preservation, and we believe their approach matches our core strategy and values.”

Sam Alexander, chair of YNYER LEP’s Skills and Employability Board, said: “The natural environment plays a vital role in our social and economic health as a region. It is crucial we have the skills available to protect Yorkshire landscapes and contribute to tree health in the UK and beyond. With the help of funding secured by the LEP, this specialist knowledge centre will build the capacity in the region to protect tree health and lead the way for others. We’re pleased to see the project reach this latest milestone.”

Minister for the Northern Powerhouse and Local Growth, Jake Berry, said: “We’re deeply committed to boosting economic growth and levelling up our communities across the North and the Midlands.

“It’s great to hear that thanks to over £285,000 from the Local Growth Fund, the UK’s first Tree Health Centre is being built right here in the Northern Powerhouse and will safeguard the region’s natural environment for future generations to come.”

Arboretum Director John Grimshaw said ‘We’ve been planning this building for ages, and it’s so exciting to see works starting at last. We look forward to delivering sound advice on the management of healthy trees for a healthy environment – a critical issue for us all.’

Click here to download the original press release.

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The Yorkshire Arboretum - Tree Health Centre Tender

The Yorkshire Arboretum is open to tenders for the construction of a new Tree Health Centre

Project summary

The project is the construction of a new tree health centre including external works and landscaping. The new building is a partially suspended timber frame with larch cladding, a flat roof and curtain wall glazing. The new centre adjoins the existing visitors centre and also creates a new feature walkway leading on to a belvedere viewing platform. For access to the planning conditions and a full ancillary of tender documentation for review and contract construction information please email business@yorkshirearboretum.org

1. The Project and Site

1.1 The Employer

The Castle Howard Arboretum Trust
The Estate Office
Castle Howard
YORK
YO60 7DA

1.2 The Design Team and Project Team

The Employer has appointed a team of professional consultants for the Project. The Design Team comprises:

The Architect/ Contract Administrator/ Principle Designer

Salt Architects
54 Lairgate
Beverley
HU17 8EU

The Quantity Surveyor

Andrew Taylor Quantity Surveying Ltd
Mill Lodge
Mill Street
Hutton
Driffield
YO25 9PU

The Structural/ Civil Engineer

Scott Hughes
Unit 3
Cross York Street Studios
Leeds
LS2 7BL

The M&E Consultant

LEDA
Micklewaite House
70 Cross Green Lane
Leeds
LS9 0DG 

Project Title, Site and Location

Project Title

Tree Health Centre

Site

The site is located within the existing visitors centre and the New Tree Health centre will extend the existing building to form one large centre. Works include formation of a new walkway with tree planting and a belvedere viewing platform.

Location

The Yorkshire Arboretum
Castle Howard
York
YO60 7BY