Killer on the Loose

Today's Guardian Country Diary is all about the fungus-like wilt disease called Phytophthora austrocedri that is killing the trees in Teesdale's famous evergreen juniper forest near High Force.

The disease was first detected here four years ago and now seems to have a fatal grip on the junipers on either side of the very popular Pennine Way footpath that leads through Moor House National Nature Reserve to High Force waterfall.

There doesn't seem to be any real prospect of controlling the disease within the forest. Visitors are asked to keep to the footpath and keep their dogs on a lead so that their pets don't carry the disease deeper into the forest but while we were there not a single dog owner complied. When you leave the forest you now need to wash your boots in the disinfectant footbath then spray the soles, so that the disease isn't carried further afield, but few visitors seem to take much notice of that.

This juniper forest is a magical place, quite unlike any other forest that I've visited. Most of the juniper trees have been prostrated by the wind and the snow that accumulates on them in winter, so with their twisted trunks each has a unique character. 

A  few achieve a conventional tree shape but never reach much more than four metres in height, so the visitor feels like a giant, especially when other points of reference are hidden.

The more prostrate trunks, especially in the wetter spots, have a fine moss and lichen flora.

The more upright trees have twisted trunks that might grace an Arthur Rackham fairy tale illustration.

And then there is the scent - a wonderful head-clearing aroma of gin, from the berries that ripen to an attractive shade of blue.

This is healthy juniper foliage ......

......... and these are twigs from trees infected with the disease. By this stage the tree is doomed, because the infective organism has already destroyed the sapwood of the tree down at the base of the trunk.

It's a sad scene beside the footpath, of trees that have shed most of their needles...

...... or are turning yellow then brown.

Open patches are beginning to appear in the forest. How far and how fast the disease will spread is anyone's guess, but it now seems to have a very firm grip. This tiny infective organism could alter the whole landscape here.

The disease organism originated on conifers in Argentina and seems to have been imported by the horticulture industry, where phytosanitary precautions are totally inadequate. Tree diseases like this have the potential to bring about major landscape changes. 

For a scathing criticism of the failings of the international trade in trees, see the late Oliver Rackham's The Ash Tree.

No one seems sure how the disease arrived here in Teesdale, but juniper regeneration has been very poor here in the past and planting of infected trees sourced from outside the forest might be the cause.

Juniper is already in decline nationally, so the arrival of this potentially devastating disease is particularly unwelcome.