Wildlife conservation charity Herts and Middlesex Wildlife Trust is having to invest thousands of pounds to combat the effects of ash dieback on nature reserves across Hertfordshire.

The disease arrived in the UK in 2012 and has since spread across the country, weakening ash trees to the point that they will drop limbs, collapse or fall.

Ash dieback is caused by a fungus which blocks water transport in the tree, leading to scratches in the bark, leaf loss and eventually killing the tree from the outermost branches inward.

Herts and Middlesex Wildlife Trust is assessing the health of ash trees on its nature reserves in the county. About half of the nature reserves that the Trust cares for are currently affected by the disease and will require ongoing monitoring and in some cases felling of ash trees.

Some nature reserves are already subject to path closures, such as Gobions Wood, Brookmans Park, where significant ash dieback has been found, while Hawkins Wood Nature Reserve, near Therfield, has had to be closed completely.

Ash dieback has also been identified at Fir and Ponds Wood in Coopers Lane Road, Potters Bar, and Balls Wood in Hertford Heath.

Affected trees near to footpaths or along roads pose a risk to public safety due to falling branches or collapsing trees.

Tree safety work related to ash dieback took place at Lemsford Springs in February.

Ian Carle, Herts and Middlesex Wildlife Trust’s nature reserve manager, said: “Ash dieback poses a significant problem for the Trust both from a visitor safety point of view and in terms of cost. We have already spent £50,000 on this work.

"Over the coming months we will be surveying our sites to fully understand the programme of work we have to carry out, but it could cost hundreds of thousands of pounds to address the ash dieback on our reserves over the next few years.”

To help keep visitors and property safe, the Trust is undertaking a targeted programme of works on its nature reserves. It is also stressing the importance for all visitors to keep to official paths and pay close attention to safety signs.

Work is being carefully timed to minimise the impact on wildlife such as nesting birds and is expected to be carried out over a number of years.

The main symptoms of the fungal disease are dead branches, blackening of leaves and discoloured stems.

Symptoms of ash dieback are easier to detect in summer when trees are in full leaf.

Once a tree is infected the disease is usually fatal, either directly, or indirectly by weakening the tree to the point where it succumbs more readily to attacks by other pests or pathogens, such as honey fungus.

While there is currently no cure for ash dieback, evidence suggests a small number of trees are resistant to the disease.

Where resistant trees are found, these will be left to support the recovery of the population of ash trees across Hertfordshire.

Ian Carle added: “Our priorities for our nature reserves are to manage them for wildlife and keep our visitors safe so that they can enjoy getting close to nature.

“Sadly, this means we will need to close some paths on our sites and in some instances close entire reserves.

"We will also monitor the ash trees in our care closely and investigate where resistant trees can support the recovery of the ash tree population.”

Visitors to affected reserves can help reduce the spread of ash dieback by cleaning footwear, bicycle tyres and buggy wheels.

The disease is primarily spread in the wind and by infected soils and other natural products of woodland being spread around such as mud, twigs and leaves. So by cleaning off debris, people can reduce the spread of the disease.

More information about ash dieback including which reserves are being affected by the disease can be found at www.hertswildlifetrust.org.uk/ashdieback.