Trees along the roads and pathways of Hertfordshire could become unsafe, as a fungus known as Ash Dieback takes hold across the county.

Ash Dieback – or Chalara – is a fungal disease affecting ash trees, which are common in woodland, hedgerows, roadsides and gardens across the county.

It causes the loss of leaves, the destruction of branches and the eventual death of the tree. If not spotted early enough – the risk is that large branches could break off onto pathways and roads.

On Friday Tree Health Officer Gemma Worswick highlighted the risks posed by the fungus to members of Hertfordshire County Council’s Environment, Planning and Transport Cabinet Panel.

“Mature trees can die within 10 years of infection,” said Gemma Worswick. “And if a tree is already under stress it can leave it more vulnerable.

“Trees become dangerous before they die because they are carrying dead wood.”

Ash Dieback was first recorded in the UK in 2012 and its spread has already been confirmed in around half of the county.

An area of mature trees in Weston Hills, near Baldock, was highlighted at the meeting as an area where the fungus was already particularly advanced.

However roadside ash trees can be more susceptible to the fungal disease because of the high salt content in soils, due to winter gritting.

It is estimated that ash trees  can become hazardous within two years of infection. And, typically, even mature trees die within 10 years.

The County Council has a legal duty to take reasonable actions to manage tree risk on its land and Gemma Worswick said monitoring was important.

“Its important to monitor the rate of decline so we can effectively monitor the risk,” she said.

There is currently no national plan to contain or eradicate Ash Dieback. And no new Ash trees can be planted in the UK.

At the meeting Ms Worswick also highlighted the continuing risk posed by the Oak Processionary Moth.

Four nests of the moths – whose toxic hairs can harm humans and animals –  were found near Watford, in 2016.

Officials from the Forestry Commission  – following two years of control and monitoring – believe that site has now been eradicated.

But male Oak Processionary Moths – who can fly several miles from their nest – have been found in Oxhey Woods, Bushey, Rickmansworth, Watford Rural, Hoddesdon, Northaw, Berkhamsted and St Albans.

And there is expected to be a further outbreak in Hertfordshire at some point in the next few years.

According to Ms Worswick, one contact with the moths can cause a reaction similar to a nettle string. But repeated exposure can increase sensitivity and reactions can become significantly more severe.

In one case a professional gardener, from Southwark, suffered a reaction similar to anaphylaxis. In another a pet dog needed significant surgery after picking up a nest in its mouth.

Ms Worswick said: “The risk is presented by the microscopic hairs, which are barbed, contain a toxin and are easily dislodged. They can get trapped in clothing and in fur and and can cause a reaction.

“Initially it can be something like a nettle sting, but repeated exposure can develop sensitivity. Landscapers, gardeners and residents in areas with high infection rates are most at risk.”

The Oak Processionary Moth and Ash Dieback were highlighted as the main risks facing trees across Hertfordshire, given the common occurrence of ash and oak trees across the county.