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Tree pests and diseases

Ash Dieback

What is it? Toggle accordion

Ash Dieback (Chalara fraxinea) is a fungal pathogen specific to Ash trees (Fraxinus excelsior) and has infected and killed a large proportion of ash trees in Europe. 

It was first discovered in the UK in nursery stock in 2009 and has recently been discovered in ash trees growing in woods and plantations especially in Eastern England.  It is thought that the fungus spores have been carried on the wind from Europe to infect trees here.

It is potentially a very serious threat to our Ash trees and it is unlikely that the disease can now be eradicated from Britain.

Ash Dieback is now firmly established in St Albans City and District and also in other local authority areas in Hertfordshire. Council officers are sharing information between authorities so that a well informed and consistent approach can be made in terms of managing the disease on publicly accessible land and also in the provision of advice to the public.

Predicted decline rates in infected trees Toggle accordion

The fungus is extremely aggressive and symptoms become obvious in younger trees within months rather than years. 

Trees cannot recover from infection, but larger trees can survive infection for a considerable time and some might have genetic resistance and not die. (Current experience from Denmark). 

In line with national guidance the District Council will retain ash trees where possible. The likelihood of infection with Chalara, is not a justification to undertake works to fell or prune ash trees. This is for three reasons:  

  • to aid identification of trees which may show genetic resistance or other ability to recover;
  • to  retain as much of the biodiversity that is dependent on ash for as long as possible in as wide a population as possible, to allow other species to bridge the biodiversity gap;
  • to allow more time for other replacement species of trees to grow, to give a more gradual transition of dominant landscape species.

Owners should seek advice from a qualified arborist if it is considered that there may be an immediate risk to public safety. The Trees and Woodlands Section has a list of approved arborists.

More detailed information on Chalara can be found on the Forestry Commission website and the following link to the Countryside Management Service.

Oak Processionary Moth

What is it? Toggle accordion

Oak Processionary Moth (OPM) is a major defoliator of oak in Europe.  The larvae (caterpillars) feed on the foliage of many species of oaks, including English, Sessile and Turkey oaks (Quercus robur, Q.petraea and Q.cerris).

What harm can it do? Toggle accordion

OPM is primarily a human health issue. The larvae (caterpillars) are covered in irritating hairs that contain a toxin and contact with these hairs, or their inhalation, can result in skin irritation and allergic reactions.  These problems are significant because oak processionary is often most abundant on urban trees, along forest edges and in amenity woodlands. See the Forestry Commission’s website for more information on signs and symptoms. 

The Forestry Commission has issued a press release to urge the public to report sightings of Oak processionary moth caterpillars.  We are now entering the greatest risk period as the caterpillars emerge between June and August to feed before turning into adult moths.  The full press release is available here.

New Defra OPM Policy - there are new changes coming into force on 24 May 2023 that impact moving large oak trees within the Oak processionary moth management zones in the South East of England.  This could affect how your source oak trees.  

How serious is the threat? Toggle accordion

We have found OPM on the Alban Way near the eastern district boundary in St Albans, and so people need to be vigilant. OPM has also been recorded recently near our boundary in Welham Green and also in the neighbouring boroughs of Watford, Hertsmere and Broxbourne over the last couple of years.  It has more recently been found in Verulamium Park, Harpenden Common and the Nickey Line.

What can the Council do to combat this? Toggle accordion

We have asked tree contractors (who are most likely to come into close contact with OPM), and our network of voluntary tree wardens to be vigilant and would ask for any suspected cases to be reported as soon as possible to the Forestry Commission (who are leading on the issue) and the District Council. A number a tree contractors have, and are developing means of treating outbreaks of OPM.

Who to contact Toggle accordion

Firstly, it is important that you do not touch the larvae or disturb the nests.  If you think you have seen OPM, its larvae or its nests you can report a sighting using TreeAlert


You can also call: 0300 067 4442 or email:

Please also contact St Albans City & District Council, Parks and Green Space

Contact us or report a problem