Brighton and Hove City Council said emergency work to try and stop the disease spreading and to save the historic tree started immediately after it was discovered at the weekend, and careful monitoring is now being carried out.
A spokesman said: "The infection has caught hold on a specific large limb within the canopy of the famous tree, which our arboriculture experts have now ‘girdled’. Girdling means removing a section of bark around the complete circumference of the affected limb to hopefully stop the spread of the disease to other parts of the tree.
"Unfortunately, we cannot guarantee girdling will save the twin as the risk of this infection spreading will continue until the end of this growing season and possibly the next two.
"We will of course be closely checking and examining the twin for any changes that may occur."
The council said the situation was complicated by the very warm weather we are having as this provides the perfect climate for the disease carrying elm bark beetles to thrive. The disease season should finish in the next 2-3 weeks depending on the weather.
Small, infected branches have already been removed and the girdled limb will also be removed once elm disease season has finished this year.
‘Extremely sad news’
Councillor Amy Heley, chair of the council’s environment, transport and sustainability committee, said: “This is extremely sad news as the Preston Park twins are not only historic, but are a very important part of the city’s cultural heritage. They have been much loved for centuries and are as treasured now by residents as they were hundreds of years ago.”
Cllr Heley, who is also a Preston Park ward councillor, added: “Our arboriculturists are all experts in their field and I know they will do everything in their power to save the remaining twin, but sadly we cannot guarantee its survival.”
The previous twin was felled in December 2019 after it was found to be infected with elm disease. Despite efforts by the council's arboriculturists to manage the spread, the disease had reached the roots of the twin making felling inevitable.
Work was also carried out to try and prevent the disease spreading to the roots of the remaining twin. Although that work may have succeeded to prevent root spread as there are no signs of disease in the roots, the council said it now knows the limb is diseased and suspect an aerial infection by beetles.
The Preston Park elms are believed to have been planted 400 years ago – four years before the Pilgrim Fathers landed in America, when James 1 was on the throne and when Shakespeare’s plays were first being performed.
One of the most likely ways for a tree to become infected is via beetles breeding on elm logs stored in the area. These logs may have been brought in from other parts of Sussex where there has been a massive rise in trees becoming infected and subsequent logs becoming available.
The council is asking people not to buy any logs for winter fuel if the supplier cannot guarantee that the wood isn't elm and not bring any elm timber into the city for use as garden ornaments, seating, etc.
The council's arboriculture team offers a free inspection of firewood and other timber. If the wood is elm, the council will dispose of it and give you a similar quantity free of charge.
The council also asked people to report any tree that has leaves turning from green to yellow or brown or having a scorched look during the spring, and any dead trees.
Similarly, if you have an elm tree in your garden that may be dead or dying, the council is happy to come out and inspect it free of charge.
Anyone wanting to use the service can contact the team by calling 01273 292929 or emailing [email protected]