Dutch Elm Disease
There are around 38,000 trees in Edinburgh's parks and streets
Whether you think that sounds a lot, thirty years ago there were 45,000 elm trees alone in Edinburgh.
Edinburgh and Brighton are 'the' place for elms - planted by the Victorians.
Edinburgh has been losing a thousand elm trees a year for 30 years, and now there are just 12,000 left.
They lost 1,200 in 2012 - probably because of a mild winter followed by a spring with temperatures at 20°C and no wind - perfect conditions for the elm bark beetle to fly off and find another elm.
The elm bark beetle is the innocent carrier of the Dutch elm disease - a fungus that spreads along the vascular system of the tree.
The tree reacts by shutting off its vascular system ahead of the fungus and so effectively kills itself.
The fungus would kill the tree anyway eventually but because of the way the tree reacts to protect itself from the fungus, a tree can sometimes go from the first signs of infection to a dead tree in a month.
The original infestation came from the Far East in the early nineteen hundreds and reached Scotland in the 1970s.
There is no treatment - a research team at the Botanic Gardens in Edinburgh tried fungicides for the fungus and pheremone traps for the beetles, but they don't work.
There are two kinds of elm in Edinburgh.
There's English elm, which breeds by suckers, so that all English elm trees are clones.
And there's Wych elm, which breed by seed dispersal.
It was thought that Engish elms being clones would be more susceptible to Dutch elm disease, but time has shown that English elm and Wych elm are both susceptible.
Edinburgh Council have tried other elm species such as Ulnus Regal - but it too has succumbed.
In the USA they are now breeding fungus-resistent elms. However, there is very little money in the UK for research.
So the Council is fighting a losing battle - a rearguard action to slow down the advance of the Dutch elm disease.