That quintessentially English poet, Rudyard Kipling, wrote, in A Tree Song, “Of all the trees that grow so fair, / Old England to adorn, / Greater are none beneath the Sun, / Than Oak and Ash and Thorn.”
Well, English oaks have suffered … and are suffering … from attack by the mysterious acute oak decline and by the oak processionary moth, which rejoices in the scientific name Thaumetopoea processionea, The breeding female moth lays a large number of eggs on the bark of oak trees in the autumn, and very large numbers of caterpillars hatch in the spring, feeding on the young leaves of the tree as they process through the foliage in the long columns that give them their name. This defoliation affects the tree’s general health and vigour but, though there is evidence that repeated attacks can harm the trees, the tree will not suffer lasting damage if the outbreak is confined to a single year (author’s italics).
But now comes news that the second of Kipling’s three species of tree is threatened by a much more serious enemy … Chalara fraxinea. This is known … I had almost written ‘commonly known’ … as ‘ash tree fungus’; and there is a serious prospect that it will become commonly known throughout the United Kingdom within the next few years, for the fungus has been found in ash trees in Norfolk, Suffolk and Kent, having arrived in England from continental Europe; and two cases have been reported in Scotland. It’s a very nasty piece of work which kills between 80% and 90% of all ash trees that become infected; and, once it starts spreading, it infects more than 90% of all the ash trees it encounters. In other words, once ash tree fungus is on the march, it will kill three-quarters of all the ash trees in its path.
In the later part of summer, the fungus begins its deadly work when what look like miniature mushrooms appear on the dead and dying leafstalks that have been discarded by trees as they shed their leaves in preparation for the coming winter. Spores from these fungal growths are picked up by the wind the following spring; and any that land on the leaves of ash trees penetrate these, allowing the fungus to start spreading through the tree. Relatively inconspicuous black spots appear on the infected leaves: but by the time these have become noticeable, the fungus has spread through the leaf stalks into the tree’s branchlets and thence into the branches, where what look like cankers betray its presence. These cankers interrupt the flow of nutrients through the branches, causing growth above them to die back, although lower branches may still appear to be healthy: but as the canker encircles the trunk, the tree gives up the ghost, having been pretty well doomed from the day of its infection.
At present, there is no effective treatment for infected trees, even if the fungus is detected at a very early stage. The only thing that can be done is to burn infected trees … and to stand any chance of containing an outbreak of ash tree fungus, every tree within a 20-mile radius of the outbreak of any infection should be reduced to ashes. Even that drastic action carries no guarantee of success for, although our European neighbours’ experience is that ash tree fungus advances about 20 miles in a year, some wind-borne fungal spores could well be blown further; and there is the ever-present chance of human agencies inadvertently spreading spores to previously unaffected areas of the country.
More than 100 000 ash trees have already been incinerated in the south-east of England, in East Anglia and in Scotland; and the import or movement of ash trees and ash tree seeds is now banned throughout the U.K. in a bid to halt the spread of this deadly fungus. Cross your fingers and hope that this disease does not spread further throughout Scotland!
Sandy Simpson, Polmont Horticultural Society