Ash die back
Ash die back is a fungal disease caused by Hymenoscyphus fraxineus not to be confused with infestation by beetle larvae of the emerald ash borer, Agrilus planipennis (see later). The disease was first identified in Britain in a Buckinghamshire nursery in February 2012. It was first reported in Poland in 1992 where ash trees died in large numbers.
Biology of the fungus
H.fraxineus spores enter through small fissures in the bark of ash trees and multiply rapidly in their asexual phase to produce a girdling effect on twigs and small branches. Filamentous threads of the fungus spread throughout the transport tissue of the tree casing dieback of young leaves, which are unable to receive or transport nutrients. Asexual spores are produced on microscopic conidia giving a powdery appearance to infected areas.
A sexual phase occurs in the infected stalks of fallen leaves. ‘Male’ and ‘female’ fungal nuclei fuse to form a zygote that develops into a fruiting body or ascocarp. The fruiting bodies are easily spotted as small mushroom-like growths on the stalks of rotting leaves and twigs. Like mushrooms, they produce millions of wind-borne spores spreading up to a ten mile radius.
Note: The sexual phase was only identified recently showing H. fraxineus to to be a genetic variant of a very similar species H. albidus. Previously it had been named as Chalara fraxinea and the disease was referred to as Chalara dieback.
New leaves at the tips of branches discolour and wounds open on the stems inviting secondary fungal and bacterial infections. It also makes them vulnerable to attack by boring beetle larvae such as the Asian emerald ash borer that has been so destructive in North America. Young trees are very susceptible and tend to die within a couple of years; mature trees are less vulnerable.
The ecological damage resulting from the loss of the third most common broadleaf UK tree species is likely to be serious. In Denmark where up to 90% of the ash trees are infected, it is estimated, for example, that around 30 invertebrate species are entirely dependent on ash for their food supply. There are clearly significant knock on effects for all members of the food web.
Economic effects are inevitable because ash is an important source of high quality timber.
Asian emerald beetle (emerald ash borer)
Native to northern regions of Russia, China, Japan and Korea, the emerald ash borer has recently invaded large areas of North America affecting ash trees from Colorado to the East Coast. The bright metallic green beetles locate ash trees by means of volatile leaf chemicals and lay their eggs in small fissures in the bark. Stressed and wounded tissues are particularly vulnerable. The young larvae that hatch from the eggs set about eating the transport tissue of the tree with devastating results as the bark peels off leaving thousands of characteristic bore trails. After a year or more in which they undergo four separate larval changes, adult beetles emerge through small exit holes in the bark. These are often the only visible signs of infection.
Effective management of both diseases requires early identification of symptoms. Infected ash trees together with those in the immediate area can be felled to create a localised quarantine. Ash borer larvae probably reached the American continent in packing cases, highlighting the need for particular vigilance in ports of entry.
The hunt is on for natural predators of the emerald ash beetle. A number of parasitic wasp species native to Asia have been trialled in North America but the introduction of biological control agents is notoriously problematic due to the complexity of food webs, not least because the wasps do not confine their dining habits exclusively to the emerald ash beetle.
A much more productive solution in the longer term will come from the identification of genetic factors which promote resistance or increase tolerance to infection.
So far, the ash dieback has been restricted mainly to young plantations and we are yet to face up to a major invasion by the emerald ash borer. The more cataclysmic news reports are premature but no less welcome by the fact that they draw necessary public attention to the conservation of our native species.