Science

Sudden Oak Death

Sudden Oak Death, the common and rather misleading name for the tree pathogen ‘Phytopthera ramorum’, is the latest arboricultural crisis hitting America and Europe.

P. ramorum was discovered in the costal regions of California, where it has caused widespread and significant death of native American oak species. It is now thought to have spread to Oregon and into parts of Canada.

The pathogen is a totally new discovery and is thought to be airborne with very few, if any, natural predators. It is therefore responsible for random attacks, rather than a clear pattern of infection.

The disease itself is fungal and attacks the main stem or trunk of mature trees and branches of smaller specimens. Symptoms include the production of dark reddish-black sap from cracks low down on the trunk (often referred to as ‘Bleeding Canker’). The canker can spread completely around the base of a tree, destroying the cambium layer (the one cell thick membrane beneath a trees bark, which is vital to its growth) and eventually girdling the tree. This will cause rapid leaf loss and ultimate demise of the specimen, hence the adopted term ‘Sudden Oak Death’ (due to the seemingly sudden way in which it dies).

Phytopthera ramorum has yet to be found in any native British oaks (Quercus robur, Quercus petraea & Quercus ilex). It has however been found in this country in non-native species of oak and imported container grown stock of Viburnum and Rhododendron. It is this nursery stock that is thought to be the reason for the diseases spread into Britain so far, hence the current ban on imports from infected areas of the United States.

Encouragingly, in tests so far, Quercus robur, the English Oak and Quercus petraea, the Sessile Oak (both British natives) have proved to have a higher level of resistance to the disease. Unfortunately though, Fagus (Beech) and Castanea (Sweet Chestnut) do seem to have a weak level of resistance. It is these species that could well be at more risk in Britain.

A rather more immediate concern in Britain, is the decline in the condition and health of mature oaks in general. This is not thought to be due to sudden oak death, but is probably a more complicated gradual decline caused by other pathogens and one major factor - the change in climate conditions. All plants have a growing season that is part of their genetic make up. Some flower in Summer, some in Winter, some keep their leaves all year and some are deciduous. Anything that occurs to interrupt this is going to have a slow and steady effect on the condition and health of a plant. Trees are more susceptible to this due to their extended longevity. A warmer, dryer climate and longer growing seasons are bound to effect adversely a tree that needs a steady Summer/Winter pattern to allow recovery and regeneration in Spring.

While there is still a lot of research needed into the potential effect of Sudden Oak Death and the general decline of oaks and other species in this country, there is one fact that is certain, we need to plant more trees. All we can be certain of is that the environment is adversely affecting our existing tree stock, which needs to be counteracted by replacing any that are in decline. Only by doing this can we assure our country’s woodland heritage.