Sandy’s Garden ... Hytophthora Ramorum – An Unwelcome Immigrant

Sandy Simpson
Sandy Simpson

Phytophthora ramorum … now there’s a name to conjure with!

And no, it’s not the name of a person; it is the name of a fungus-like pathogen which causes extensive damage and even mortality to a wide range of trees and other plants. But yes, it is an immigrant, and a very successful one at that for, after the first report of its arrival in Britain … on a viburnum bush in Sussex in 2002 … it has spread very successfully throughout the United Kingdom. No-one is sure where Phytophthora ramorum originally came from: but most authorities agree that Asia is its most likely ancestral home. We do know that the disease had reached Europe by 1993 when the at-that-time unidentified pathogen was found in Germany and in The Netherlands, initially in young rhododendron bushes and larch trees, this latter species being particularly susceptible to the disease, a circumstance which led to Phytophthora ramorum acquiring the common name of ‘larch tree disease’ or ‘Japanese larch disease’ in this country.

It became clear however, that larch tree disease was the same infection known in the United States as ‘sudden oak death’, although the strains of the disease found in Britain have, so far, had only a limited effect on mature oak trees growing in these islands since the discovery of an infected 100-year-old tree in November 2003. What did become clear was that the spores of the fungus were persistent, lingering where infected larch trees had been felled in ground in which species such as Douglas fir, Noble fir and western hemlock were subsequently planted, infecting these trees in turn. However, UK authorities were still confident, a decade ago, that sudden oak death … as it was becoming known, using the common American name … could be contained by meticulously clearing all trees affected by an outbreak of the disease and sterilising infected ground. Sadly, this proved to be an over-optimistic assessment of the problem as it became increasingly clear that air currents, watercourses, vehicle tyres and even animals’ paws spread the disease, not to mention the unintentional movement or import of infected plants, for sudden oak death has been found in nursery stock in some European countries from which British arboriculturists buy plants.

So what does the disease do to trees? The bark of infected trees ‘bleeds’ a black gunge from lesions … one might call them wounds, ulcers, abscesses, or tumours … a gunge which dries to form a crust. The inner bark beneath such lesions is dying; and, as the infection spreads through the bark, the tree dies. Despite its common name, sudden oak death does infect other plants, ornamental shrubs such as rhododendron, viburnum, pieris and camellia being vulnerable. Phytophthora ramorum causes wilting shoots, blackened leaves and partial die-back in these shrubs, but is seldom fatal. However, the infected spores will, of course, spread to susceptible trees in the neighbourhood; so any outbreak of the disease is almost as much bad news for gardeners as it is for the owners of woods and forests.

Sadly, the National Trust for Scotland (NTS) recently reported an outbreak of sudden oak death in its wonderful Inverewe Gardens, near Poolewe in Wester Ross. The infected trees are, unsurprisingly, larches: but the risk of the infection spreading to other plants has led to the issuing of two Statutory Plan Health Notices for the felling of the affected trees, giving the NTS gardeners until the end of February to fell and dispose of all the trees within a 250-metre radius of the infected larches and to dispose of all nearby rhododendron bushes. Not, I should stress, that the Trust is in any way disinclined to deal with the outbreak; the statutory notice is a procedural move which underlines the seriousness with which the authorities view the occurrence. Yes, Phytophthora ramorum is definitely an unwelcome immigrant!