The results of his survey indicate that Massaria is well established in the City with 40 per cent of the susceptible planes infected with Massaria and bearing approximately 60 decayed branches at, or well toward, the point of failure.
The City of London Corporation has removed the infected branches.
The disease is most frequent in the historic burial ground of Bunhill Fields and the north-west quarter where 50 per cent of the susceptible planes are infected.
Most trees only exhibit one infected branch but, less frequently, up to five branches were found in any one tree. Analysis of these branches shows most are in the 25mm to 100mm band but that several are greater than 150mm in diameter; one infected branch was recorded at circa 300mm diameter.
Keen who is also Arboricultural Association vice chairman said: "The decision now is how to manage the problem in the long term. We know it is present, we know it has the propensity to cause branch failure that may lead to damage or injury and we know it is expensive to monitor; the only practical way is to climb, inspect and remove the branches. And make no mistake, when I say the trees need to be inspected aerially I can assure you the infection is not visible from ground level as I inspected all these trees from the ground and saw no signs that those 60 branches were infected.
"The City of London Corporation is now giving thought to how it manages its plane trees in the long term. A reactive approach of monitoring and pruning when the problem is spotted will be needed but it is hoped to move toward a pre-emptive approach, within an adaptive management wrapper, so we learn and adjust as the problem is managed.
The London Tree Officers’ Association (LTOA), in conjunction with the Arboricultural Association, has established a working group for Massaria.
The City of London Corporation’s Director of Open Spaces has presented the findings of the study to the LTOA and it is hoped that the research will complement that of others to inform policy and procedures that will emerge before too long, said Keen.
"The threat is real and it is necessary to adopt the City of London Corporation’s example of early action" he added.
Keen said the Arborists’ Working Group is producing an information note for climbers so that they are aware of the dangers presented by Massaria to climbing operations.