Henry Kuppen was at Barcham Trees in November to deliver a seminar on ‘Reasonable management of pests and diseases, and upcoming threats to urban trees’.
He is the director of the Dutch company Terra Nostra, which he terms a knowledge centre for trees and soil in the urban environment. He began his career as an arborist in the 1980s, and has a particular interest in the oak processionary moth.
The day began with a consideration of the relationship between trees and their pests and diseases. Henry told delegates more pest threats are coming to Europe because of trade, transport and the continent becoming warmer. While years ago most tree pests and diseases came from America, today they tend to come from Asia due to changing trading patterns. He said that as pathogens are a component of the natural world we must accept this fact; their role includes taking out weak individuals and preying on parasites. In forests of mixed composition pests and diseases are seldom a problem, but humans are keen to eradicate them in the ‘urban forest’.
Integrated pest management
For a pest to thrive it must have a suitable climate and because it also needs food this often opens the door to disease. Henry is firmly of the opinion that monoculture encourages pathogens. Trees with genetic uniformity, especially cultivars, offer little resistance to pests, but because seedlings have variation they tend to be more tolerant of pests and diseases.
Henry is an advocate of integrated pest management in four stages: we need to differentiate, register, analyse and then act or accept. Differentiation means dividing the spectrum of risk into categories. The first is for quarantine organisms, such as Xylella and Asian longhorn beetle, followed by those which are a risk to human health, such as that posed by oak processionary moth (OPM).
The third category is for those which cause a tree to die, such as ash dieback, while the fourth is for those which constitute a safety risk, such as ash dieback and Massaria. Some diseases, such as horse chestnut bleeding canker, adversely affect a tree’s condition, and some pests, such as aphids, are in the ‘nuisance’ category, while horse chestnut leaf miner is in the final category of ‘none’.
When registering we need to ask where the problem is and what is its spread. Analysis should involve asking these questions:
What are the effects and potential of the pathogen?
What are the risks?
What needs to be done to comply with legal obligations?
How should we act? Henry remarked,
‘We hardly ever consider accepting a pest or disease. Instead we feel we must act!’
A tree’s risk profile is high if falling branches in public spaces could land on places such as paths, roads, play areas, parking spaces and seats. Its profile is moderate if branches are likely to fall within 3m of these, and low if further than 3m. The Dutch legally have a duty of care in public health, safety and nature conservation, while since 2013 the European Union has said the use of pesticides in public spaces should be minimised or banned. Henry believes the target of integrated pest management should be the best management choices within the legal framework, with the lowest risk and environmental impact – at the lowest cost.
Ash dieback: spread and management options
The next part of the day involved looking at a range of pests and diseases, with suggestions on how best to deal with them. We began with two fungi: Hymenoscyphus fraxineus (formerly Chalara fraxineus) and Massaria. Ash dieback appears in two of Henry’s risk categories – causing a tree to die and a safety risk. Originating in Poland, it is believed to have reached the Netherlands in 2010 and the United Kingdom in 2012.
The spores of Hymenoscyphus fraxineus can travel up to 30km and remain viable, and in July or August the fruiting bodies produce around 1,500 spores per hour, 24 hours a day for about two weeks. Henry says the decision to stop ash movements will do nothing to stop the problem because the spores are almost everywhere. He then gave us an insight into how the disease is managed in the Netherlands. Most Fraxinus was planted in the 1960s to the 1980s, after Dutch elm disease killed off the dominant Ulmus. Today Fraxinus is the dominant genus in the western Netherlands and the second most dominant, after Quercus, in the east of the country. Fraxinus excelsior is the main species within the genus.
The city of Utrecht, the fourth largest in the Netherlands, had a tree population of 160,000 in 2014, of which 13% were of the Fraxinus genus. Fraxinus excelsior comprised 88.7% of this ash population. A follow-up study in 2015 graded Fraxinus species according to their susceptibility to ash dieback. It found that Fraxinus excelsior ‘Jaspidea’ had an infection rate of 90.9%, with Fraxinus excelsior ‘Pendula’ showing 100%. At the other end of the scale, Fraxinus angustifolia had an infection rate of 9.1% and Fraxinus excelsior ‘Atlas’ just 2.8%. But Henry urged a note of caution here. ‘Do not plant “Atlas” too widely, or it too may one day become more susceptible due to its monoculture!’
In addition to a large range of species susceptibility, the Utrecht study found trees under 40 years old were more susceptible to ash dieback than were older ones, and that trees growing in natural surroundings, such as those in the city’s Amelisweerd woodland estate, were the most susceptible of all. 80% of Amelisweerd’s ash population was affected, especially young seedlings and mature trees, combined with honey fungus (Armillaria). Henry admitted that although there is no treatment for ash dieback and that while urban trees may do better than those elsewhere, tree officers do need to develop a management plan.
His suggestions include determining the degree of defoliation, with the ‘turning point’ around 50%, monitoring the degree of regrowth, as tolerant trees are the future, and using branch diameter as a definition of risk, with those of 4cm and more being most at risk. Henry also recommends increasing the survey frequency of moderately and highly infected trees, pruning affected trees with branch diameters of more than 4cm in moderate and high risk areas, and felling highly infected trees after individual consideration. He mentioned that Utrecht needed €3million to manage its ash trees in 2017.
The target of integrated pest management should be the best management choices within the legal framework, with the lowest risk and environmental impact – at the lowest cost.