A Seminar led by Henry Kuppen at Barcham Trees, 22 November 2017
Report by Colin Hambidge
Henry Kuppen is the director of the Dutch company Terra Nostra (www.terranostra.nu), 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. Although this was first seminar at Barcham Trees’ Cambridgeshire nursery, his reputation had gone before him as the audience of tree officers, contractors, consultants and landscapers filled the lecture room.
Barcham’s Keith Sacre had already met him, but it was at a recent tree conference in the Russian city of Voronezh where both were presenters that Keith persuaded Henry to visit the nursery and lead a seminar.
The day began with a consideration of the relationship between trees and their pests and diseases. Henry told delegates more 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 rôle 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’.
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. Away from the natural forest, a tree’s condition can be weakened by human activity such as pipe and cable laying, mowing and parking cars. 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, registrate, 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, decrease 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 registrating we need to ask where is the problem and what is its spread? Analysis should involve asking these questions: What is the effect 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 places and seats. Its profile is moderate if branches are likely to fall within three metres of these, and low if further than three metres. 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.
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 30 kilometres and remain viable, and in July or August the fruiting bodies produce around 1500 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 per cent were of the Fraxinus genus. Fraxinus excelsior comprised 88.7 per cent 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 per cent, with Fraxinus excelsior ‘Pendula’ showing 100 per cent infection. At the other end of the scale, Fraxinus angustifolia had an infection rate of 9.1 per cent and Fraxinus excelsior ‘Atlas’ just 2.8 per cent. 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. Eighty per cent 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 per cent, 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 more than 4cm in moderate and high risk areas, and felling highly infected trees after individual consideration. He mentioned that Utrecht needs €3,000,000 to manage its ash trees in 2017.
Massaria disease is caused by the fungus Splanchnonema platani, formerly known as Massaria platani. Native to the USA and countries bordering the Mediterranean Sea, It was first recorded in the Netherlands in 2007 and in the UK two years later. As its specific name implies, it affects Platanus x hispanica, Platanus occidentalis and Platanus orientalis. It results in large lesions, often reaching several feet, forming on the upper surfaces of large branches.. The lesions begin as pinkish strips with black, flaky bodies. Branches in the lower canopy drop their leaves in the middle of summer, the lower inner canopy dies back and the affected bark can drop off in just two weeks, with the tissue turning brown. Affected wood soon becomes short-fibred. On a brighter note, Massaria-affected plane trees can soldier on for many years, and it is not always necessary to remove damaged branches.
Henry told delegates the problem tends to be related to drought-stress, with trees growing in paved areas at particular risk, so it is important to note the place in which an affected tree is growing. Surveys should be conducted from a platform, and it is important to registrate not only the growing position, but also the diameter of the trunk and the diameter of dead branches, as most infestation occurs in branches with a diameter less than 2cm. He also mentioned that 40-70 year old trees tend to be the most vulnerable.
Moving from fungi to bacteria, we then turned to Pseudomonas syringae pv Aesculi or bleeding canker of horse chestnut. Henry soon told delegates he believes the problem is related to cultivars. For example, Aesculus hippocastaneum ‘Baumannii’ is widely grown in the Netherlands, and is highly susceptible to the disease. Aesculus carnea and Aesculus hippocastaneum are also high in the susceptibility list, while Aesculus parviflora is very low in the table, and both Aesculus glabra and Aesculus flava are also good choices for those wishing to grow horse chestnuts.
Believed to have begun in India, by 2002 bleeding canker of horse chestnut was in both the Netherlands and the United Kingdom. We do not yet how the bacterium spreads, but it can affect the bark of trees of any age. When the disease is confirmed in a tree, Henry advises an annual inspection, and points out that a secondary infestation indicates the disease is in advanced state. Trees with a diameter of up to 50cm, therefore usually young ones, are most susceptible, closely followed by any tree in poor condition. Fractures of large horizontal branches becomes likely and although the disease can cause the tree to die, some have recovered. Henry suggests surveying susceptible trees with a high risk profile twice a year – once in June or July and again in September – and those with a moderate risk profile annually in September.
It has been scientifically proven that the Pseudomonas bacterium is rendered inactive if the cambium is warmed at a temperature of between 39 – 42°C for 48 hours, but the development of the bacterium on untreated parts of the tree and the degree of re-infection in the treated parts are being evaluated.
Attention was then turned to an insect pest – Thaumetopoea processionea, oak processionary moth (OPM). It was discovered in the Netherlands in 1988 and by 2005 this serious threat to human health was all over the country. In the UK, it was first spotted in west London, where it is believed to arrived on Dutch nursery stock. Henry gave a thorough and fascinating insight into the moth’s life cycle, which is well documented.
The urticating hairs of the larvae are 100-150 microns long, and have barbs which attach them to human skin. These hollow hairs are filled with the foreign body protein Thaumetopoiene, which can cause a pseudo-allergic reaction, which may result in an anaphylactic shock. Contact with the hairs can result in severe skin rash, vomiting, dizziness and fever, and they can also affect the eyes. Many animals, such as horses, dogs, cattle and sheep, are also sensitive to them, and as the urticating hairs can remain active for seven years burning them is the only way of destroying them.
Pheromone traps, which attract the male moths, give a valuable insight into populations, which can be translated into management measurement for the upcoming season. The moth’s predators include great tits, blue tits, parasitic flies, parasitic wasps and bats. The pest can be managed by spraying, picking them from trees, sucking the nests or by just accepting the problem. Henry talked through the spraying options open to Dutch and British contractors and the best protective wear.
The day concluded with discussion of three upcoming threats to trees. Xylella fastidiosa, common to Brazilian coffee plantations, is, Henry told delegates, the highest risk disease in Europe – and with 150 host species at risk it would be difficult to argue with his opinion. It can be spread by plants, xylem-sucking insects such as aphid and froghopper, and by root-grafts. Xylella fastidiosa has four sub-species, of which the most threatening is Xylella fastidiosa multiplex. The bacterium invades and blocks the xylem vessels of woody plants and broad-leaved trees, it can be in the host for up to three years without damage being shown, and it requires winter temperatures of -18°C to kill it.
The pine processionary moth, Thaumetopoea pytocampia, a native of southern Europe, is widespread in Italy, Spain, Portugal and Switzerland, and preys upon Cedrus, Larix and Pinus. The moth generally makes its nest on twigs in the canopy, and it may pupate in the soil if the weather gets too hot. Its urticating hairs are similar to those of the OPM, with very similar affects on humans.
And finally to Agrilus planipennis, the emerald ash borer. This Asian native is spreading west, reaching the USA in 2002 and Moscow in 2007. Depending on temperature, it can live one or two years, surviving down to temperatures of -33°C. It makes semi-circular holes in the cambium zone. Females produce 60-90 individual eggs on the bark or in groups in cracks in bark, usually towards the top of a tree. It can take up to five years before an infestation is noticed. In both the USA and in Russia its spread tends to follow roads and railway lines. Affected trees may re-grow, but they will still die from the pest. It has a predator in the wasp Spathius galinae, and trees may be give a stem injection of emmamectin, which has been presented as a solution on an individual basis, but is unlikely to be the answer for the reasonable long-term management of all ash trees.