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Plane truths about tree disease

By Colin Hambidge
A recent seminar on a disease that threatens plane trees highlighted some important points about how we think about our forests and about commerce
Last autumn, Lucio Montecchio, professor of forest pathology and ornamental tree pathology at the University of Padua in Italy led a seminar on canker stain of plane trees at Barcham Trees. There could be nobody better qualified to do so, since he has been studying the topic for nearly 30 years. 

The threat posed to Britain’s plane trees by canker stain of plane is growing, as it works its way northwards through France, travelling at a faster rate than previously thought. The problem is caused by an ascomycete fungus (Ceratocystis platani), which originated in the eastern United States. It reached the ports of southern Europe, most probably on infected crating material, during World War Two, spreading rapidly through Italy, Switzerland and France. It was also reported recently in Greece and Albania. 

Montecchio began by challenging the way we look at the threats facing our trees. When we talk of ‘fungal invasions’, do we really mean ‘invasions of cheap commodities’, he asked. The concept of plant protection and the legislation relating to it come from an anthropocentric perspective – not from a wish to save plants, but rather to reduce production losses. Do we really care about the health of forests as a whole? Montecchio suggested that what we actually care about is the loss of productive species. For example, we care for hazels when they are planted for nuts, but dislike them when they compete with beech in a first-generation forest. 

He pointed out that we can go as far as to suggest deliberately spreading lethal, epidemic parasites in order to ‘biocontrol’ undesirable species. For example, inoculation with oak wilt fungus (Ceratocystis fagacearum) and treatment with chemical silvicides were compared as a means of eradicating oak trees in central Minnesota, USA, from 1953 to 1967. Inoculation with the fungus resulted in 92 to 99 per cent mortality, while application of chemical silvicides resulted in lower mortality and higher cost. 

If a forest is regarded as 100 per cent a natural ecosystem producing social and ecosystem services, then there are no crops to protect. In an unmanaged natural forest, a native parasite causes sustainable level of damage. These parasites invade and ‘de-invade’ forests, allowing both natural selection of the best biologically performing genotypes, and the geographic distribution of species. 

Or, Montecchio asked, is a forest actually a human-assisted ‘income machine’? In that case where there is one product and one enemy, such as Chalara in an ash plantation, there is a need for crop protection. He suggested that in order to hide human responsibility, we call imported exotic parasites ‘invaders’. But is it a biologic invasion or an invitation to spread? 

All living beings seek out the best. All species, from fungi to humans, look for space and nutrients in order to survive, reproduce and spread. Where there is a high level of competition, migration is the answer. Colonisation strategies are the same in all creatures able to move spontaneously. But fungi have no legs or wings; they cannot travel thousands of miles in a few days to invade and colonise our plants. 

Montecchio told his audience that it is the import of cheap commodities which is invasive, not exotic fungi; these behave merely as pioneers in a new world. When something is cheap, we buy it, including the poorest commodities, such as waste wood for biofuel, firewood and bark. Trade is the main means of the unnatural introduction of known and unknown pathogens. Most of the fungi with epidemic potential were and are imported from similar climatic areas through different commodities such as nursery stock, logs, seeds and packaging material. 

An exotic, artificially introduced fungus can spread epidemically in a new environment, initially with no visible symptoms. Early warning, surveillance, contingency plans, plus rapid and efficient eradication measures, are needed at this stage. 

But in a few years, the susceptible species decline and die, by which time it is often too late for anything other than containment. Of course, early warning, eradication and containment do not come free of charge, Montecchio reminded delegates. 

What of the dozens of unknown species regularly imported and then reported as pathogens? Eradication measures could work, but not one of the quarantined forest fungi has been 100 per cent eradicated in the whole European Union. We are, he suggested, less efficient than legless microscopic fungi. What can we realistically do? We can decrease the actual probability of introduction and spread. 

First, in a science-based approach, known parasites must be studied before their arrival, according to climatic and environmental features, looking for vulnerabilities. There could be early warning and detection through multi-level surveillance, including at citizen level and through social media. We also need inspectors with the right ‘toolbox’, including information, training, laboratories and other resources. It also needs a quick response at supranational level, so we are ready to move according to common contingency plans, have the financial resources available in advance and carry them right through to post-eradication surveys, with no overlaps in responsibilities. 

Platanus acerifolia (the London plane), a vigorous, hardy hybrid of Platanus occidentalis and Platanus orientalis, was produced around 1670 in the Oxford Botanical Garden. The selection of a few, quite similar genotypes across Britain accelerated during the Industrial Revolution, mainly thanks to their tolerance of air pollution.

In a few decades, the London plane was planted all over Europe and the USA – in gardens, parks and avenues. But with clonal selections, important features are enhanced to the detriment of others, such as susceptibility to unknown parasites. 

The later arrival of chainsaws, and the large, slow-closing cuts they produced became an open invitation to wound parasites such as Ceratocystis platani. The artificial spread of London plane in southern Europe and the trans-oceanic import of Ceratocystis platani resulted in the meeting of two perfectly compatible partners. 

The spread of the fungus in Europe was mainly through infected sawdust and pruning tools, Montecchio emphasised. The lack of disinfection of tools after pruning infected trees is the main infection pathway across long distances, due to the long-term viability of the fungus on sawdust residues on chainsaws. ‘The disease is effectively spread by uninformed contractors involved in infected plane pruning, and their infected chainsaws’, he said. 

There is no cure for this human-assisted disease. Prevention and containment are the only efficient ways to slow down the spread of this parasite. Movement of plants for planting and non-squared wood from countries where the disease occurs is prohibited.

Colin Hambridgeis a gardening writer.

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