Ash dieback fuels fungicide debate

The arrival of ash dieback disease, caused by the fungus Chalara fraxinea, has added to the growing list of pests and diseases arriving in the UK over the last decade. Of the UK’s 80m ash trees, at most 10m will be resistant; the rest will die, says plant scientist Glynn Percival, a visiting research fellow at Reading University, pointing to the global trade in trees and a lack of inspections as the causes.

‘Everyone is being told there is no cure, but that is not true... We have fungicides, so of course there is a cure,’ he adds, referring to commercially available crop treatments as obvious candidates for testing. He blames resistance within government advisory organisations on the failure to license fungicides for ash dieback.

However, many experts are unconvinced. ‘There is no chance for a fungicide application at large scale,’ says Ottmar Holdenrieder at the University of Zurich, Switzerland, who has studied ash dieback disease in Europe. The fungus forms spores during the summer and develops resistant thread-like growths in the tree from which fruit bodies (ascomycetes) emerge. ‘Poisoning the whole forest to get rid of this pathogen would not be a good idea. The therapy shouldn’t kill the patient,’ Holdenrieder warns.

In a nursery situation, any fungicide active against ascomycetes would work, but Holdenrieder has concerns that this would produce susceptible plants. The answer, he believes, is breeding resistant trees and replacing ash by other species. Relying on remnant ash trees resistant to ash dieback is not fool-safe, argues Percival, as the remaining trees may be susceptible to a different ash disease.

‘It is necessary to save as many trees as possible. We don’t think it is a good idea just to let them all go,’ says Tim Mott, director of Natural Ecology Mitigation, a company that may have a solution. The company is working with Imperial College London and has been lab-testing a formulation based on copper sulphate and other minerals and hopes soon to be working with the Forestry Commission, with field trials set to resume in Spring 2013 at Imperial’s Silwood Park Campus.

Copper is used in several commercial fungicides and works by disrupting cellular proteins. ‘CuP33 consists of very low elemental copper, typically 150ppm, and nutrients. Trees accept the treatment as a nutrient and the copper kills the fungi/bacteria,’ Mott explains. The group has been successfully lab tested on a species of Phytophthora mould that is found in oak trees and a species of Pseudomonas that grows on horse chestnut.

The formulation could be either aerially sprayed or injected into trees, with the ‘costs more to do with men and machines than anything else,’ Mott says. However, some experts worry about using a copper-containing spray as copper is a heavy metal and does not break down in the environment.

For now, there are no approved solutions for ash dieback in Europe. But Euros Jones of the European Crop Protection Association is hopeful that the fungus will be susceptible to chemical treatments. ‘You can have a fungicide that works on many different types of fungal problems in plants. So you might have a solution already on the market,’ he says. For now, Jones says member companies have looked at the problem but there isn’t an obvious solution.

John Lucas at Rothamsted Research compares ash dieback with Dutch elm disease. In the latter case, high value specimen trees in the UK were injected with fungicide suspensions. But it was difficult to get the fungicides into the crown of large trees and eventually treated trees began to suffer infections in more distant branches and treatment was abandoned.

Lucas is optimistic a treatment exists for Chalara, but is not hopeful that it will solve the impending ash calamity. ‘There are now more chemical groups of systemic fungicides available for treating diseases caused by fungi, including the azoles, strobilurins and SDHIs (succinate dehydrogenase inhibitors). I would expect one or more of these to have activity against Chalara. The problem, however, is how to treat large numbers of ash trees dispersed across most of the UK in hedgerows and woodlands.’

The Chalara fungus infects leaves as well as trunk tissue, so might be difficult to target. And infected leaves lie on the forest floor. Lucas believes spraying trees is not feasible and questions what protection this would offer. Some tree diseases can be controlled by trunk injection, but this is expensive.

Nevertheless, Percival says people should be given an option to treat their ash trees. ‘At the moment we are just relying on one option, burning and felling badly infected trees,’ he says, but other options must be considered.

‘I believe some advisors are putting their personal bias against professional neutrality. I am a scientist and I don’t like fungicides. I grow my own fruit and vegetables, but I will use fungicide rather than lose my whole crop.’

A spokesperson for Defra commented: ‘We’re using government scientific experts to approach companies with proposed treatment solutions and rapidly review and evaluate them to see whether they have potential for further testing and development.

‘We are following up potential products in conjunction with CRD [Chemicals Regulation Directorate] and plant health experts in order to see whether these products are likely to meet the criteria.’