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Despite earlier fears, the unhealthy plane trees in Dunedin's Octagon are not dead or dying and no primary pathogens are present, the Ministry of Agriculture and Fisheries (Maf) recently advised the Dunedin City Council.
However, the trees appeared to be susceptible to secondary pathogens, a weakness that could be due to their urban environment.
In January, council community and recreation services manager Mick Reece said he was "seriously worried" the trees had fallen victim to a new, unidentified fungal disease.
Four of the 16 plane trees which have lined the carriageway through the Octagon for more than 100 years did not come into leaf in summer this year, and others showed signs of infection.
He warned at that stage that at worst all 16 trees could die and have to be replaced with another type of tree.
The trees were further tested by Maf.
Mr Reece said this week a Maf report showed the trees were not dying but their soil needed to be "actively maintained" to enable them to defend themselves against pathogens, given their urban setting.
The overall ministry findings were "good news, albeit a bit of a wake-up call for us".
"We now need to work out how to respond appropriately to achieve longevity for our city's street trees."
Planning implications also included having "suitable replacement trees" ready for planting in key areas, perhaps with more urgency than previously expected.
The report suggested three stages of action to support the "future health of Dunedin's street trees":
• Physical modifications, such as removing surface plantings.
• Changing maintenance practice, such as gentle or no pruning until they recover.
• Therapeutic treatments, such as feeding the soil.
The programme of work, yet to be finalised, would be non-chemical as there were no pathogens to kill.
There would be a need to investigate which were the most suitable trees to be planted in urban areas, or whether trees were suitable for the Octagon at all.
"We may also get to the situation where we are nursing some of these older affected trees for several years and they don't improve and it may still be necessary to cut off the life support system."