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Council tree officer Elena O'Neill said scientists from the Ministry of Agriculture and Forestry in Auckland had managed to isolate and grow a contagion from samples taken from ailing plane trees in the Octagon last month.
Initially, it had been hoped results identifying the exact type of disease attacking the trees would be available late last month.
However, Ms O'Neill said the scientists were still growing the sample before positively identifying it, and results were not expected for another week.
"At this stage, they haven't given us any answers," she said.
In January, the Otago Daily Times reported four of the Octagon's 119-year-old trees were showing signs of infection, which Dunedin tree consultant Frank Buddingh feared could be due to a vascular disease new to New Zealand and known as Ceratocystis fimbriata.
The disease blocked the trees' vascular system, robbing them of nutrients and causing their near-dead appearance.
Three plane trees at the Clutha District Council's offices in Balclutha had also developed symptoms at the same time as the plane trees in the Octagon, in November last year, he said.
Late last month, Otago and Southland District Health Boards staff confirmed a plane tree in Frederick St, outside the Dunedin Hospital entrance, appeared to have the same symptoms.