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Two of the trees - at the southern end of the Octagon carriageway - were showing few signs of life yesterday, as leaves opened on the other 14 trees.
That was despite a $50,000 programme to rehabilitate the trees unveiled by Dunedin City Council staff in May this year, following earlier fears they could be under attack from a mysterious fungal infection.
Ministry of Agriculture and Forestry test results released in April appeared to rule that out, and the trees' urban environment was instead believed to be making the trees vulnerable to secondary pathogens.
However, council parks and reserves team leader Martin Thompson said yesterday the two obviously ailing trees did not appear to be responding to the treatment, and were showing signs of an unknown fungal infection.
The remaining trees had come into leaf, although "probably not as vigorously as we would like", he said.
Fresh samples had been taken and would be sent to Ministry of Agriculture and Forestry scientists in Wellington. Results were expected in several weeks.
Tests carried out earlier this year had been "pretty inconclusive", but the ailing trees were not showing the usual signs of anthracnose, a common disease for plane trees.
"It usually doesn't prevent the leaves coming out entirely. What it usually does is wait until the leaves are out and then attack [those] leaves," Mr Thomspon said.
"It would be unusual if it was anthracnose, and other than that, we're not entirely sure."
Ivy and 5cu m of topsoil had been removed from the base of each tree as part of the council's rehabilitation programme. Lime and rock dust had been added around the roots.
The first application of a compost tea solution was sprayed into the trees' foliage on Monday night, and would be repeated every five or six weeks while the trees were in leaf, Mr Thompson said.
It was too soon to say whether any trees might need to be removed, if the rehabilitation programme did not work.
"Trees can sometimes take a season or two to respond to treatments like that. There's a little bit of water to flow under the bridge before we make that call."