Experts get to the root of Hong Kong tree disease problem

New approach aims to quickly prevent and control cancer plaguing urban forest in districts where removal has been the norm

PUBLISHED : Monday, 03 April, 2017, 12:06am
UPDATED : Tuesday, 04 April, 2017, 5:45pm

A new way to prevent and control the spread of a deadly disease plaguing city trees is being tested as the government revamps its approach to the urban forest.

Brown root rot disease, a ­fungal infection often referred to as “tree cancer”, has no known cure and is one of the top tree killers in Hong Kong and tropical ­regions around the world.

Diagnosing the disease in its early stages has not always been easy, because symptoms – abnormal crown conditions and the presence of fruiting bodies – do not appear until it is too late.

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Three experts tested out the new method, which aims to identify the disease quickly and accurately, in a dig at a heritage tree in Kowloon late last month.

The men, who sit on a government panel, found signs of decay further down the root system.

The method, previously used by arboricultural scientist Paul Barber in Vietnam, differs from the traditional approach that mainly relies on observing symptoms above ground and sending samples for laboratory diagnosis.

“Trees may look OK, but if they’re not OK they’re infecting other trees. That’s why we must isolate the disease and identify it as quickly as we can,” arborist Kevin Eckert said.

Trees may look OK, but if they’re not OK they’re infecting other trees
Kevin Eckert, arborist

Implementing the new technique is key to preventing and controlling the disease, which is known to affect at least nine ­species and has already hit at least 10 districts across the city.

Spread through root-to-root contact, the disease may also be transmitted by those who visit infected sites.

Some 22 of the 480 city trees that are listed as old and valuable are infected with the root disease, according to official figures.

Over the years, at least 19 heritage trees have died from the ­disease. In the past two decades infected trees have collapsed and killed at least three people.

The work of the menwill be ­included in an updated version of government guidelines on the disease, which was first released in 2012.

But experts warn there is no “one size fits all” solution.

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Diseased treeshave to be isolated from others, mainly through severing their roots or chemically.

“The challenge we face here, is that we have trees in parks, pits, surrounded by concrete and on slopes.

Each situation is different. We need to diagnose, just like any patient, based on the different species, condition, its site and treat it appropriately,” Eckert said.

The discovery comes as the government hopes to draw from global best practice to take better care of city trees.

In January, the government established an 18-member urban forestry advisory panel to assist ­officials on policy and operational aspects of tree management, in the hope of bringing in new ­perspectives and professional experience.

Some experts and academics previously criticised officials for failing to properly manage and conserve old and valuable trees, opting instead for a choice of ­removal rather than long-term monitoring.

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The government chopped down 75 diseased trees last year, none of which were listed as old and valuable.

Forest pathologist Philip Cannon, who works with the US ­Department of Agriculture, said cutting down trees is necessary when there is danger of the ­disease spreading.

“We may sacrifice a few trees, but that’s for the benefit of the whole forest, for all the rest of the trees in Hong Kong,” Cannon said.