Brown root rot disease could be spread by humans
Specialists say hikers, imported plants and even monkeys could have a part to play in dispersing the disease that is killing the city's oldest trees
The wide spread of the virulent brown root rot disease in Hong Kong's trees may have been caused by human mistakes made worse by monkeys.
Tree specialists say contamination of hikers' boots, use of improperly quarantined ornamental plants and soil carried by monkeys could all be factors.
Conservancy Association chief executive Ken So Kwok-yin said there was a "comparatively low" chance of spreading the disease by air, so soil was the most likely carrier.
"[Air transmission] usually happens at a very late stage when the spores are dispersed from the fruiting bodies grown on the infected trees," So, an adviser to the government's Tree Management Office, said.
"The transmission could be caused by human activities and their lack of awareness, which brought infected soil from elsewhere, for example through imported plants which were not properly quarantined," he said.
Small flowering plants placed near the heritage trees for decoration could bring about the infection if they were already contaminated by the fungus.
University of Hong Kong tree specialist, Professor Jim Chi-yung, said hiking could also spread the fungus.
"The infected soil debris might have followed hikers' clothes and shoes from urban areas to the countryside."
Asked if the monkeys commonly found in Kam Shan Country Park could become a source of transmission, Jim said: "It could happen if they have contact with the infected soil and move around in forests."
A spokeswoman for Agriculture, Fisheries and Conservation Department said plants imported from the mainland did not require permits or quarantine.
The Tree Management Office, set up in 2010 under Development Bureau to co-ordinate tree work done by more than 10 separate departments, was unable to explain the spread.
But it said it had used a basket of measures to tackle the disease since December last year.
These included adding pictures to guidelines for inspectors, organising workshops for staff and strengthening the reporting system by asking frontline inspectors to identify the disease.
But a representative of staff at the Leisure and Cultural Services Department, which is responsible for the largest number of trees, said it would be hard to make the measures effective.
"A workshop of one or two days will not help. How can we distinguish one mushroom from another without special training?" Cheung Siu-wing, chairman of the Leisure Services Staff General Union asked.
Cheung, who received eight months' horticultural training in Britain in the 1980s, said frontline staff were already overloaded, with each inspecting 20 hectares of green areas a day. "We don't even have proper equipment to check if the roots are rotted."
Under the guidelines for controlling the disease, and the practice in Taiwan, the infected tree, its remains and the soil should be incinerated. But the Tree Management Office said Hong Kong did not have a suitable incinerator so the remains were sent to landfills. So questioned whether contractors were careful enough to prevent the spread during transport and burying.
Observations by the Post this month found branches of an infected banyan in Happy Valley in its planter, loosely covered by green canvas, after the tree was removed in July last year.
The office said it would remove the stump and soil shortly.