Brown root rot is caused by a fungus with strong virulence named Phellinus noxius, which prefers acidic, hot and humid conditions. The disease has been found in the United States and tropical and subtropical regions in Asia, including southern Japan, the mainland, Taiwan, Malaysia and Singapore.
It has been reported that over 200 plant species are hosts for the fungus. It has attacked at least nine species in Hong Kong, with the initial infection in a Chinese Banyan in Kowloon Park in 2007.
According to a tree specialist at the University of Hong Kong, Professor Jim Chi-yung, the fungus attacks weak trees. That is why many heritage trees in poor health have fallen prey to it.
A banyan next to the greenhouse at Hong Kong Park is the latest infected heritage tree recommended for removal.
The fungus can release enzymes that eat away at tree roots and weaken the wood’s texture. It spreads via spores carried in the air, water, soil and even by human contact.
During the initial stages of infection, the symptoms are difficult to detect, meaning a tree may not display signs it is in a weakened state and could collapse at any time. Confirming the disease requires laboratory work.
Officials in Taiwan were forced to remove plantations and natural forest to prevent the spread of the disease. The infected remains and soil must be incinerated.
No effective cure has been found for the disease. The Tree Management Office said it had commissioned a local university to look into the possibility of using another fungus, Tirchoderma, to suppress the disease. But Jim said a study into such a solution, which is under way in Australia, may take years to test in the environment, and was not a short-term solution.
“There’s a school of thought in Taiwan that the government shouldn’t waste time finding a cure for the disease. Instead, the government should focus on prevention. Plant strong trees and improve the health of existing trees,” he said.