Brown Root Rot Disease

1. What is Brown Root Rot disease?

Brown Root Rot disease has led to the rapid health deterioration of the India-rubber Tree in the photo.

Phellinus noxius (有害木層孔菌) is a white simultaneous rot fungus that causes Brown Root Rot (BRR) disease (褐根病), which could result in rapid health and structural deterioration of trees and may lead to tree failure.

The disease is prevalent in tropical and subtropical areas worldwide and affecting trees in Hong Kong, nearby regions (e.g. Taiwan, Macau, Singapore, Malaysia, Japan), and other places across the world (e.g. Australia, Central America, Africa and Oceania). It has been reported that over 200 plant species are hosts to Phellinus noxius worldwide. According to international literature, there is no direct remedy for BRR infected trees at present.

The situation in Hong Kong and other places in the region:

The Chinese Banyan is one of the tree species susceptible to BRR infection in Hong Kong. The Chinese Banyan is one of the tree species susceptible to BRR infection in Hong Kong.

(a) In Hong Kong

Since 2010, there has been increasing number of reported BRR cases across the territory, partly due to improved diagnostic knowledge and techniques. As at May 2014, there are around 100 trees confirmed with BRR infection.

(b) In Macau

According to the Department of Gardens and Green Areas, the Civic and Municipal Affairs Bureau of Macau, around 250 numbers of BRR infected trees have been removed each year since 2011.

(c) In Taiwan

According to the information from the Public Works Department of Taipei City, as at October 2012, there are over 6,200 numbers of BRR infected trees identified in Taipei City.

(d) In Singapore

Brown Root Rot problems in Singapore have been known for decades and recent incidents of BRR infection have also been reported.

2. How does Brown Root Rot disease spread?

The disease mainly spreads through root-to-root contact or through infested wood debris in soil. Hence, personal gear such as shoes (sole) and tools such as adze, knives, scissors and shovels that have been used on confirmed/suspected BRR infected trees or sites should be carefully and thoroughly cleaned and sterilised by disinfectant. The disease may also be transmitted through the long range dissemination of basidiospores from fruiting bodies.

Brown Root Rot disease has led to the rapid health deterioration of the India-rubber Tree in the photo.

3. Hong Kong SAR Government’s work in controlling the Brown Root Rot (BRR) disease

The Tree Management Office (TMO) of the Greening, Landscape and Tree Management Section of the Development Bureau serves at the policy level to provide advice and support on tree management by government departments.

(a) Diagnosis of Brown Root Rot Disease:

Early diagnostic symptoms of BRR disease are often difficult to detect although the disease can cause a rapid decline in tree conditions within a short period of time. The TMO has spearheaded local researches on BRR to better manage the disease. One of the most important applications is the early confirmation of BRR diagnosis through molecular (DNA) analysis provided by local institutions so that follow-up actions to manage the spread of the disease could be timely undertaken.

Primordial stage of fruiting bodies of Phellinus noxius. Mature fruiting bodies (bracket forms) of Phellinus noxius.

Diagnosis of BRR disease in trees is usually undertaken in three steps:

  1. Identification of Symptoms (typical symptoms such as unusual thinning and yellowing of tree crown, diminished leaves or shedding of leaves);
  2. Field diagnosis of signs (presence of adhesive soil particles, mycelial netting on the mycelia sheath and fruiting bodies of Phellinus noxius); and
  3. Laboratory tests including fungal culture isolation and/or molecular analysis.

(b) Guidelines on Brown Root Rot Disease:

To ensure the control of the BRR disease in Hong Kong, with the advice of the Expert Panel on Tree Management, the TMO has promulgated a precautionary and preventive approach for BRR management through the release of the leaflet on Introduction to Brown Root Rot Disease (2011) and Guidelines on Brown Root Rot Disease (2012). The Guidelines provide ready information on the identification of suspected BRR cases, management strategy, removal procedures, etc., supplemented with a pictorial guide and a set of FAQs on BRR disease.

(c) Education to professionals and the public:

Seminars and workshops have been organised to strengthen government departments' capacity in handling the disease and the public's awareness of the disease. The TMO will continue to provide training to staff of departments to equip them for field diagnosis of BRR disease and arrange seminars/ talks on BRR disease to members of the industry and the public.

(d) Educational Video on Brown Root Rot Disease:

To further promote the knowledge on BRR disease, the TMO has produced an educational video on "Brown Root Rot Disease - Diagnosis, Control and Management Strategy" through vivid illustrations and animations on the identification of suspected BRR cases, the management strategies and removal procedures.

(e) Local Research on BRR Disease:

Although there is no known direct remedy for the BRR disease so far, there has been overseas research information to suggest the possible employment of antagonistic fungal species (particularly Trichoderma strains) as biological agent to control the BRR disease. In light of this, the TMO has, in collaboration with local universities, commissioned research studies in this aspect. The study demonstrated successful isolation of local strains of Trichoderma which could effectively suppress the BRR fungus under laboratory conditions. The actual application in the field has yet to be determined through field trials, although initial trials gave promising results.

(f) Exchange and Collaboration:

The TMO has been communicating actively with local tree experts as well as overseas tree specialists in sharing information and experience on BRR management. They include government officials, academics, researchers, municipal tree experts as well as arboricultural professionals from Hong Kong, Macau, Mainland China, Taiwan, Malaysia, Singapore, Australia, Europe and the USA.

4. Overall Summary: