Landcare Research is spearheading work to help understand and halt the spread of a destructive disease that is killing one of New Zealand’s iconic native tree species, the kauri.
The disease, kauri dieback, is caused by a pathogen (a disease-causing agent) belonging to the genus Phytophthora (from the Greek for ‘plant destroyer’), a notorious group of organisms that cause major plant diseases around the world. The species affecting kauri, which appears to be new to science, has been called Phytophthora taxon Agathis (PTA) as an interim measure, until its precise relationships are clarified. Biosecurity New Zealand has declared it an ‘unwanted organism’ and is facilitating a response with the Department of Conservation and the northern regional councils.
PTA was first recognised in 1974 on Great Barrier Island, and it is probable it was introduced to New Zealand some years earlier. However, it behaved as a ‘sleeping disease’ and the seriousness of the problem is only now becoming apparent.
Symptoms include yellowing of foliage, canopy thinning, death of branches and the eventual death of the tree. Kauri dieback is also known as kauri collar rot, as it kills the living tissue of the major roots and the lower trunk, eventually ring-barking the tree. Associated with this girdling, affected trees often show excessive bleeding of gum through the bark near the base of the tree trunk.
Dr Ross Beever from Landcare Research says that present evidence suggests PTA is a soil-borne disease that is spread by soil and soil-water movement, through underground root-to-root contact. As well it is likely to be transported around on human footwear and by animals such as wild pigs.
Researcher Stan Bellgard says there are currently three areas of work into the disease.
The first is a project determining the effectiveness of the prescribed hygiene treatments currently being used to prevent human transfer of the pathogen, and to compare these with treatments based on household bleaches and Citricidal® (an organic grapefruit seed extract concentrate),’ Dr Bellgard says.
We are also working in collaboration with SCION and Plant & Food Research to optimise soil and lesion-based detection of PTA. The research aims to provide a scientifically robust method to detect PTA in soil and in affected trees displaying typical symptoms near the collar of the tree.’
The third research area is a collaborative project between the Auckland Regional Council and Landcare Research trialling the use of stream-based surveillance of Phytophthora species in the Waitakere Ranges. If successful, such a surveillance method may provide a way of determining the presence or absence of PTA at a landscape scale.
This is all being undertaken to combat a disease whose long-term effect may be to change the composition of kauri-dominated forests to forest dominated by podocarps, and in doing so significantly alter biodiversity values. Infection could also lead to premature death and the loss of giant trees that are cultural icons.
Dr Ross Beever
Dr Stanley Bellgard