Kauri
forest threatened by kauri collar rot disease
Communities are being asked to take special measures when entering the Waitakere Regional Parkland to help stop the spread of the newly-identified deadly kauri disease Phytophthora taxon agathis (PTA), commonly know as kauri collar rot.
Dying trees were first noticed by a Piha resident on Maungaroa Ridge Track two years ago, but the disease is now better understood after ground-breaking research by Dr Ross Beever, Sarah Tsai and Dr Nick Waipara of Landcare Research and others.
There were already four known types of phytophthora in kauri, some associated with occasional tree deaths.
It is suspected that PTA, which is more deadly, is an introduced disease, possibly of relatively recent origin here. It affects trees of all ages, from seedlings to giants, and once infected, trees are unlikely to recover.
The disease is soil or water-borne and enters the trees through the roots. Telltale signs are huge bleeding lesions near the base of the trees, which can girdle and ring bark the tree, yellowing foliage, and ‘stag-horns’ where leafless canopy branches seen from above have the appearance of stags’ antlers. Tree death can occur quite quickly. PTA is also known at Great Barrier Island and from soil at Trounson Kauri Park.
Stands of dead and dying trees covering 10 to 20 hectares each have been found at Cascades, Huia (west of Huia Bridge), Piha (Maungaroa Ridge), Karekare (between Karekare and Farley Streams) and Anawhata (Kuataika Track). People are being asked to report any affected trees they see on the park, and on their own private property.
Kauri are currently regenerating from milling, which largely ceased with the closure of the Piha operation in 1921. So what will happen if many kauri die? The forest structure would change with a predicted increase in podocarp trees such as rimu. Rimu does not appear to be affected by PTA.
Spread of the disease requires a vector agent. In the case of PTA this could be by feral pigs, recreational visitors, researchers, tourism, film or sporting concessionaires, volunteer pest control programmes, or ARC staff and contractors.
The ARC plans to increase funding and efforts to kill feral pigs which have been on the increase and has commited $90,000 for each of the next two years to carry out further research, including surveys to establish just how widespread the disease is.
For all human vectors, new practices need to be adopted to try and halt the spread. Spread is more likely in wet weather when tracks are boggy. Going from stream bank into water, and vice versa is another particular threat.