Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Phytophthora 'Taxon Agathis', a new pathogen of kauri? No, just an old one under a different name

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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 199, October 2009.

There has been considerable publicity recently about a “new” fungus* (see note) that is associated with the death of kauri (Agathis australis) in Auckland and Northland. There have even been suggestions that the survival of kauri is at stake. Reports in the media and in information items produced by the Auckland Regional Council (see www.kauridieback.co.nz) give a clear impression that a virulent pathogenic fungus has been recently introduced and that it is spreading rapidly.

The fungus is not new. It was first found in New Zealand in 1972 by scientists from the Forest Research Institute (now trading as ‘Scion’) in a small patch (ca 1.5 ha) of dying kauri saplings and rickers on Great Barrier Island. Isolations made from basal cankers and soil from the diseased area and from soil in two healthy areas (one adjacent to the diseased area and another 4 km away) consistently yielded a species of Phytophthora which was identified as Phytophthora heveae by the Commonwealth Mycological Institute.

Pathogenicity tests showed that this fungus was capable of killing kauri seedlings (see Gadgil 1974 for full details). As P. heveae was isolated not only from the diseased area but also from soil from areas with healthy kauri, it was apparent that the fungus was pathogenic only under a specific combination of environmental conditions.

Wet soil appeared to favour pathogenic activity as most of the affected trees on Great Barrier recovered during the following dry summer. Because it was causing only minor damage in a small area in which it was known to be present, P. heveae came to be regarded as an insignificant pathogen of kauri and except as a record in the archives, was largely ignored.

Interest in the fungus was revived in 2006 when Landcare Research scientists investigating death of kauri in the Waitakere Ranges isolated a species of Phytophthora that was morphologically identical with the Great Barrier isolates identified as P. heveae. A comparison using molecular techniques of the Great Barrier and Waitakere isolates with P. heveae isotype isolates from Malaysia showed that all New Zealand isolates were identical and close to but distinct from the Malaysian P. heveae.

Acceptance of the modern tendency for basing species separation on phylogenetic analysis rather than morphology meant that another name for the New Zealand isolates was required. More work is needed to determine whether the New Zealand species is new and until that decision is made, an informal name, Phytophthora ‘taxon Agathis’ is being used (for full details see Beever et al. 2009).

This interim name has no status under the Code of Botanical Nomenclature and its adoption gives no indication of the connection between the recent Waitakere isolates and the Great Barrier isolates identified as P. heveae. This has given rise to an almost universal misapprehension that a new, recently introduced pathogen has been found.

Let us be quite clear. The fungus now known as Phytophthora ‘taxon Agathis’ is the same species identified as Phytophthora heveae from Great Barrier Island in 1974. It is not a recent introduction. It is not a rapidly spreading virulent pathogen. A survey of the original disease site on Great Barrier Island in 2007 showed that the diseased area now occupied ca. 10 ha (Beever et al. 2009). The fungus was already present in the adjacent area in 1972, so the extension of the diseased area does not indicate spread of the fungus.

Its detection in the Waitakere Ranges (and more recently, in Trounson Kauri Park) is unlikely to represent recent introductions. Although most likely to have been present, it was not detected before because environmental conditions were not suitable for it to have caused serious damage. Its presence in sites containing healthy kauri and the recovery of affected trees in a dry season on Great Barrier Island has shown that the fungus is pathogenic only under a specific combination of environmental conditions.

Peter Gadgil

*Note: In this article, the word ‘fungus’ is used in its general sense, broadly defined as “an organism traditionally studied by mycologists”. Strictly speaking, species of Phytophthora should be called Oomycete fungi (Brasier 2009). I consider such pedantry unnecessary in a general article - P. Gadgil.

Phytophthora References

Beever, R.E.; Waipara, N.W.; Ramsfield T.D.; Dick, M.A.; Horner, I.J. 2009: Kauri (Agathis australis) under threat from Phytophthora? pp. 74-85 in “Phytophthoras in Forests and Natural Ecosystems”, Proceedings of the Fourth Meeting of IUFRO Working Party S07.02.09, (Goheen, E.M.; Frankel, S.J. tech. cords.), General Technical report PSW-GTR-221, USDA Forest Service,
Albany, CA, USA.
Brasier, C. 2009: Phytophthora biodiversity: how many Phytophthora species are there? Pp. 101-115 in “Phytophthoras in Forests and Natural Ecosystems”. (full reference above).
Gadgil, P.D. 1974: Phytophthora heveae, a pathogen of kauri. New Zealand Journal of Forestry Science 4: 59-63.

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