The genus Agathis (Araucariaceae) includes about 13 species of tropical to warm temperate trees found from Malesia, through Australia to New Zealand. Kauri (Agathis australis) is a dominant tree in lowland forests of northern New Zealand (Steward and Beveridge 2010). Giant individual trees can reach over 4.5 m in trunk diameter and exceed 1,000 years in age, and are cultural icons (Beever et al., 2009). In 1972 a Phytophthora was associated with dead and dying trees in a kauri forest stand on Great Barrier Island, an island off the northern New Zealand coast (Gadgil, 1974). Symptoms included yellowing of foliage, canopy thinning and occasional tree death. The causative organism was identified as P. heveae by J. Stamps of the Commonwealth Mycological Institute (Gadgil, 1974). In 2006, Phytophthora ‘taxon Agathis’ was reported from kauri in a forest west of Auckland on regenerating and mature trees (Beever et al., 2009). The original identification of the causative organism as P. heveae was questioned, as the ITS–sequence of the isolate obtained from the Great Barrier Island and those from the mainland since 2006 was identical to P. castaneae. This raised the possibility that the kauri Phytophthora was a new species within Clade 5 of the genus (Blair et al. 2008). The ‘Kauri killing’ Phytophthora organism has been now formally described as Phytophthora agathidicida B.S. Weir, Beever, Pennycook & Bellgard (Weir et al., 2015), distinct from P. castaneae and other species of Phytophthora.
Differential oogonium ornamentation, together with the size of the oospore, was first recognised to be a diagnostic characteristic by Beever et al. (2009) in their study of representative isolates of the members of Clade 5. The mean oospore widths for P. cocois and P. heveae were not significantly different from each other nor from the Clade 5 mean, but oospore width of P. agathidicida is significantly larger than all other species and P. castaneae is significantly smaller than all other species (Weir et al., 2015). Oogonium wall ornamentation of P. agathidicida is mildly stipulate. Oospores nearly fill the oogonia with a mean width of 27.7 µm, and ranging between 19.8 –35 µm. Antheridia are amphigynous, globose; some have knots at the base. P. agathidicida is homothallic.
Sporangia are globose to ovoid-ellipsoid, papillate, borne terminally from long, thin, branched sporangiophores and could be formed via internal proliferation. Sporangia are non-caducous (although some isolates have a somewhat defined septum near the base of the sporangium). Sporangia have a mean width of 28.4 µm, ranging between12.4–50 µm, and a mean length of 39.6 µm, ranging between 14.9–75 µm.