Kauri (Agathis australis) Under Threat From Phytophthora?

Five species of Phytophthora have been recorded from Agathis australis (kauri) or soil in kauri forests: P. cinnamomi, P. cryptogea, P. kernoviae, P. nicotianae and Phytophthora taxon Agathis (PTA) initially recorded as P. heveae. P. cinnamomi has been found widely in natural stands and has been linked with ill-thrift and occasional tree death, especially in regenerating stands on poorly drained sites. PTA, which is known from fewer natural stands, is associated with a collar rot, causing large bleeding lesions near the ground, yellowing foliage, and tree death. The other three species have only been reported once. ITS sequence studies of PTA show it belongs with, but is distinct from, P. heveae in ITS clade 5. Cultural and molecular studies indicate a close relationship with P. katsurae. It is proposed that PTA may be introduced to New Zealand, but too few isolates are available to determine whether genetic variability of this species provides support for this hypothesis. Recent surveys have found collar rot is widely distributed across the natural range of kauri. Typically, affected stands are relatively small in size. Size class distributions indicate trees of many ages are affected and a disease front can sometimes be detected. Pathogenicity tests show PTA is highly pathogenic to kauri. We propose that collar rot caused by PTA is an emerging disease caused by an introduced pathogen that is spreading slowly from widespread disease foci. It poses a threat to kauri, both at the individual icon level and at the population level, with flow-on effects to kauri ecosystems.