Kauri Dieback Symposium 30 November 2013
Talk 9 of 13

Kauri dieback, caused by Phytophthora taxon Agathis (PTA) is a serious threat to the health and survival of kauri trees in New Zealand. Phosphite (phosphorous acid) has been used successfully against a number of Phytophthora diseases in horticultural and forest settings; it is a potential tool for controlling PTA. In January 2012, trials were established in Auckland (Huia and Whatipu) and Northland (Raetea and Omahuta) kauri forests. Chemjet® stem injectors were used to inject trees with phosphite (Agrifos®600) at concentrations of either 7.5% or 20%, while control trees were left untreated. Phytotoxicity symptoms were noticed with the high application rate. After one year (January 2013), half of the previously injected trees were re-injected, in all cases with 7.5% phosphite. In assessments made 17 months after the initial treatment, most trial trees had canopy health and vigour similar to or slightly worse than that at the start, with no obvious differences between treatments. However, treatment differences were obvious in trunk lesion assessments. Many more lesions remained active (expressing ooze, continued expansion) in untreated trees (53%) than in phosphite treated trees (2%). There were no obvious differences in lesion activity among the different phosphite rates or regimes. Many unanswered questions about phosphite treatment remain, including; doses for large trees, longevity of effect, curative ability on badly diseased trees, unwanted side effects etc. Forest trials are continuing and will be monitored every six months for a number of years, to determine both the long-term effectiveness of phosphite treatments in kauri and the necessity of repeat treatments.

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