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New Zealand Plant Protection 66 (2013): 383

Phosphorous acid for controlling Phytophthora taxon Agathis in kauri: field trials

I.J. Horner and E.G. Hough

ABSTRACT

Phytophthora taxon Agathis (PTA) threatens the health and survival of New Zealand kauri trees. Longterm trials testing phosphite (phosphorous acid) for control of PTA in infected kauri were established in four Auckland and Northland forest sites in January 2012. All 160 trial trees (girths from 40 to 120 cm) showed symptoms of PTA infection. At the start of the trial, tree canopy health was assessed, basal trunk lesions were measured and marked, and reference photographs were taken. Trees were injected with either high (20%) or low (7.5%) concentrations of phosphite (20 ml/20-cm trunk circumference) or left untreated. Treatment with the higher concentration resulted in moderate phytotoxicity symptoms, with leaf yellowing, browning and some premature twig drop. These symptoms were no longer apparent in most trees after a few months. Re-assessments of canopy health and trunk lesion activity or spread are being made 6-monthly. One year after initial treatment application, canopy health was generally similar to or slightly worse than at the start of the trial (using baseline photographs), regardless of treatment. On average, more lesions were active (expressing fresh ooze) in untreated controls than in phosphitetreated trees. Lesion advance was greater in untreated trees than in phosphite-injected trees. Phosphite applications will continue either annually or at longer intervals in various treatment regimes.

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