Urban Tree Pathogens: The Impact of Neofusicoccum luteum on Araucaria heterophylla (Norfolk Island Pine)

Abstract Araucaria heterophylla is a widely planted evergreen ornamental species in urban and coastal landscapes worldwide. Increasingly, these trees have been exhibiting stress symptoms, including canopy dieback, browning of foliage tips, defoliation, and reduced needle length. Previous research has shown that these symptoms may be the result of predisposing abiotic stressors, such as extreme weather events, soil compaction, and proximity to urban infrastructure. Building on this, the present study examines the trees’ foliar fungal communities and potential pathogens. Thirty-two (32) mature A. heterophylla trees of varying health growing on the foreshores of Gold Coast, Australia, were selected for isolation of fungi from foliage samples. Sixteen fungal species from ten families were identified. Neofusicoccum luteum , the most commonly represented species and previously unreported in A. heterophylla , was isolated from eight trees (25%) and selected for six-week-long pathogenicity testing under controlled glasshouse conditions. To assess pathogenicity, forty two-year-old trees were inoculated with either N. luteum or sterile agar (control). Half of these trees received limited water, and the other half was watered regularly. The infection rate exceeded 90%, and lesions developed in both groups, but lesions in the control groups were significantly smaller ( p < 0.05) than in trees inoculated with N. luteum . Re-isolation at the end of the pathogenicity tests had a higher success rate in drought-affected trees (75%) despite the smaller size of the lesions. Although no mortality occurred, trees inoculated with the pathogen were significantly ( p < 0.05) shorter than the control trees under the regular water conditions. These results suggest that N. luteum may contribute to A. heterophylla decline and reduced growth rate, but is unlikely to be the sole causal agent and is unable to kill a tree rapidly. Strategies for improving A. heterophylla health and management emerging from this study include the need for: (a) supplem.entary irrigation during prolonged drought to prevent the pathogen from entering through damaged bark; and (b) consistent tree health monitoring.