Abstract
Ironwood (Casuarina equisetifolia subsp. equisetifolia) trees on Guam have been in decline since 2002. This study applied proportional odds logistic multiple regression modeling to a set of biological variables in order to find significant decline predictors as a first step towards identifying pathogenic contributors. Based on the analysis of a set of 77 medium and large trees, the bacterium Ralstonia solanacearum and the fungus Ganoderma australe species complex were found to be significant predictors, with p values of <0.001 and < 0.008, respectively. Their respective values for symptomless trees were 18% and 0%, compared to 80% and 35% for those nearly dead trees of which 30% had both organisms and 15% had neither. More complex models were also fit, that included interactions. The R. solanacearum by G. australe interaction was not a strong effect, indicating additive rather than multiplicative behavior of these effects on decline severity (p value>0.087). When nine covariates were applied univariately to a data subset of 30 tree cross-sections, the significance of R. solanacearum was strongly upheld while that of G. australe was moderately reduced. Also significant were percent cross-sectional area with bacterial wetwood and the formation of ooze within 24 h. Wetwood bacteria of Klebsiella spp. and K. oxytoca were found across all levels of decline and were not significant predictors. Other enteric bacteria identified included Kosakonia, Enterobacter, Pantoea, Erwinia, and Citrobacter.
Highlights
Casuarina equisetifolia subsp. equisetifolia, often referred to as ironwood, is indigenous to Southeast Asia, Malaysia, Northern Australia, Oceania (Potgieter et al 2014) and possibly to Guam and the Northern Mariana Islands (Safford 1905)
Ralstonia solanacearum was found in 18% of the symptomless trees, while neither organism was found in 15% of the nearly dead trees
Though completing Koch’s is beyond the scope of this study, evidence to support G. australe as a likely causal agent includes the presences of fruiting bodies in ironwood tree decline (IWTD) tree sites, the significants of fruit bodies as predicitors of decline in the study and previous studies (Schlub 2010; Schlub et al, 2011a)
Summary
Casuarina equisetifolia subsp. equisetifolia, often referred to as ironwood, is indigenous to Southeast Asia, Malaysia, Northern Australia, Oceania (Potgieter et al 2014) and possibly to Guam and the Northern Mariana Islands (Safford 1905). Cross-sections of these trees exhibited areas of wetwood that were dark, water infused, and radiated from their centers (Mersha et al 2009; Mersha et al 2010a; Schlub et al 2011a). More numerous at the farm, and appearing elsewhere on Guam, were trees with the same cross-sectional symptoms but accompanied by thinning foliage and a lethal progressive dieback. This latter condition is referred to as ironwood tree decline (IWTD) (Mersha et al 2009; Mersha et al 2010a; Schlub et al 2011a)
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