Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree (Casuarina equisetifolia) decline in Guam

Abstract

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.