Mimosa pigra dieback in the Northern Territory, Australia: Investigation into possible causes

Abstract

In this study, Mimosa pigra dieback is described as plants or population of plants which appeared to suffer from ill health, death of their growing points and the emergence of green reshoots coming from the base. Dieback is an important phenomenon, not only in many crops and natural forests worldwide, but also in many invasive trees and shrubs in Australia. The dieback phenomenon is thought to be caused by complex and interacting factors and causes are often difficult to disentangle. In some cases it is a result of an interaction between both biotic and abiotic factors. Since 1988, dieback has been widely observed in Mimosa pigra L. (Fabaceae), a woody shrub that is one of the most serious exotic invaders of floodplains and riparian areas in the Top End of the Northern Territory. However, there has been no conclusive evidence of links between M. pigra dieback and specific causal factors. In addition, the dieback phenomenon has not been previously quantified. This thesis aimed to describe M. pigra dieback phenomenon at plant and population levels and its effect on plant growth, survival and demography. Specifically, the aim of this thesis was to evaluate the role of biotic factors (insect borers and fungal pathogens), and abiotic factors (soil pH and salinity), in relation to the development of M. pigra dieback. In addition, the thesis examines the pathogenicity of the fungal endophytes/pathogens associated with dieback-affected and healthy M. pigra. M. pigra has been a focus of biological control programs for more than 30 years, but it still remains a serious threat in the wetlands and conservation areas in the Top End of the Northern Territory. While extensive studies have been conducted to evaluate the impact of biological control programs, little is actually known about dieback and its causal factors. To evaluate the causal factors of M. pigra dieback, repeated ground surveys, plant health assessment and specimen collection of stems and soils, were conducted in the period 2011 to 2013 in established M. pigra sites on Melaleuca Station, Mary River, Northern Territory. Insect borers were quantified and putative fungal pathogen abundance were surveyed. A combined morphological and phylogenetic approach was used for fungal pathogens/endophyte identification. Using a subset of isolates, trials were performed to examine differences in the pathogenicity of the recovered fungal pathogens/endophytes. Results of field research showed that dieback in M. pigra is highly dynamic and fluctuates in its severity. Different sites showed increases and/or decreases in plant health over time. Dieback affects plants in all age classes, from seedlings, juveniles to adult plants, resulting in partial mortality of plants in all age categories over the trial period. Contrary to expectations, none of the biotic and abiotic factors examined showed a direct relationship to the changing population dynamics of M. pigra dieback over time. Nevertheless, fungal isolations demonstrated that M. pigra is a host to a diverse range of endophytes, with the most frequently isolated taxa belonging to the Botryosphaeriales, Xylariales and Diaporthales. Results of the fungal identification studies showed that M. pigra harbours diverse endophyte communities. The use of morphological characters was crude and unreliable. However, species identification using the ITS sequence data was also insufficiently discriminatory while inclusion of sequence data based on EF1-a allowed the discrimination between sibling species, especially among the confusing taxa within the Botryosphaeriaceae. The pathogenicity trials undertaken revealed the potential of fungal species from Botryosphaeriaceae to kill seedlings and cause lesion development in juvenile and adult plants in both controlled and field conditions. Overall, the research findings highlight the challenges in attributing a single factor as being responsible for the dieback phenomenon. This is the first study to investigate M. pigra dieback and the results reveal that M. pigra dieback has potential dramatic plant population-level effects. The results also provide evidence of the ubiquity of the phytophagous insects and fungal endophytes across all sites and seasons. Furthermore, among the 1,352 fungal isolations, it is likely that new fungal species will be identified. However, a number of factors were able to be ruled out in relation to potential causes of M. pigra dieback. Continued efforts are needed to focus on interaction studies of biotic and abiotic factors in relation to M. pigra dieback. The fact that population (stand) dieback in M. pigra occurs in multiple catchments in the Northern Territory, and over large areas, makes the dieback phenomenon to be of considerable ecological importance.