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 diebackaffected 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.iii 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 endop...

First report of oomycetes associated with the invasive tree Parkinsonia aculeata (Family: Fabaceae)

“…Aboveand below-ground biomass of blackberry decreased during regular flooding events when infected with these species, which is consistent with the common method of Phytophthora dispersal (Aghighi et al 2015). In other research on woody weed dieback in arid zones, Phytophthora species were likely assumed to be unimportant since many dieback-affected invasive tree species are found in very dry climatic conditions (Raghavendra et al 2017) with little evidence of root disease, and few studies focused on sampling roots (Haque 2015;Sacdalan 2015;Diplock 2016). If dieback is observed in regions with intermittent flooding or extended wet seasons, it is more likely that Phytophthora species are involved.…”

“…Dieback in several important woody weedy trees and shrubs in arid, semi-arid and wet-dry tropical habitats is a widely-researched phenomenon due to the potential for the biotic causal agent(s) of dieback to be used in biological control of these invasive trees (Raghavendra et al 2017). However, most invasive plant dieback research has focused on putative pathogens derived from stem tissue, in the form of endophytic higher fungi (Haque 2015;Sacdalan 2015;Diplock 2016), and few have considered the involvement of Phytophthora species. An exception is the research on European blackberry (Rubus anglocandicans) decline in Western Australia (WA; Aghighi et al 2015), associated with Ph.…”

Use of Stem Implanted Bioherbicide Capsules to Manage an Infestation of Parkinsonia aculeata in Northern Australia

“…An inoculation methodology, progressively developed in [ 6 ], proved successful in establishing dieback within healthy individuals using French white millet (grain) colonised with certain species selected from the fungal bank. Subsequent research developed an advanced process [ 8 ] in which colonised millet was formulated in either gelatin or hypromellose pharmaceutical capsules which has been utilised by other members of this research group [ 9 , 10 , 11 ]. This technology, based on Australian patent # AU 2009201231 B2, was further developed into a mechanised delivery system [ 12 ], which was used in this research.…”

Application technology for bioherbicides: challenges and opportunities with dry inoculum and liquid spray formulations

“…indica 13 and Mimosa pigra. 14 2.1 Case study: Parkinsonia aculeata control A field study was conducted at the 16.4 km 2 Alexandria Station in the Northern Territory of Australia. A test site adjacent to a water body (Corporal Dam) comprising two sections was selected for the research.…”

“…Examples of other woody species controlled with similar bioherbicide systems from the University of Queensland research include mimosa bush, Vachellia farnesiana , 11 Chinee apple Ziziphus mauritiana , 12 prickly acacia Vachellia nilotica subsp. indica 13 and Mimosa pigra 14 …”