European Blackberry (Rubus anglocandicans A. Newton) is one of the top 20 Weeds of National Significance in Australia. It is a major weed of conservation areas, particularly in wetter regions, and is also a major weed of forestry and agriculture. Rubus anglocandicans is the most abundant and widespread species of European blackberry in the south-west of Western Australia (WA). Herbicides and cultural control methods are generally ineffective, or require multiple applications; however, this weed is often located within inaccessible areas, which limits control options. Therefore, biological control has been identified as the main option for the control of blackberry in Australia.\ud \ud Biocontrol started in the 1980s, initially with the appearance a strain of the host-specific rust Phragmidium violaceum in Victoria. This, so called “illegal strain” and later the official strain of the rust were eventually spread to WA, but they failed to provide control. The blackberry species specifically targeted was R. anglocandicans – the main species in the Manjimup-Pemberton area and along the Warren and Donnelly Rivers in the south-west of Western Australia. New strains of the rust were released in 2004 and 2005. In some areas the levels of rust developed on blackberry was high, at least initially.\ud \ud While monitoring for rust control, the presence of blackberry decline was noticed. The extent of the disease, with noticeable changes to vegetation structure, from an impenetrable tangle of vegetation to a parklike setting of trees and grass, following the disappearance of dense blackberry infestations, has lead to it being called “blackberry decline”. More detailed examination of the decline sites suggested that the decline was not due to the inoculated rust, but possibly due to a root pathogen.\ud \ud Surveys between 2010 and 2012 led to the recovery of ten different Phytophthora, nine Pythium species and Cylindrocarpon species. The surveys also identified other abiotic and biotic factors such as landforms and grazing that appear to be associated with the decline of blackberry. The Phytophthora species isolated included a new species from Phytophthora clade 6 which was described as P. bilorbang as a part of this study. The other Phytophthora species included P. cinnamomi from decline-free sites, and P.amnicola, P. cryptogea, P. inundata, P. litoralis, P. multivora, P. taxon personii, P. thermophila, and a P. thermophila-amnicola hybrid from decline sites. Primocane under-bark inoculations and pot infestation trials in the glasshouse provided evidence of the pathogenicity of P. bilorbang and P. cryptogea to R. anglocandicans. In a dual combination trial to examine synergistic effects between Phytophthora species, disease severity increased by combining at least two to three species including P. amnicola, P. bilorbang, and P. cryptogea under a regime of regular waterlogging. In an in planta under-bark inoculation trial in the field to confirm the pathogenicity of Phytophthora species in the blackberry decline with and without application of phosphite, phosphite reduced the size of lesions caused by all Phytophthora species. Extensive ‘on-ground’ surveys showed the “decline” to extend along at least 64 km of riverbank, and at present is only known from the Warren and Donnelly River Catchments. \ud \ud In this project, the etiology and epidemiology of the decline distribution have been investigated and a conceptual model, a “blackberry decline spiral” is proposed to describe the key factors that are hypothesised to be involved in the decline phenomenon of R. anglocandicans. This model includes predisposing, inciting and contributing factors. It is assumed that predisposing or stress factors such as periodic flooding set the stage for inciting factors (e.g. lack of genetic potential in R. anglocandicans and grazing by animals). Whilst contributing factors (e.g. Phytophthora species as root pathogens and leaf rust) included in the blackberry decline spiral all have a role in this syndrome, the involvement of the hypothesized predisposing and inciting factors are also essential for the expansion of the decline. \ud \ud This thesis has shown blackberry decline to be a complex syndrome made up of a number of factors, most significant of which are periodic flooding and damage to the roots by at least two Phytophthora species, P. bilorbang and P. cryptogea
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