The prospect of biological control of Mimosa pigra with fungal pathogens in Australia

During surveys in Central and South America two fungal pathogens were identified as high-potential classical biological control agents for Mimosa pigra L. in Australia. The first, a wet-season fungus Phloeospora inhnosae-pigrae H.C. Evans & G. Carrion, was released in Australia in 1994 and the second, the dry-season rust Diabole cubensis (Arthur & J.R. johnst.), in 1996. Field observations in Central and South America and pre-release evaluations in England indicated promising impacts by the fungal pathogens as classical biological control agents, but both failed to establish long-term in Australia. The coelomycete anamorph (asexual form), P. inhnosae-pigrae, was successfully mass-cultured in liquid medium and a standardised culturing and application protocol was developed and tested for large-scale field applications during the wet seasons from 1996 to 1998. The fungal pathogen established temporarily in the field and caused a considerable reduction in growth of mimosa seedlings. However, mature plants were less affected by P. mimosae-pigrae due to premature leaf-drop of diseased plant tissue. In addition, the fungal pathogen failed to develop its sexual form (teleomorph) in Australia, which is assumed vital for its survival. Post-release evaluation concluded that while P. inimosae-pigrae was unable to self perpetuate under the conditions in Australia, and thus was not suitable as a classical biological control agent, it has suitable characteristics and potential to be developed and used as a mycoherbicide for the control of mimosa. The environmental conditions in northern Australia also appeared to prohibit the long-term establishment of the dry-season rust D. cubensis. Mass-production of spores of the rust was labour intensive, yields were inconsistent and spores rapidly lost their viability. The rust caused disease symptoms on plants of mimosa after field inoculations but failed to spread and reinfect new growth of plants.