284 Sixteenth Australian Weeds Conference Summary Parkinsonia (Parkinsonia aculeata L., Fabaceae) is an exotic, leguminous woody weed that threatens much of rangeland Australia. A dieback dis- order has been widely observed on parkinsonia plants in Australia. This study focuses on screening isolates, collected from affected trees in the field, against ger- minating parkinsonia seedlings. Preliminary results show that some of these isolates are able to cause death within 12 days of inoculation, indicating potential for development of a pathogen-based mycoherbicide to reduce recruitment. Keywords Parkinsonia, dieback, seedlings, isolate screening, biocontrol agent. INTRODUCTION Parkinsonia is a weed of national significance that impacts on the environment and the pastoral industry in rangeland Australia (Deveze 2004). A dieback disorder which causes high mortality of parkinsonia plants has been widely observed in Australia for at least several decades. As part of a study of this phenom- enon, approximately 200 fungal accessions have been isolated from dieback-affected trees across Australia (N. Diplock unpublished data). To date, three different genera from this isolate bank have been shown to be capable of inducing significant stem lesions on parkin- sonia trees after inoculation (N. Diplock unpublished data), thereby supporting the possibility of developing mycoherbicides based on endemic fungal pathogens. Parkinsonia seed is protected by a tough coat and is tolerant of most environmental conditions. Dormancy can be broken by hot, wet conditions (van Klinken et al. 2006) and fire (Deveze 2004). Seed germination in northern Australia occurs during the wet season (generally December to March) (Deveze 2004). Ecological research has shown that seedlings are particularly sensitive to fire, drought or inundation (van Klinken unpulished data). In this paper, 84 fungal isolates were carefully selected from an isolate bank maintained at the Univer- sity of Queensland (Diplock et al. 2006) to represent different collection sites and morphological charac- teristics to ensure the widest range of possibilities. These isolates were screened for pathogenicity against Evaluation of fungal isolates for potential use as mycoherbicides for seed bank reduction of Parkinsonia aculeata Ruey Toh 1 , Victor J. Galea 1 , Naomi Diplock 1 and Rieks D. van Klinken 2 1 School of Land, Crop and Food Sciences, University of Queensland, Gatton Campus, Queensland 4343, Australia 2 CSIRO Entomology and CRC for Australian Weed Management, Indooroopilly, Queensland 4068, Australia Email: s4088734@student.uq.edu.au germinating parkinsonia seedlings using a specific bio- assay. The aim of this study was to determine whether any of these isolates were lethal to seedlings, and therefore might have potential as mycoherbicides. MATERIALS AND METHODS Preparation of inoculum Millet seed was rinsed twice in distilled water, soaked for 24 hours, and rinsed for a third time before being transferred into plastic McCartney tubes (11.60 g prepared millet seed per tube), and autoclaved twice within 24 hours (N. Diplock unpublished method). Fungal isolates were initially divided into 16 locations according to the GPS coordinates of the collection sites (N. Diplock unpublished data), and replicates were removed from the list. Sub-culturing was carried out from master culture plates using half- strength PDA (potato dextrose agar), according to the selection list. The fungi were incubated at 25°C. In cases where bacterial contamination was present, full-strength PDA with antibiotics was used. This medium contained Penicillin (Sigma ® , Penicillin G sodium salt) 300 ppm and Streptomycin (Sigma ® , Streptomycin sulfate salt) 200 ppm. The tubes containing autoclaved millet were in- oculated with a 10 × 10 mm piece of actively growing culture and incubated at 25°C until fully colonised. The tubes were then placed unsealed in the laminar flow cabinet and allowed to partially dehydrate and were then stored at 4°C. Seed germination Intact seeds (collected from the Caerphilly Station near Charters Towers, QLD) were washed with sterile water, bleached for five minutes in 2% NaOCl, and washed again with sterile water, then dried in the laminar flow cabinet before being clipped at the end nearest to the embryo groove, with a disinfected nail clipper (J. McKenzie personal communication). Clipped seeds were germinated in surface-sterilised Petri dishes on moist sterile filter paper in darkness at 25°C. Seeds were deemed ready for transplanting once the radicle extended over half the length of the seed (generally after two days of incubation).