Nassella Trichotoma (Nees) Hack Ex Arechav. - Serrated Tussock Nassella Neesiana (Trin. & Rupr.) Barkworth - Chilean Needle Grass

Nassella trichotoma (serrated tussock) and Nassella neesiana (Chilean needle grass) (Poaceae) are significant agricultural and environmental weeds in Australia and New Zealand. A biological control program was initiated in 1999 with detailed field exploration in Argentina and study of pathogens on both species. The potential biological control candidates identified for serrated tussock were either not host-specific (Puccinia nassellae), not sufficiently pathogenic to Australian accessions of the weed (P. nassellae, Tranzscheliella spp.) or their biology and life-cycle could not be fully determined (P. nassellae, Tranzscheliella spp., Corticiaceae sp.), precluding further work. Concurrently, P. nassellae ex Chilean needle grass and two other rust fungi, Puccinia graminella and Uromyces pencanus, were identified as potential biological control candidates for Chilean needle grass. P. nassellae ex Chilean needle grass causes premature senescence of foliage and can be very damaging to this grass in the field. It has proven to be extremely host-specific, to a point where only three out of seven Australian Chilean needle grass accessions have been susceptible to one of the various isolates with which they have been inoculated. Mass rearing and storage of this rust have proven problematic and consequently it has not undergone comprehensive host-specificity testing. P. graminella was found to be damaging to Chilean needle grass and was common and widespread during the initial surveys. However, it later became scarce, probably due to prevailing drought conditions. A large number of Chilean needle grass plants within each accession from both Australia and Argentina were found to be resistant to this rust, thus complicating mass culturing for host-specificity testing. To date, the most promising candidate agent for Chilean needle grass is U. pencanus: it was host-specific during preliminary testing; it is easy to mass culture; its spores can be frozen for later use; and it can be very damaging to Chilean needle grass populations in the field. It has now been approved for release in NZ, which is a historical decision as this is the first pathogen biological control agent in the world approved for release on a grass. The next phase of this project will be to seek permission to release this fungus in Australia.