Biological Control of Chilean Needle Grass (Nassella neesiana, Poaceae) in Australasia: Completion of Host Range Testing

Nassella neesiana (Trin. and Rupr.) Barkworth (Chilean needle grass, CNG, Poaceae) is a Weed of National Significance in Australia and a declared pest plant in parts of New Zealand. Studies have been conducted in Argentina to identify potential biological control agents (pathogens) for this species. The rust Uromyces pencanus Arthur & Holw has been selected for having the greatest potential: it is highly host specific and can cause significant damage to the target weed. Most of the host range testing for the selected isolate (UP27) is now complete. No pustules have developed on any of the test species other than the target. However, there has been some development of the rust, with formation of haustoria, in Piptatherum miliaceum (L.) Cosson and several Austrostipa spp. In addition, some of the inoculated plants of P. miliaceum showed peculiar macrosymptoms (“blisters”) somewhat resembling pustules. Resistance mechanisms were observed to occur in all these species, probably explaining why the rust could not develop further to produce spores. As Austrostipa and Piptatherum are very closely related to Nassella (all in the tribe Stipeae) these results are not unexpected. It is surprising though that penetration, with little further development, was observed in other congeneric Nassella spp. tested. Testing of various isolates of U. pencanus revealed they varied greatly in their ability to attack different populations of CNG. U. pencanus isolate UP27 is able to infect eight out of the nine tested Australian populations and one out of three from New Zealand. An isolate has yet to be found that is able to infect those CNG populations resistant to UP27: those tested so far have failed. Authorities in New Zealand have recently approved the importation of U. pencanus for release in that country. An application to release the rust in Australia will be prepared soon.