Biological control of Chilean needle grass (Nassella neesiana, Poaceae) in Australasia. Application to Release.

ANDERSON FREDA E.; LUCRECIA GALLEGO; ROMINA M. SÁNCHEZ; JANE BARTON; DAVID A. MCLAREN

Melbourne

Congreso; 18th AUSTRALASIAN WEEDS CONFERENCE; 2012

Council of Australasian Weed Societes Inc.

Nassella neesiana (Chilean needle grass) is a significant agricultural and environmental weed in Australia and New Zealand and the two countries have worked together for ca. ten years to find a suitable biological control agent to use against it. The only suitable agent found in the home range of the weed (South America), and the subject of this paper, is the rust fungus Uromyces pencanus (Anderson et al. 2010). Most of the pathogens that have been used for classical biocontrol of weeds world-wide have been rusts, and they have never caused unpredicted non-target damage in the field (Barton 2004). Uromyces pencanus has been observed killing infected leaves, and reducing seed production, of N. neesiana in the field in Argentina (Anderson et al. 2006). The rust is particularly damaging in dry weather. Laboratory experiments have confirmed the rust can reduce the growth of infected plants (Giordano et al. 2009). A rust may complete its life cycle on only one host, or it can form some spore types on one host and other types on another (not closely related) host (Kirk, et al. 2008). Uromyces pencanus has been reported in the literature to form three types of spores on N. neesiana: urediniospores, teliospores and aeciospores (Arthur 1925). We have previously shown that aeciospores often found on N. neesiana belong to the life cycle of another rust (Puccinia graminella) (Anderson et. 2010). Teliospores of U. pencanus appear to have lost the ability to produce basidiospores and therefore, the nature of its life cycle can not be categorically proven. It is believed to cycle as urediniospores on its grass host (Anderson et al. 2010). Mycoparasites that exist in Australia are similar to those that exist in Argentina, so there is no reason to believe they will significantly hamper the activity of the rust here. It is extremely unlikely that the introduction of U. pencanus would lead to adverse impacts on native rusts through hybridization. Extensive host range testing has been conducted with a single strain of U. pencanus (UP 27). It was applied to 79 taxa including 14 populations of N. neesiana, 2 populations of the weed serrated tussock and 7 cultivars of wheat (in total, 60 species were tested). UP 27 was shown to be highly host specific, however it did form pustules on two Austrostipa species, A. compressa and A. macalpinei. On these species, spore formation was 30 and 10 times less respectively than on the target plant. When the U. pencanus spores collected from Austrostipa compressa were applied to N. neesiana plants, no infection resulted. Climatic isolation from current and future spread of N. neesiana plants, no infection resulted. Climatic isolation from current and future spread of N. neesiana (Gallagher and Duursma 2012) makes it unlikely that Western Australian populations of A. compressa or A. macalpinei will ever be in close association with N.