Summary
Invasive weedy unpalatable grasses constitute arguably the worst biological problems in agriculture and the environment. They out-compete both improved pasture grasses in agriculture and native grasses, which are the most threatened group of the Australian flora apart from orchids (Dept of the Arts, Heritage and Environment 2006). In agriculture, they constitute economic loss due to reduction in carrying capacity for livestock, land value, direct costs of labour and herbicides used to control them (Natural Resources and Mines 2001), and indirect environmental costs of off-target effects such as run-off into streams (Dept of the Arts, Heritage and Environment 2006) and uptake by invertebrates and other components of the food chain. Control currently relies on prevention (efforts to avoid introducing weeds into new properties), eradication (grubbing before seeding) and, if that is not possible, chemical control by broad-acre spraying or weed-wiping (Bray and Officer 2007).
There are few selective herbicides for grasses within pasture, which is composed primarily of grasses. Using non-selective herbicides such as glyphosate (trade names Roundup and Zero) leaves bare ground that is even more susceptible to dominance by the weedy grasses as their seeds germinate and out-compete other grasses (Bray and Officer 2007). Using herbicides leads over time (it is estimated about 17 successive applications are needed) to herbicide resistance, which exacerbates the problem (Storrie 2007). The weedy Sporobolus grasses (WSG) comprise a group of about five species that have been introduced to Australia and have invaded northern NSW and Queensland (Bray and Officer 2007). They have a current distribution of almost half a million hectares and a potential distribution of over 60% of Queensland and 30% of NSW. All are large (1-2 m tall) tussocky grasses with tough foliage that is hard for cattle to chew and digest and leads to reductions in productivity and condition (Betts and Officer 2001). All also seed prolifically from a young age, with an average-sized tussock producing about 200 seed heads annually, each distributing hundreds of tiny (1-2 mm long) seeds.
WSG have been declared noxious and regionally prohibited in both states (Australian Weeds Committee 2010). Two of the most successful are S. fertilis (Giant Parramatta Grass GPG) in northern NSW and southern Queensland and S. pyramidalis (Giant Rat's Tail grass GRT) throughout Queensland). The most popular method of chemical control is the Group J herbicides, flupropanate and 2,2-DPA, which kill these and some other tough weedy grasses selectively but are slow to act, have long withholding periods, long residence times in soil and cannot be used with lactating cattle (Natural Resources and Mines 2001).
Recently, GPG was shown to be resistant to both of these selective herbicides in the Grafton area in northern NSW (Ramasamy 2008; Ramasamy et al. 2007b; Ramasamy et al. 2008) and it is likely that such resistance has already been selected in other WSG, as in serrated tussock in Australia (McLaren et al. 2006). An ideal solution to the problem of controlling the WSG, in particular GPG and GRT, is biological control, which has the potential to be selective and long-lasting. Selective agents are rare among grasses because of the close relationships among the grass family (Grass Phylogeny Working Group 2001), but some have been developed (Yandoc et al. 2005; Yobo et al. 2009). A search for potential biological control agents for WSG in South Africa failed to find any (Palmer et al. 2008). Recently two fungi have been observed in Australia as potential biocontrol agents for GPG: Nigrospora oryzae and Fusarium sp.
