Alismataceae weeds in Australian rice crops: Current control methods and the potential for biological control

The Alismataceae family of aquatic plants, consisting of the natives Alisma plantago aquatica and Damasonium minus and three introduced species, Alisma lanceolatum, Sagittaria montevidensis and Sagittaria graminea are considered the most important aquatic broadleaf weeds of rice in Australia (Cox, 1984). Irrigation development, aerial sowing and the introduction of semi-dwarf rice varieties have reduced the competitive ability of the rice plant and contributed to the spread of these weeds throughout the rice growing districts of southern New South Wales. Within the last 20 to 30 years the three introduced species have become prominent throughout the region, having not yet reached their potential distribution (McIntyre and Newnham 1988). Current control methods are largely reliant on the use of a single herbicide, Londax ®. High rates, and often multiple applications have been necessary to control problem weeds and maximise yield potential. Whilst advances in herbicide technology have maintained, if not improved weed control, intensive herbicide programs have led to the development of herbicide resistance in several aquatic species including Sagittaria montevidensis (Arrowhead) and Damasonium minus (Starfruit). Furthennore, social and environmental concerns have slowed the development and registration of rice herbicides and increased the costs of weed control. With fewer herbicides and a cultural system vulnerable to weed induced losses, integrated weed management strategies including the potential for biological control agents will be needed to solve weed problems in rice. The endemic fungal pathogen Rhynchosporium alismatis which produces necrotic and chlorotic lesions on mature leaves and less frequently on petioles and inflorescence stalks of Damasonium minus and Alisma species is being investigated as a potential biological control agent for these weeds. By increasing the pathogenicity and widening the host range of the fungus the use of a fungal bioherbicide to control broadleaf weeds in rice in Australia would become a viable option.