Efficacy of a potential mycoherbicide for control of Alisma lanceolatum and Damasonium minus in Australian rice crops

Abstract

Alisma lanceolatum (Alisma) is becoming a problem weed in NSW rice crops because it is invasive and tolerant of currently used herbicides. Darnasonium minus (starfruit), a widespread native, is a serious competitive weed in rice. The endemic fungus Rhynchosporiurn alismatis, which causes necrotic lesions on leaves and scapes of Alisma and starfruit, is being studied for potential development as a biological control agent for these weeds. Spray application of a conidial suspension to Alisma seedlings in the field at the 2-3 leaf stage, in 2 seasons, resulted in significant suppression of plant growth and markedly retarded development. Lesions developed on emergent leaves and caused premature death. In 1993 assessment of treatments 53 days after inoculation showed that although total leaf number per plant was reduced by 20%, total leaf dry weight was reduced by 34%, and root dry weight by 32%. Inflorescence development was also markedly reduced in both years; inflorescence number was reduced by an average of 77% in 1992-93 and by 52% in 1993-94. In 1994, inoculation reduced the dry weight of healthy leaves at 63 days after treatment by 49%, although root dry weight was not significantly reduced. Competition from a developing rice crop would be expected to reduce plant size and vigour further. Inoculation of glasshouse-grown starfruit seedlings produced similar trends. Spray-inoculation of seedlings 33 days after sowing resulted in 45% reduction in total plant biomass (dry weight, measured 64 days after inoculation) compared with 19 and 25% reduction in biomass in seedlings sprayed 23 and 50 days, respectively, after sowing. Inoculation of starfruit seedlings in the field significantly reduced biomass production by 21% and increased 6-fold the number of necrotic inflorescences. These experiments indicate that R. alismatis has potential for development as a mycoherbicide and a possible role in integrated control of these weeds.