Biological control of water hyacinth by using pathogens: opportunities, challenges, and recent developments.

There is good justification to renew concerted efforts to develop pathogens for biological control of water hyacinth (Eichhornia crassipes, (Mart.) Solms-Laub. ). Among the most promising pathogens are Uredo eichhorniae, suitable as a classical biocontrol agent, and Acremonium zonatum, Alternaria eichhorniae, Cercospora piaropi, Myrothecium roridum, and Rhizoctonia solani, which are widely distributed in different continents, as bioherbicides. Other, less widely distributed pathogens, notably species of Bipolaris, Drechslera, and Fusarium, may hold promise, but further studies are needed to confirm their usefulness. Ongoing studies on the biology of U. eichhorniae are expected to help resolve the biocontrol potential of this rust fungus in the near future. It is also anticipated that recent advances in bioherbicide technology, and previous experience with pathogens of water hyacinth, will enable development of effective bioherbicides. Success in this effort will require the use of highly virulent pathogens and pathogen strains as well as novel formulations that help to counter water hyacinth’s ability for rapid growth under different site conditions and assist the pathogen to overcome environmental limitations. In Florida, USA, our aim is to develop a bioherbicide that could be used in integration with the existing suite of introduced arthropod biocontrol agents and improve the overall effectiveness of the biological control strategy under different weed-control scenarios. In this attempt, unlike in previous unsuccessful attempts, we are aided by the availability of a large and diverse collection of highly virulent pathogen strains to choose from, the facility to formulate effective pathogens in newer materials to assure consistency of performance, the ability to integrate dual and multiple pathogens, and a ‘biofriendly’ posture of regulatory agencies toward the development and registration of bioherbicides. Results from our ongoing field trials suggest the feasibility and commercial potential of our current approach. THERE are several good reasons to consider pathogens as biocontrol agents: pathogens can cause significant reductions in water hyacinth biomass, especially following natural disease outbreaks, after severe insect attacks, or when used as inundative bioherbicide agents (Charudattan et al. 1985; Shabana et al. 1995b). Published accounts of natural declines in water hyacinth populations following natural disease outbreaks further confirm the potential of pathogens in limiting water hyacinth populations (Martyn 1985; Morris 1990). Controlled experimental studies have confirmed the potential of Acremonium zonatum, Alternaria eichhorniae, and Cercospora piaropi to control (i.e. reduce weed biomass) water hyacinth (Martyn and Freeman 1978; Charudattan et al. 1985; Shabana et al. 1995b). In addition, it has been well proven that pathogens can be successful as classical or inundative (bioherbicide) agents. Currently, worldwide about 15– 20 weeds are biologically controlled with pathogens (Rosskopf et al. 1999). Therefore, there is clear justi* Center for Aquatic and Invasive Plants, Plant Pathology Department, University of Florida, Gainesville, FL 326110680, USA. Email: rc@gnv.ifas. ufl.edu