3.8.2S
EPIDEMIOLOGY OF ZOOSPORIC PATHOGENS IN HYDROPONIC SYSTEMS -- THE ROLE OF BIOSURFACTANTS IN DISEASE SUPRESSION

ME STANGHELLINI

Department of Plant Pathology, University of California, Riverside, CA, 92521, USA

Background and objectives
Most of the destructive diseases in hydroponics have been attributed, either directly or indirectly, to root-infecting zoosporic pathogens. These pathogens occur primarily in the following genera: Pythium, Phytophthora, Plasmopara, and Olpidium. Once introduced into a recirculating cultural system, these infectious agents are favored as a result of (i) the abundance of a genetically uniform host, (ii) physical environment with a relatively constant temperature and moisture regime optimum for sustained pathogen reproduction, and (iii) a mechanism for the uniform dispersal of the primary inoculum throughout the entire system via the recirculating nutrient solution. Although numerous disease control strategies have been proposed, most have relied upon the use of chemical pesticides. However, in many countries these chemicals are not registered for use on vegetable crops. Additionally, frequent use of these chemicals has led, in many instances, to the development of pesticide-insensitivity by the target pathogens. Thus, global efforts have been directed towards the search for alternative disease control strategies. One such strategy is biological control, which is the use of antagonistic microorganisms (primarily bacteria and fungi) to either directly or indirectly control target pests. Known mechanisms of antagonism include the following: competition, hyperparasitism/predation, induced resistance, and the production of extracellular metabolites such as antibiotics, hydrogen cyanide, and siderophores. While each of these mechanisms has been implicated as operative in soil systems, their involvement in soilless systems has not been realized. The reasons for their probable ineffectiveness in soilless systems will be reviewed. Recently, we discovered that certain plant-inhabiting bacteria produce novel extracellular metabolites which lyse zoospores. Zoospores, (i.e. unicellular, motile asexual spores), have been documented as the primary, if not sole, life stage responsible for both the spread of these pathogens and the location of appropriate infection sites on roots. Zoospores are also the 'weak link' in the life cycle of these pathogens in that zoospores, as opposed to other life stages, lack a cell wall and are encased solely by a plasma membrane. The absence of a cell wall imparts a high degree of vulnerability to this critical spore stage to the lytic activity of these extracellular metabolites. Our objectives were to identity of these metabolites, to determine their mode of action, and evaluate their potential efficacy for disease control in recirculating cultural systems.

Results and conclusions
We discovered that certain rhizoplane/phylloplane-inhabiting bacteria, primarily Pseudomonas spp., produce extracellular metabolites which have been identified as biosurfactants (1). Purified biosurfactants were shown to selectively destroy the permeability of the plasma membrane of zoospores, the weak-link in the life and disease cycle of zoosporic pathogens. Loss of permeability results in rapid death of the zoospore. Our preliminary results also show that these biosurfactants are produced in situ when these bacteria are added, in the presence of a suitable carbon substrate, to the recirculating nutrient solution. Sufficient quantities of biosurfactant are produced to provide disease control in recirculating cultural systems. Progress and problems in the use of these bacteria for biological control of zoosporic pathogens in recirculating cultural systems will be presented.

References
1. Stanghellini, M.E. and Miller, R.M. 1997. Plant Dis. 81: 4-12.