2.10.15
BIOCHEMICAL CHARACTERIZATION OF ANTAGONISTIC PSEUDOMONAS FLUORESCENS STRAINS INVOLVED IN BIOLOGICAL CONTROL OF BROWN SPOT OF PEAR

J ALEMANY, A BONATERRA, J CAMPS, E MONTESINOS

Institute of Food and Agricultural Technology-CERTA, University of Girona, Avda. Luis Santala, s/n. 17003, Girona, Spain

Background and objectives Several strains of Pseudomonas fluorescens and Erwinia herbicola were isolated from a wide range of environments and were characterized for their in vitro antagonism to Stemphylium vesicarium, the causal agent of brown spot of pear, Penicillium expansum and other plant pathogens. These strains were grouped in relation to the utilization of several nutritional media, and to their antagonistic capacity. These strains have been also characterized in their ability to produce cyanide and it has been correlated to their antagonistic capacity, biovar and origin. The aim of this work was to characterize the production of antifungal metabolites produced by the most antagonistic strains P. ;fluorescens EPS288 and P. ;fluorescens EPS375.

Materials and methods Different growth conditions such as temperature and growth media have been assayed in order to determine the pattern of cyanide production by the strain EPS288. Cyanide was detected by the alcaline picrate and spot test methods. Culture supernatants and the extracts resulting from an organic solvent extraction of some media where the strain EPS375 has grown, have been assayed for antifungal activity against S. ;vesicarium and P. ;expansum by different bioassay tests [1].

Results and conclusions
The strains that produce cyanide were isolated mainly from the roots of different plants. Strain EPS288 is the most effective in the biological control of brown spot of pear under controlled environment conditions and produces cyanide in the late exponential phase and at different levels depending on the growth media (up to 1 ;mM). The media conductive to cyanide production were Luria Beriani, King B and growth media at 21C with different carbon sources. Strain EPS375 was also very efficient in the biological control of brown spot of pear but do not produces cyanide, although it is antagonistic to S. ;vesicarium in vitro. Strain EPS375 produces some metabolites that can be isolated by organic solvent extraction. These extracts inhibit the germination of fungal conidia in bioassays. The metabolites are now being characterized.

References
2.Cartwright DK,.Chilton WS and Benson DM 1995. Applied Microbiology and Biotechnology, 43, 211-216.