1Postharvest Unit, and 2Dept. of Food Technology, UDL-IRTA, 177 Rovira Roure Ave, 25198-Lleida, Catalunya, Spain

Background and objectives
The growing concern for reduction in the use of pesticides, and the development of resistance to chemical compounds by the major post-harvest pathogens, have resulted in interest in biological control methods to control post-harvest diseases. Candida sake strain CPA-1 has been demonstrated to have antagonist activity against the major post-harvest pathogens of apple fruits [1] and is being registered in the EU for this purpose. An epiphytic bacterium isolated in Lleida, Spain from apples, identified as Pseudomonas syringae strain CPA-15, was also a very effective agent of post-harvest diseases of apple and pear fruits. The objective of this research was to employ this bacterium as biocontrol agent and use it in combination with C. sake strain CPA-1, to improve their control of major post-harvest diseases on apples and pears fruits.

Materials and methods
Laboratory screening: fruits were wounded (3x3x3 mm) at the stem and calyx end. P. syringae alone, or in combination with C. sake, were applied to each wound followed by inoculation of pathogens, and stored at 20C for 7 days.
Small-scale trials: fruits were treated as described previously to evaluate the effectiveness of P. syringae alone or P. syringae+C. sake for control of P. expansum stored at 1C for more than 60 days.
Population dynamics: the population dynamics of P. syringae were studied in vitro in NYDB medium at 25 and 1C. At refrigerated temperature the population dynamics were studied on wounded pears dipped in P. syringae and P. expansum. These fruits were stored under controlled and conventional atmosphere conditions.
Population dynamics of the mixture: cultures of P. syringae and C. sake were grown in NYDB medium in different combinations, at 25C.

Results and conclusions
The best results were obtained against Botrytis cinerea on pear cv. Bianquilia, against Rhizopus stonolifer on Golden Delicious apples, and against P. expansum on pears cvs Bianquilia and Conference, and Golden Delicious apples, in laboratory tests at 20C for 7 days. P. syringae (CPA-1 5) reduced blue mould 98% in pear cv. Bianquilia and 72% in Golden Delicious apples after 60 days at 1C. The antagonists controlled blue mould on apples stored for 70 days at 1C more effectively when combined, than individually. In combination the rot diameter reduction was 90%, while when treated with C. sake alone it was 77%, and with P. syringae, 63%. The maximum population at 1C was 3.8x107 cfu/ml after 23 days and at 25C, 2x108cfu/ml after 31 h. In vitro assays demonstrated P. syringae can grow actively at refrigeration temperatures. Populations of C. sake (CPA-1) alone in in vitro assays reached the same level as in the mixture with P. syringae (CPA-15) after 36 h at 251C. However, populations of P. syringae (CPA-15) in the mixture were consistently lower than when used alone.

< b> References
1. Vinas I, Usall J, Teixido N, Sanchis V, 1998. International Journal of Food Microbiology (in press).