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The production of rhodotorulic acid, a siderophore synthesized by Rhodotorula strains, was improved with the objective of achieving the biocontrol of phytopathogenic moulds. Rhodotorulic acid increased up to 60% in the presence of urea as a nitrogen source, pH near to 8 and a C:N ratio of 8:1. The siderophore-containing spent medium showed in vitro antifungal activity against important plant pathogens including Botrytis cinerea, which causes grey mould on a wide variety of host plants including numerous commercial crops. The antifungal activity was related to siderophore concentration.

Antagonistic activity of the mixtures Rahnella aquatilis-Rhodotorula glutinis and R. aquatilis-Cryptococcus laurentii was assessed against Penicillium expansum (cause of blue rot) and Botrytis cinerea (cause of grey rot) on apple fruit at 4°C and 95% relative humidity (RH). Under these cold-storage conditions, the mixture R. aquatilis-R. glutinis inhibited the development of B. cinerea and P. expansum in apples stored for 40 days and reduced the incidence of disease produced by these moulds to nearly zero. The other mixture, R. aquatilis-C. laurentii, was less effective; the incidences of the grey rot and the blue rot were about 25% and 15%, respectively. Population dynamics of the mixtures showed that the growth of R. aquatilis was strongly stimulated by the presence of the yeast R. glutinis, but in the case of the mixture R. aquatilis-C. laurentii, the same effect could not be observed. In this study, it was demonstrated that the combination of two microorganisms with different requirements and antagonistic abilities resulted in a successful mixture against the two pathogen molds, B. cinerea and P. expansum. In addition, it was also proved that it is possible to improve the biocontrol of these pathogens without increasing the inoculum size of the antagonist (alone or in mixtures), which was always 10 6 cell/ml, despite the high concentration of the pathogen (10 6 conidia/ml) utilized.

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