Antagonistic action of siderophores from Rhodotorula glutinis upon the postharvest pathogen Penicillium expansum V. Calvente, D. Benuzzi, M.I.S. de Tosetti* Area de QuõÂmica Tecnolo Âgica y MicrobiologõÂa Industrial, Departamento de QuõÂmica, Facultad de QuõÂmica, BioquõÂmica y Farmacia, Universidad Nacional de San Luis, 5700-San Luis, Argentina Received 19 August 1998; received in revised form 7 January 1999; accepted 15 February 1999 Abstract Rhodotorulic acid produced by Rhodotorula glutinis strains improved the biological control of blue rot caused by Penicillium expansum in harvested apples. The production of the siderophore was closely associated with the iron concentration in the medium. Thus, very low additions of the metal reduced the siderophore production considerably. The antagonistic eect of R. glutinis and rhodotorulic acid was studied by using in vitro and in vivo assays. In the in vitro assays, rhodotorulic acid reduced the growth of P. expansum, whereas the chelate (rhodotorulic acid plus iron) did not. Siderophore antagonism was then related to competition for iron. In biocontrol assays on apple wounds, the blue mold was more eectively controlled by the antagonistic agent plus siderophore than by the antagonistic agent alone. The disease incidence (DI: percentage of treated wounds that developed rot) was 34% when apples were protected by R. glutinis alone, whereas it was 6% when the fruits were protected by R. glutinis plus rhodotorulic acid. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biocontrol; Siderophores; Postharvest diseases 1. Introduction The decay of harvested fruits and vegetables is the cause of major losses in food production. Penicillium expansum and Botrytis cinerea are well-known posthar- vest pathogens. They produce blue and gray rot, re- spectively (Goepfert, 1980). The control of these phytopathogens is commonly made by using synthetic fungicides. As the use of synthetic pesticide is being restricted (Hileman, 1993), it is necessary to develop alternative methods for controlling postharvest dis- eases. Biocontrol, an alternative to fungicides, employs saprophytic microorganisms that protect fruits and vegetables from infection. However, it appears that control of postharvest diseases with antagonistic microorganisms is not often as consistent as control with synthetic fungicides. Therefore, it may be necess- ary either to improve the activity of antagonistic microorganisms or to combine antagonistic microor- ganisms with other forms of biological control (Wilson et al., 1994 ). Knowledge of mechanisms by which antagonistic microorganisms aect plant pathogens may be useful in the improvement of biocontrol. Competition is believed to be one of these mechanisms (Wilson and Wisniewski, 1994). In particular, competition for iron may play a role in the biocontrol interactions (Raaska and Mattila-Sandholm, 1995). Siderophores, low molecular weight ferric chelating agents, may enhance the eectiveness of a biocontrol agent. There is substantial evidence to believe that siderophores play some role in the biocontrol of phy- topathogenic microorganisms by sequestering iron, and thereby inhibiting pathogen growth or metabolic activity (Riquelme, 1996). However, the role of sidero- phores produced by biocontrol organisms other than International Biodeterioration & Biodegradation 43 (1999) 167±172 0964-8305/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0964-8305(99)00046-3 www.elsevier.com/locate/ibiod * Corresponding author. Ca tedra de MicrobiologõÂa Industrial, Facultad de QuõÂmica, BioquõÂmica y Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, 5700-San Luis, Argentina. Fax: +54-2652-430224. E-mail address: msanz@unsl.edu.ar (M.I.S. de Tosetti)