The objectives of the present study were to purify and assess the killer toxin effect produced by Aureobasidium pullulans under casual agents of green mold (Penicillum digitatum) and sour rot (Geotrichum citri-aurantii). Initially, different methods of protein precipitation were tested. The proteolytic activity and the presence of proteins acting on cell wall receptors, β-1,3-glucanase and chitinase were determined, and toxin purification was conducted by Sephadex G-75 gel exclusion chromatography and cellulose chromatography (medium fibers). Subsequently, purification was confirmed by polyacrylamide gel electrophoresis, and the detection of killer activity was performed in solid YEPDmethylene blue buffered with citrate-phosphate (0.1 M, pH 4.6). Toxin identification was performed by liquid chromatography-mass spectrometry. The results showed that the best protein precipitation method was 2:1 ethanol (vol/vol ethanol/supernatant). It was possible to observe the presence of enzymes with proteolytic activity, including β-1,3-glucanase and chitinase. During the purification process, it was verified that the killer toxin produced by the yeast has a low-molecular-weight protein belonging to the ubiquitin family, which presents killer activity against P. digitatum and G. citri-aurantii.

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Considering the importance of citriculture for the country, this segment of Brazilian agribusiness faces phytosanitary problems that affect quantity and quality of the fruits, reducing nutritional and market values. Fungi are considered the main cause of fruit losses in the post-harvest, and the main diseases that affect citrus are: blue mold (Penicillium italicum), green mold (P. digitatum) and sour rot (Geotrichum citri-aurantii). Chemical control using Imazalil is the most effective method for controlling molds, however, its excessive and indiscriminate use has led to the proliferation of resistant strains, in addition to the persistence of residues of this fungicide on the fruits. Although the fungicides guazatine and propiconazole control the sour rot, they are only registered in countries in Europe, South Africa and Australia. Our review deals with the use of yeasts as biological control agents as an alternative to chemical control, covering: (i) their main mechanisms of action, such as competition for space and nutrients; (ii) production of siderophores; (iii) biofilm formation; (iv) production of antifungal compounds, hydrolytic enzymes and killer toxin; (v) mycoparasitism; and (vi) resistance induction. In addition, procedures for the development and registration of bioproducts are presented and discussed. KEYWORDS: Geotrichum citri-aurantii, mechanisms of action, Penicillium