Black Aspergillus grape rot and ochratoxin A
Occurrence of ochratoxin A in grapes and grape derivatives
In 1966, Zimmerli and Dick (1996) reported a high incidence of ochratoxin A contamination in Swiss table wine that sparked widespread interest in ochratoxin A in wine and grape juice. The more southerly in Europe the wine originated the higher the incidence and level of ochratoxin A contamination, particularly in red wines. These findings resulted in a major panic amongst consumers, wine producers and national and European regulatory authorities. A number of research programs resulted to increase knowledge of the frequency and levels of contamination, to identify the etiological fungal agents, and to develop possible tools to control ochratoxin A contamination in grapes and grape derivatives. Confirmation of high levels of ochratoxin A in wine produced in the most Southern European regions was provided by market surveys of retail wines in Germany, Italy, Spain, and UK (Battilani and Pietri, 2002). In addition to wine, ochratoxin A also was found in dessert wines, grape juice (especially from red grapes), vinegar (especially balsamic vinegar), and dried vine fruits (Battilani and Pietri, 2002). There have been numerous reports since the mid-1990s of ochratoxin A in wines at levels above the 2 |g/l legal limit. Contamination is related to geographic origin and wine color with the highest ochratoxin A contamination in southern products, and with red wines more contaminated than rosé wines and rosé wines more contaminated than white wines (Battilani and Pietri, 2002).
Ochratoxin A is produced mainly by species in the genera Aspergillus and Penicillium. Initially, A. ochraceus and P. verrucosum were considered to be responsible for the production of the toxin in grapes, similar to the problems in wheat. However, several recent investigations have shown that ochratoxin A in wine is due to contamination by black Aspergilli, primarily strains of A. carbonarius and others belonging to the Aspergillus niger species aggregate that cause a "black rot" disease (Battilani and Pietri, 2002). The disease is particularly severe in the warmer grape-producing southern portions of Spain, France, Italy, and Greece. The black, heavily melanized spores are highly resistant to sunlight and survive sun-drying. The fungi survive on plant debris in the soil and the conidia are disseminated in vineyards by air currents. These fungi infect via injuries caused by careless handling, insects and previous fungal pathogens, usually in mature berries, and then spread throughout the bunch.
The taxonomy of Aspergillus section Nigri is not resolved and the correct identification of the species responsible for ochratoxin A accumulation in grapes remains problematic (Perrone et al., 2006). This point is critical since each species usually has a specific toxin profile and accurate identifications could be used to predict potential toxicological risks. Recently, DNA sequence polymorphisms have been used to improve identifications and help distinguish species in this section (Perrone et al., 2004; Patino et al., 2005). Other studies have combined ochratoxin A production with DNA analysis, to better identify the fungal producers of ochratoxin A (Cabanes et al., 2002; Perrone et al., 2006). In particular, Perrone et al. (2006) used Amplified Fragment Length Polymorphisms with genomic DNA sequences from and toxin production by 77 black aspergilli strains from grape berries, to show that Aspergillus tubingensis is a distinct ochratoxin A-producing species. Thus, A. tubingensis, A. carbonarius and A. niger all can lead to ochratoxin A contamination of wines. Aspergillus carbonarius is currently viewed as the most important fungal source of ochratoxin A contamination since it occurs the most frequently and has the highest proportion of toxigenic strains in field populations (Battilani et al., 2003; Perrone et al., 2006).
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