| Summary: | Alternaria brown spot, caused by <i>Alternaria alternata</i> (Fr.) Keissl, is an important disease in tangerines and their hybrids, affecting leaves, twigs, and immature fruit. Differences in susceptibility to this pathogenic fungus have been described for different Citrus species. In this paper, the expression of the mycotoxins alternariol and alternariol monomethyl ether in different <i>A. alternata</i> isolates was analyzed by HPLC-MS. A correlation was observed between the mycotoxins content and the pathogenicity of each isolated of <i>A. alternata</i> used, suggesting that the mycotoxins may be involved in the evolution of brown spot in ‘Fortune’ fruits caused by this fungus. The increased expression of the above mycotoxins was associated with the end of mycelia growth, high sporulation, and an increase in hyphal melanization in the fungus. On the other hand, the presence of laccase activity in the xylem of ‘Fortune’ fruits inoculated with <i>A. alternata</i> suggests that this is the way the fungus propagates in the plant. These results add to our knowledge of the pathogenesis of <i>A. alternata</i> in Citrus.
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