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<i>Alternaria alternata</i> is one of the most devastating phytopathogenic fungi. This microorganism causes black spots in many fruits and vegetables worldwide, generating significant post-harvest losses. In this study, an <i>A. alternata</i> strain, isolated from infected pears (<i>Pyrus communis</i>) harvested in Italy, was characterized by focusing on its pathogenicity mechanisms and competitive exclusion in the presence of another pathogen, <i>Botrytis cinerea</i>. In in vitro assays, the fungus produces strong enzymatic activities such as amylase, xylanase, and cellulase, potentially involved during the infection. Moreover, it secretes four different toxins purified and identified as altertoxin I, alteichin, alternariol, and alternariol 4-methyl ether. Only alteichin generated necrotic lesions on host-variety pears, while all the compounds showed moderate to slight necrotic activity on non-host pears and other non-host fruit (lemon, <i>Citrus limon</i>), indicating they are non-host toxins. Interestingly, <i>A. alternata</i> has shown competitive exclusion to the competitor fungus <i>Botrytis cinerea</i> when co-inoculated in host and non-host pear fruits, inhibiting its growth by 70 and 65%, respectively, a result not observed in a preliminary characterization in a dual culture assay. Alteichin and alternariol 4-methyl ether tested against <i>B. cinerea</i> had the best inhibition activity, suggesting that the synergism of these toxins and enzymatic activities of <i>A. alternata</i> are probably involved in the competitive exclusion dynamics in host and non-host pear fruits.