Morphological and chemical characterization of Alternaria populations from apple fruit

Alternaria is a frequent contaminant of apple fruit, causing severe economic losses. It can produce external lesions and mouldy core, characterized by a rotten area in the apple core.

In the present study, morphological and chemical characterization of Alternaria from apples was performed, evaluating differences related to agricultural practices and type of disease.

A low morphological diversity was observed; most of the isolates were identified as A. tenuissima sp.-grp. (95 %). A. arborescens sp.-grp. and A. gaisen sp.-grp. were present in a proportion of 1 %, and 3 % of the isolates showed intermediate characteristics between these sp.-grps. and were identified as Alternaria sp. The chemical diversity was greater; 27 secondary metabolites were produced by the apple isolates. The most frequents were altertoxin-I (85 %), altechromone A (76 %), tentoxin (69 %), and tenuazonic acid (68 %). The alternariols were produced in a lower frequency when comparing with isolates from other crops; alternariol, 58 % and alternariol monomethyl ether, 57 %. The predominant secondary metabolite profile included compounds from different chemical families, such as dibenzopyrones, tetramic acids, perylene quinones, and cyclic tetrapeptides.

A wider metabolomic capacity was observed in isolates from conventional apples when compared to those from organic fruit, with the predominance of strong producers of altertoxins and alternariols. The isolates from mouldy core showed higher ability to produce metabolites from different chemical families than those from external lesions. The wide chemical diversity of the Alternaria apple population should be considered to assess the health risk associated with apple by-products.