Alternaria toxins and plant diseases: an overview of origin, occurrence and risks

The genus Alternaria includes both plant-pathogenic and saprophytic species, which may affect crops in the field or cause harvest and postharvest decay of plant products. The taxonomy of the genus Alternaria is not well-defined yet. A polyphasic approach based on morphological features, phylogeny and toxin profiles could be the key to a correct identification at species level and the evaluation of mycotoxin risks associated with fungal contamination. Species of Alternaria are known to produce many metabolites, mostly phytotoxins, which play an important role in the pathogenesis of plants. However, certain species, in particular the most common one A. alternata, are capable of producing several mycotoxins in infected plants and/or in agricultural commodities. The major Alternaria mycotoxins belong to three structural classes: the tetramic acid derivative, tenuazonic acid; the dibenzopyrone derivatives, alternariol, alternariol monomethyl ether and altenuene; and the perylene derivatives, the altertoxins. The toxic effects of the Alternaria toxins have not yet received the same attention as the biological activities of other mycotoxins. However, the Alternaria mycotoxins should not be underestimated since they are produced by several Alternaria species frequently associated with a wide range of diseases in many plants of a high agrifood value. The major problems associated with Alternaria mycotoxin contamination of agricultural products are illustrated by focusing on various crops and their relevant diseases, e.g. black rot of tomato, olive, and carrots; black and grey rot of citrus fruits; black point of small-grain cereals; and Alternaria diseases of apples.