Chemotaxonomy of the genus Stemphylium

The filamentous fungal genus Stemphylium is often found on various crops, and especially the common animal feed plant Medicago sativa (alfalfa) is often infected by this plant pathogen. Some members also contaminate food products such as tomatoes and cherries. With this in mind it is important to consider what consequences such an infection/contamination can have, e.g. production of mycotoxins. Identification of such pathogens is pivotal to determine if an infection is to be further handled [1]. Traditionally Stemphylium isolates have been identified using morphological characters, however, correct identification takes a specialist. Particularly S. herbarum, S. botryosum, S. vesicarium, S. alfalfae and S. gracilariae have very similar conidial appearance [3]. Phylogenetic studies using DNA sequencing of Stemphylium have shown that some species within the genus are distinguishable via phylogeny (ITS, GPD) while others are not [4, 5]. Especially, a group of S. herbarum, S. vesicarium, S. alfalfae, S. tomatonis and S. sedicola are almost identical when only considering phylogeny. The purpose of this project was to utilize current chemical analytical methods, to build a profile of secondary metabolites (chemotaxonomy), to attempt to distinguish the species in the genus Stemphylium. The previously mentioned species, which are hard to distinguish in conventional identification methods, were of special interest. A total of 90 isolates were analyzed for chemical production along with morphological characters. Some of the detected compounds were stemphol, alterporriols and altersolanols, of which the last two are shared with the closely related genus Alternaria. Both datasets (morphology and chemical production) were clustered to produce phenograms of the isolates. Some isolates from previous studies [1, 2] were also used in the current study and phylogenies were used to support findings in the current study. The chemotaxonomy did not have a strong resolving power to differentiate the species in question, and it is possible that the previously proposed differences in morphology are not a proof of separate species, but rather variation within the same species.