Phylogenetic analysis of Alternaria spp. associated with apple core rot and citrus black rot in South Africa

Abstract

Dry core rot of apple (DCR) and Alternaria black rot of citrus (ABR) have in the past respectively been ascribed to Alternaria alternata and A. citri. In recent years, however, it has been speculated that several other species of Alternaria could also be associated with these diseases. In an attempt to elucidate the identity of these taxa, 25 isolates associated with DCR, and 26 isolates associated with ABR were selected for molecular characterisation. Nucleotide sequences of 1116 sites including the histone gene section and the internal transcribed spacers (ITS 1 and 2) of the rRNA gene were determined for these isolates. The gene trees generated from the individual and combined data sets using maximum parsimony, maximum likelihood and neighbour-joining analysis methods distinguished five clades with strong bootstrap support, namely Alternaria sp., A. arborescens, A. infectoria, A. tenuissima, and a clade containing isolates variable in morphology, referred to as the Alternaria group. In the alignment of the combined ITS and histone data set, unique transition/transversion substitutions, as well as positional insertions and deletions were observed for each of the above clades. In addition, key sequences in the form of serial composing nucleotides in both the ITS and histone sections of the alignment were also discovered for the molecular identification of A. arborescens, A. infectoria and A. tenuissima. The final phylogeny also indicated that no host specificity existed among the species associated with these two post-harvest disease complexes. Contrary to the host specificity observed on leaf diseases of these hosts in the field, it appears that the post-harvest diseases are the result of adverse storage conditions and opportunism of different small-spored Alternaria spp.