Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 7, 2001
Publication Date: N/A
Interpretive Summary: Species of Stemphylium are microscopic fungi that cause many of the most destructive diseases in agricultural crops. These fungi are traditionally characterized by morphological features that are sometimes obscure, making classification of these organisms difficult. In addition, there has been no consensus using morphology to infer the levels of genetic relatedness (phylogenetics) among these species. The identification of a pathogen is important in order to be able to develop appropriate disease control strategies, to make predictions about the potential destructiveness and the disease-causing capacity of the fungus, and to determine it's relatedness to other pathogens. In this study, we clarified the phylogenetic relatedness of 43 members of the genus Stemphylium and determined the taxonomic significance of host, geographic origin, and asexual stages. The phylogenetic relationships based on the DNA sequence data agreed with the morphological species concepts of most Stemphylium species included in this study. However some characters currently used in the taxonomy of this genus were not useful. This information will be useful to scientists, plant breeders, and regulatory agencies seeking to determine the diversity, international movement, and disease potential of these fungi.
The phylogenetic relationships of 43 isolates representing 16 species of Stemphylium were inferred from ITS and glyceraldehyde-3-phosphate dehydrogenase (gpd) gene sequence data. The results generally agreed with morphological species concepts. Species that were primarily pathogenic to alfalfa were resolved into two separate groups. Stemphylium botryosum and two isolates with morphological characters similar to S. globuliferum had identical sequences at both loci. These two loci in S. vesicarum, S. alfalfae and S. herbarum were nearly identical but differed from S. botryosum. Stemphylium lycopersici and S. xanthosomatis had a single nucleotide difference in the gpd region and identical ITS sequences. Morphological and developmental characters such as sizes and shapes of the conidia, the conidiophores and the ascospores and the size and time of maturation of pseudothecia are useful for diagnosing species. However, other morphological characters such as septum development and small variations in conidial wall ornamentation were not as useful.