BACKGROUND : Accurate delimitation of plant pathogenic fungi is critical for the establishment of quarantine
regulations, screening for genetic resistance to plant pathogens, and the study of ecosystem function.
Concatenation analysis of multi-locus DNA sequence data represents a powerful and commonly used approach to
recognizing evolutionary independent lineages in fungi. It is however possible to mask the discordance between
individual gene trees, thus the speciation events might be erroneously estimated if one simply recognizes well
supported clades as distinct species without implementing a careful examination of species boundary. To
investigate this phenomenon, we studied Colletotrichum siamense s. lat., which is a cosmopolitan pathogen causing
serious diseases on many economically important plant hosts. Presently there are significant disagreements among
mycologists as to what constitutes a species in C. siamense s. lat., with the number of accepted species ranging
from one to seven.
RESULTS : In this study, multiple approaches were used to test the null hypothesis “C. siamense is a species complex”,
using a global strain collection. Results of molecular analyses based on the Genealogical Concordance Phylogenetic
Species Recognition (GCPSR) and coalescent methods (e.g. Generalized Mixed Yule-coalescent and Poisson Tree
Processes) do not support the recognition of any independent evolutionary lineages within C. siamense s. lat. as
distinct species, thus rejecting the null hypothesis. This conclusion is reinforced by the recognition of genetic
recombination, cross fertility, and the comparison of ecological and morphological characters. Our results indicate
that reproductive isolation, geographic and host plant barriers to gene flow are absent in C. siamense s. lat.
CONCLUSIONS : This discovery emphasized the importance of a polyphasic approach when describing novel species
in morphologically conserved genera of plant pathogenic fungi.