Conidium and appressorium variation in Australian isolates of the Colletotrichum gloeosporioides group and closely related species

Abstract

Australian collections of the Colletotrichum gloeosporioides group and closely related species were studied to assess the suitability of existing taxonomic criteria and to examine the possibility of using alternative characters in the delimitation of taxa within the group. Conidia produced on free hyphae in slide cultures were consistently more variable than those produced in conidiomata in pure culture. Because of this, only dimensions of conidia from conidiomata should be used in taxonomic work. Appressorium morphology but not size was a useful addition to existing taxonomic criteria, with some isolates producing only unlobed or slightly lobed appressoria and others deeply lobed appressoria. On the basis of dimensions of conidia produced in conidiomata and appressorium morphology three biological groups emerge: isolates with mean conidial widths between 3.0 and 4.2 µm, with either unlobed or slightly lobed appressoria; isolates with mean conidial widths between 4.5 and 5.5 µm, with unlobed or slightly lobed appressoria; and isolates with conidial widths between 4.5 and 5.5 µm, with obviously lobed appressoria. Hyphal conidiogenesis appears to be useful in delimiting taxa only in C. crassipes where hyphal conidia were borne on branched conidiophores that were relatively short and stout. All other collections examined produced hyphal conidia on long, unbranched conidiophores, indistinguishable from normal hyphae.