The cacao pathogen Moniliophthora roreri (Marasmiaceae) produces rhexolytic thallic conidia and their size is influenced by nuclear condition

•Moniliophthora roreri produces asexual spores in a thallic and rhexolytic manner.•Conidia and hyphal cells have variable number of nuclei, mainly one or two.•Binucleate are larger than mononucleate spores.•Binucleate spores are produced at different proportions regardless of variety.

Moniliophthora roreri, the causal agent of frosty pod rot of cacao, is a member of the mushroom-forming family Marasmiaceae (Marasmiineae, Agaricales, Basidiomycota). Yet, M. roreri has never been observed to produce a mushroom fruiting body, but rather produces billions of spores on the surface of infected pods. The question of whether these spores are produced via meiosis or mitosis has been the subject of some speculation. However, numerous molecular-based studies have been unable to support a hypothesis of sexual recombination for this fungus. We re-examined sporogenesis and the nuclear condition of hyphae and spores in M. roreri via nuclear staining and spore germination studies. Conidia are produced asexually in a thallic and rhexolytic manner as is true for other Marasmiineae species such as M. perniciosa, Flammulina velutipes and Marasmius puerariae. We also found that hyphal cells as well as spores harbor one or two nuclei, rarely three, that conidium size is influenced by number of nuclei within, and that individual isolates produced consistently and significantly different proportions of binucleate and mononucleate spores regardless of varietal group.