Ultrastructure of collarette formation in Fusarium sect. Liseola and some taxonomic implications

Collarette characteristics have not played a significant role in Fusarium taxonomy. The ultrastructural characteristics of first conidium formation on phialides of F. annulatum, F. anthophilum, F. fujikuroi, F. moniliforme, F. proliferatum, F. subglutinans and F. succisae have been studied by transmission electron microscopy. The outer wall of the phialide covers the first microconidium and does not constitute any part of the wall of the conidium. This results in enteroblastic conidiogenesis of the first conidium. The apical portion of the outer wall disintegrates and the remnants constitute the collarette. The phialide of F. moniliforme has a thin and delicate collarette while those of the other species are more robust to varying degrees. F. fujikuroi differs significantly from F. moniliforme in collarette characteristics to preclude the suggested synonymy between these species. Further taxonomic implications are discussed.