Scanning Electron Microscopy of Sclerotia of Sclerotinia trifoliorum and Related Species

The microanatomy of immature ‘white’, ‘slightly pigmented’ and mature, 1‐month‐old ‘black’ sclerotia of Sclerotinia trifoliorum , S. sclerotiorum , and S. minor were studied by scanning electron microscopy (SEM). A surface mycelial network was present over sclerotia at maturity. Also dried exudate on the superficial, sclerotial cells at maturity was observed. At this stage of morphogenesis an outer layer of the wall of medullary hyphae was synthesized. Two zones (i.e., rind and medulla) of hyphal tissue in sections of mature sclerotia were distinguished. The wall of rind cells was thick and one‐layered, whereas the wall of medullary hyphae was thick and bi‐layered. No lacunac (intercellular spaces) in sclerotial rind were found but the sclerotial medulla appeared to be lacunate in all three species. At the SEM level the structural organization of sclerotia of S. trifoliorum was identical to that one of sclerotia of S. sclerotiorum and S. minor. Thus, in the conducted investigation of the sclerotial stromata, a unique, structural characteristic of taxonomic importance to distinguish S. trifoliorum from the other Sclerotinia species was not found. Observations on the sclerotial morphogenesis in S. trifoliorum and the related species agree with and supplement the light and transmission electron microscope studies of other researchers.