In vitro effects of methylene bisthiocyanate (MBT) on hyphal morphology and ultrastructure of Ophiostoma floccosum were examined using differential interference contrast, epifluorescence and transmission electron microscopy (TEM). Differential interference contrast microscopy suggested that MBT caused rapid changes to the morphology of O. floccosum resulting in excessive formation of vacuoles and granules within the cytoplasm. Epifluorescence microscopy indicated that damage to the plasma membrane occurred in the MBT treated, but not the control, hyphae. TEM showed that in MBT treated hyphae, the plasma membrane had retracted from the cell wall. Cytoplasmic aggregation and increased vacuolation were also observed. Furthermore, complete disintegration of the cytoplasmic organelles was seen in more advanced stages of MBT induced damage. In contrast, plasma membrane of untreated hyphae closely adhered to the cell wall showing cytoplasma with typical cell organelles. Based on the results from this study we suggest that the primary site of mode of action of MBT occurred at the plasma membrane, which then triggered subsequent changes within the cytoplasm of the test fungus.

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