Morphological variation in isolates of Verticillium albo-atrum R. & B.

The morphology of resting mycelium in isolates of Verticillium albo-atrum varied according to cultural conditions; on natural media hyphae were modified to form unbranched, swollen, pigmented, resting mycelium whereas on artificial media the hyphae became highly segmented, torulose, and intertwined to form mycelial knots. In one isolate, aerial hyphae sometimes developed external bands, or 'blisters,' of electron-dense material deposited in discrete regions along the length of the hyphae. The function of these modified aerial hyphae is probably different from that of resting mycelium and might represent an early stage in the formation of some hitherto undescribed structure.