Structure of microsclerotia of <i>Verticillium dahliae</i> in roots of 'Russet Burbank' potatoes

Abstract

Microsclerotia of Verticillium dahliae in potato (Solatium tuberosum) roots were examined, primarily with the transmission electron microscope. These resting structures were found in all tissues except the xylem, even though the tracheary elements of some microsclerotium-containing roots were infected. Microsclerotia were occasionally present within 9 days of inoculation, but they became more numerous with time. Usually, the microsclerotia consisted of a peripheral region composed of degenerate cells and cells with cytoplasmic contents of moderate electron density and an inner region of cells with very electron-dense cytoplasm. Melanin coated all cells but was more abundant over cytoplasm-containing cells. Roots containing large microsclerotia were moribund. Several living roots contained small microsclerotia, some of which produced penetration hyphae. Living host cells responded to attempted penetrations by producing lignitubers which ensheathed the penetration hyphae, as was previously described by the authors.