Sclerotium rolfsii is a necrotrophic, soilborne fungal plant pathogen that produces abundant white mycelium on infected plants and in culture. Advancing mycelium and colonies often grow in a distinctive fan-shaped pattern and the coarse hyphal strands may have a somewhat ropy appearance. Cells are hyaline with thin cell walls and sparse cross walls. Main branch hyphae may have clamp connections on each side of the septum (Figure 1).[1] In agar plate culture, sclerotia are not formed until the mycelium covers the plate. In vitro or in vivo, sclerotia begin as small tufts of white mycelium that form spherical sclerotia 0.5 to 1.5 mm in diameter (Figure 2). Sclerotia darken as they mature, becoming tan to dark brown in color (Figure 3). Young sclerotia often exude droplets of clear to pale yellowish fluids. Mature sclerotia are hard, slightly pitted, and have a distinct rind. Although most sclerotia are spherical, some are slightly flattened or coalesce with others to form an irregular sclerotium. S. rolfsii does not form asexual fruiting structures or spores.