Click on any
image for a more
detailed view

Pathogen Biology

Sclerotium rolfsii (teleomorph Athelia rolfsii)

Asexual Stage (anamorph)
Sclerotium rolfsii produces an abundant white, coarse mycelium on infected host tissues usually 3-4 days after infection when conditions are warm and humid (Figure 19). The main branch hyphae are relatively large (5-9 microns in diameter) compared to many other fungi that more typically have hyphal diameters of 2-4 microns. These main branch hyphae are described as hyaline (colorless) and thin-walled with infrequent cross walls and clamp connections (Figure 20). Smaller diameter (2-4 microns) hyphal cells, called ‘feeding branches’, arise from and penetrate the plant tissue. When observed with the naked eye, the mass of hyphae appears white, and the thick, large hyphal cells will be abundant enough on infected tissues to form a white fungal mat on lower stems and at the soil surface.


Figure 19	Figure 20

About seven days after infection, the hyphae begin to form sclerotia. Spherical, fuzzy bodies begin to form from closely packed areas of hyphae or where two hyphal strands cross each other. Gradually the bodies become smooth and change in color from white to light tan to brown and possibly to black (Figure 15 ). Mature sclerotia typically consist of an outer thickened and tough rind (2-4 cells thick) which surrounds a cortex of thin-walled cells (6-8 cells thick). The center of the sclerotium contains loosely arranged filamentous hyphae. Sclerotia are typically 0.5 - 2 mm diameter, but some can become as large as 8 - 10 mm diameter. Segments of hyphae can serve as inoculum, and the fungus can overwinter as mycelium in infected plants, plant debris, or as sclerotia. Sclerotia may remain viable for several years in soil, potting media, or on plant debris in areas with mild winters. Studies have shown that S. rolfsii sclerotia survived when buried just below the surface, at 15 cm and at 30 cm depths for one winter in two locations in Iowa. S. rolfsii does not produce any asexual spores.

Sexual Stage (teleomorph)
In 1926, the sexual stage of this basidiomycetous fungus was first described in Japan. The currently accepted name for the teleomorph is Athelia rolfsii. The sexual stage is not commonly seen. As in other Basidiomycetes, A. rolfsii produces a structure called a basidium in which meiosis occurs. Four haploid basidiospores are produced at the tips of small structures on the basidium called sterigmata (Figure 21). Athelia rolfsii produces basidia in an unprotected layer (hymenium), which develops under humid conditions at the margins of lesions. The hymenium appears as a white, yellow, or buff-colored granular or encrusted area with a slightly wavy surface. The basidia are obovoid (oval shaped with one end being narrower than the other), 7-9 microns long and 4-5 microns wide. When mature, the basidiospores are forcibly discharged.

Figure 21

RETURN TO TOP