A Histological Study of Infection of Host Tissue bySclerotium rolfsii

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Abstract
Hyphae from germinating sclerotia of Sclerotium rolfsii ramified over various host tissues (sugarbeet leaves, bean hypocotyls, and carrot roots or petioles) within 24-48 hr following inoculation. Hyphae frequently coalesced to form mycelial aggregates that functioned as infection cushions. Crystals of calcium oxalate were associated with these aggregates and with individual hyphae growing over the host surface. Penetration occurred after death and collapse of cells beneath infection cushions and from appressoria that formed at the tips of individual hyphae. Penetration pegs (1-3 .mu.m thick) formed below infection cushions and from appressoria. The cuticle was apparently not dissolved, but was pulled away from the epidermis, and subsequent subcuticular hyphal growth occurred both inter- and intracellularly. Hyphal growth was parallel to the longitudinal axis of host cells and to the tissue surface. Cell wall components that stained positive with ruthenium red and alizarin red S (pectic materials and calcium, respectively) were depleted from cell walls in advance of the mycelium. Cells distal to the hyphae were necrotic and stained positive with thionin. Crystals of calcium oxalate stained black with AgNO3-dithiooxamide and were produced in abundance in necrotic tissues. Infection cushions produced by S. rolfsii appeared to facilitate infection of host tissue by secreting enzymes and oxalic acid that macerated and killed tissue in advance of fungal penetration.