Histopathology of the Interaction of Sorghum bicolor and Sphacelotheca reiliana

Abstract

Sphacelotheca reiliana, the causal organism of sorghum head smut, develops only in actively growing host meristematic tissue. Three types of mycelium are found in a developing sorus. A thin parasitizing mycelium was intercellular with haustoria. A large, reproductive mycelium produced teliospores from the centers of the intervascular regions. The peridium consisted of a partially segmented, nonreproductive mycelium surrounded by a thin layer of host cells. The distribution of mycelium in the apical meristem determined the type of sorus produced. This mycelium was carried by the elongating cells to areas in the developing inflorescence at the initiation of the host reproductive cycle. Widespread colonization of the host apical meristem at the beginning of elongation and floral differentiation resulted in the formation of a smooth sorus. When the mycelium was limited to the basal regions of the apical meristem before elongation and differentiation, the floral primordia were not colonized, and the result was a sterile or vegetative (phyllodied) head. If the mycelium was carried into the lowest regions of floral primordia, a small sorus was produced at the base of a sterile inflorescence. Widespread but sparse mycelium at the time of elongation resulted in a sorus in the shape of a partially differentiated inflorescence. Reproductive and peridial cells of the fungus in a developing sorus were mainly uninucleate.