Leaf Sheath Blights ofSorghum bicolorCaused bySclerotium rolfsiiandGloeocercospora sorghiin South Texas

Abstract

S. rolfsii (causal agent of southern sclerotial rot) and G. sorghi (causal agent of zonate leaf spot) caused leaf sheath blights on grain sorghum (S. bicolor) in fields and nurseries in southern Texas. S. rolfsii infected and killed one to all leaf sheaths of susceptible sorghums growing in saturated soil under hot, humid conditions. Sclerotia of S. rolfsii were often present on the outside of necrotic leaf sheaths. G. sorghi also infected basal and underlying leaf sheaths on susceptible cultivars under hot, humid conditions. Sclerotia were produced in large quantities and gave a gray appearance. Infection of sheaths by G. sorghi either preceded or occurred simultaneously with infection of leaf blades. Free and residue-associated sclerotia of G. sorghi were isolated from field soil and germinated readily in culture. Three field soils had 4-17 germinable sclerotia/g of air-dry soil. Occurrence of southern sclerotial rot was sporadic and predominantly in poorly drained areas, but zonate leaf spot on sheaths was present in most fields. Neither disease was economically important, but some cultivars were extremely susceptible to either one or both diseases in the field.