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Haustorial Sheath Formation in Cowpea Leaves Immune to Rust Infection. Michele C. Heath, Postdoctoral Research Fellow, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30601; Phytopathology 61:383-388. Accepted for publication 12 November 1970. DOI: 10.1094/Phyto-61-383.

One immune cowpea cultivar, Queen Anne, showed a unique dimorphic reaction to infection by the rust Uromyces phaseoli vignae; haustorial formation induced either rapid host cell necrosis or the formation of a calloselike sheath which grew up from the host cell wall to completely enclose the haustorium. In 14 other immune cultivars examined, resistance was expressed only by the more typical resistant response of hypersensitive death of invaded host cells. Although sheathed haustoria did not die immediately as did unsheathed haustoria in necrotic cells, the majority of infection hyphae in Queen Anne, like those in all the immune cultivars, ceased to grow after the formation of the first haustorial mother cell. The formation of a haustorium appeared to be essential for necrosis of either host or parasite in all immune cultivars, including Queen Anne. The unusual dimorphic reaction of Queen Anne may result from a greater tolerance of this variety to the toxic effects of the haustorium. The fact that the host remains immune in the absence of host cell necrosis suggests that the hypersensitive response is not the only effective means of limiting fungal growth in rust-infected tissues.