VIEW ARTICLE

Ultrastructure of Host and Nonhost Reactions to Cowpea Rust. Michele C. Heath, Postdoctoral Research Fellow, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30601; Phytopathology 62:27-38. Accepted for publication 12 July 1971. DOI: 10.1094/Phyto-62-27.

An ultrastructural survey of resistant reactions induced by the cowpea rust, Uromyces phaseoli var. vignae, revealed that the nonhost, Phaseolus vulgaris, responded to each infection hypha by the deposition of electron-opaque material on and within the surrounding host cell walls. These deposits apparently prevented haustorial formation in 90% of infection sites. In contrast, no signs of resistance were detected in either of the two immune cowpea cultivars until the formation of the first haustorium. The subsequent reaction of one of these cultivars closely resembled that already described for another immune cowpea, and included a distinctive response of the host plasmalemma surrounding the haustorial body. In the other, haustorial development was retarded, and all host membranes bore small areas of electron-opaque material. In both immune cultivars, these initial responses were quickly followed by the rapid and simultaneous disorganization of both haustorium and host cell. In contrast with these immune responses, the cowpea cultivar giving an intermediate (necrotic fleck) reaction showed no signs of resistance during the initial stages of penetration or haustorial formation. The eventual slow disorganization of invaded cells involved a dissection of the peripheral cytoplasm, followed by a gradual disintegration of most of the cell membranes. Haustorial disorganization did not immediately follow that of the host cell. In both the resistant host cultivars and the nonhost, death of the haustorium and the haustorial mother cell did not result in the immediate death of the intercellular mycelium. It is possible that starvation was the primary cause of the cessation of fungal growth.