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Haustoria and Intracellular Hyphae in the Rusts. F. H. J. Rijkenberg, Lecturer, Department of Plant Pathology and Microbiology, Faculty of Agriculture, University of Natal, Private Bag 9021, Pietermaritzburg, Republic of South Africa; Susarah J. Truter, Professor, Department of Plant Pathology and Microbiology, Faculty of Agriculture, University of Natal, Private Bag 9021, Pietermaritzburg, Republic of South Africa. Phytopathology 63:281-286. Accepted for publication 2 September 1972. DOI: 10.1094/Phyto-63-281.

Prior to cell penetration by Hemileia vastatrix, a thick electron-opaque layer is formed in the haustorial mother cell against the wall in contact with a host cell. A thin, less dense zone is subsequently laid down over this layer. Coffee leaf cells respond to the presence of a haustorial penetration tube by producing a large host collar that reacts weakly, if at all, to callose-specific staining. Callose apposition onto the body of the haustorium appears to be linked to incipient necrotization, since only haustoria in senescent cells stain callose-positively. These haustoria have thick sheaths, whereas haustoria associated with cells of normal appearance are thin-sheathed and react negatively for callose. In the pycnial intercellular hyphae of Puccinia sorghi, no prepenetration layers are apposed against the wall in contact with a host cell. Penetration is apparently partly enzymatic, and the middle and inner layer of the wall are continuous with the innermost layers of intracellular fungal structures. The latter are filamentous, often septate, and may coil extensively around host organelles. Penetration tubes, neckbands, and haustoriumlike dilations are absent. A dearth of intracellular fungal structures, relative to a well-developed intercellular thallus, suggests that efficient use is made of nutrients diffusing from host cells. Preliminary studies on one other pycnial and three aecial stages indicate that these characteristics may well be typical of many pycnial and aecial stages. On the basis of the presented evidence, it is proposed that intracellular structures of this type be designated “intracellular hyphae” rather than “haustoria”.