Transmission electron microscopy was used to examine details of the host-pathogen interface in daylily leaf cells infected by the rust fungus Puccinia hemerocallidis. Samples were prepared for study by high-pressure freezing followed by freeze substitution. The outstanding preservation of ultrastructural details afforded by this fixation protocol greatly facilitated the study of this host-pathogen interface. The extrahaustorial membrane that separated each dikaryotic haustorium from the cytoplasm of its host cell was especially well preserved and appeared almost completely smooth in profile. Large aggregations of tubular cytoplasmic elements were present near haustoria in infected host cells. Many of these tubular elements were found to be continuous with the extrahaustorial membrane and conspicuous electron-dense deposits present in the extrahaustorial matrix extended into these elements. The use of gold-conjugated wheat germ agglutinin for labeling of chitin revealed that these deposits were not part of the haustorial wall. Portions of many of the tubular elements associated with haustoria were conspicuously beaded in appearance. Some tubular elements were found to be continuous with flattened cisternae that in turn bore short beaded chains. Distinctive tubular-vesicular complexes previously reported only in cryofixed rust haustoria also were found in the haustoria of P. hemerocallidis.

A combination of scanning and transmission electron microscopy was used to examine the host-pathogen relationship in leaves of Duchesnea indica (Andrz) Focke infected by the rust fungus Frommeëla mexicana var. indicae McCain & Hennen. Samples for transmission electron microscopy were prepared using high pressure freezing followed by freeze substitution. This protocol provided excellent preservation of both host cells and fungal haustoria. Each haustorium of F. mexicana var. indicae possessed a long slender neck with a neck band and an expanded body that contained two nuclei positioned close together. The haustorial body was lobed and sometimes even branched but lacked septa. Details of the extrahaustorial membrane that separated each haustorium from the cytoplasm of its host cell were particularly well preserved. Extensive labyrinth cell wall ingrowths developed around haustorial necks, as well as elsewhere, in infected cells. These ingrowths appeared to be identical to those present in plant transfer cells. Transfer cells are thought to be involved in intensive solute transfer over short distances. This appears to be the first report of the development of transfer cells in response to infection by a plant pathogenic fungus.Key words: haustoria, transfer cells, freeze substitution, electron microscopy.

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