The surface of Erysiphe graminis and the production of extracellular material at the fungus – host interface during germling and colony development

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Abstract
Low temperature scanning electron microscopy was used to study the development of Erysiphe graminis DC f.sp. avenae Marchal from germination through infection to sporulation of the fungal colony. To clarify resolution of the fungus–host interface and facilitate interpretation of fungal surface structures, epicuticular waxes were removed from host leaves before inoculation. Whereas conidia were covered in spinelike protrusions or globular bodies, young germ tubes, appressoria, and hyphae were initially smooth walled, but by 15 h after inoculation, wartlike bodies, resembling globular bodies on conidia, were present on first appressorial lobes; these increased in number, eventually covering the appressorium surface and appearing on contiguous hyphae. Wartlike bodies also appeared at junctions of hyphal branches, on hyphal appressoria, and on conidiophore basal cells. Their function, if any, is unknown. The meristematic zone, at the apex of the conidiophore basal cell, remained smooth walled, but globular bodies appeared on the wall of young conidia as soon as the limiting septum had formed. Observations with the fungus in situ revealed the presence of amorphous extracellular material around primary germ tubes and appressorial lobes. Extracellular material was also present beneath appressorial germ tubes and hyphae but it was hidden unless the fungus was displaced. It could not be seen beneath conidia. The extracellular material appeared to be adhesive, sticking the fungus firmly to the host surface. Removal of the fungus showed that the extracellular material was deposited close to the tip of developing germ tubes and hyphae. It was particularly thick around primary germ tubes and appressorial lobes, and a discrete ring of extracellular material was often visible around penetration pores (holes in the leaf surface seen beneath primary germ tubes and appressorial lobes). In addition to its adhesive properties, the extracellular material may act as a matrix in which fungal enzymes are sited and focused for attack on the host. Key words: Erysiphe graminis, low temperature scanning electron microscopy, extracellular material, fungal adhesion, fungal surface morphology.

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