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Cytology and Histology

Timing and Significance of Papilla Formation During Host Penetration by Olpidium brassicae. James R. Aist, Assistant Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; Herbert W. Israel, Senior Research Associate, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 67:187-194. Accepted for publication 7 September 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-187.

Olpidium brassicae has been reported to incite host cells to form papillae at penetration sites before penetration, and it has been suggested that the papillae stopped the advancing pathogen. By means of interference contrast microscopy, we investigated timing of papilla formation in relation to growth of O. brassicae penetration tubes, from zoospore cysts, through walls of living kohlrabi (Brassica oleracea) root hairs. Most papillae were initiated after penetration tubes appeared, were conical, and were attached to the tubes (tube papillae). Those initiated before tube development were disk-or dome-shaped and were attached to the host walls (wall papillae). Tube and wall papillae both were present at a few encounter sites. Although wall and tube papillae were initiated at about the same absolute time after inoculation, cysts which induced wall papillae were significantly later than other cysts in producing tubes. Therefore, production of tubes was already tardy when wall papillae were initiated. Furthermore, failure of some cysts to penetrate was clearly unrelated to papilla formation. On the other hand, penetration efficiency of cysts that induced wall papillae was merely half that of cysts which induced only tube papillae. Further experiments will be required to determine whether penetration failures are caused by the papillae or by an inherent inability of certain cysts to complete penetration.