E. O'Gara

The infection of non-wounded and wounded periderm tissue at the lower stem of Eucalyptus marginata by zoospores of Phytophthora cinnamomi, in a rehabilitated bauxite mine

Artificially introduced zoospores of Phytophthora cinnamomi were shown to invade non-wounded and deliberately wounded periderm at the lower stem and collar of one-year-old seedlings of Eucalyptus marginata (jarrah) growing in a rehabilitated bauxite mine, during late autumn/early winter. Lesions were not produced in the majority of seedlings despite the demonstrated presence of the pathogen in the symptomless stem tissue.

Immunocytochemical comparison of peripheral vesicles in zoospores of Phytophthora and Pythium species

An ultrastructural and immunocytochemical comparison has been made of vesicles in the peripheral cytoplasm of zoospores of Phytophthora cinnamomi and species of the related Oomycete genus, Pythium. Our results give evidence of three morphologically and immunologically distinct vesicle populations in Pythium aphanidermatum and Py. butleri. Large peripheral vesicles can be recognized by their size and morphology, and by labeling with monoclonal antibody, Cpa-2 raised against a P cinnamomi antigen. They occur predominantly on the dorsal surface of the zoospores and are retained within the cell during encystment. After encystment, the large peripheral vesicles move away from the plasma membrane and become distributed throughout the cyst cytoplasm, in a manner similar to that observed in P. cinnamomi. There are also small vesicles in the Pythium zoospore cortex. Some are identified as ventral vesicles through their reaction with monoclonal antibody Vsv-1, raised against a P. cinnamomi antigen. The ventral vesicles are concentrated along the ridges of the groove on the ventral surface of the zoospores, and their contents are secreted during encystment. The secreted material remains localized on the cyst surface and marks the site of germ tube emergence, as it does in P. cinnamomi. Other small peripheral vesicles occur on the dorsal surface of the Pythium zoospores. These vesicles are not labeled by any of the three antibodies tested but disappear during encystment, suggesting that their contents are secreted. Our results give strong support to the idea that three types of peripheral vesicles are a common feature of zoospores throughout the Peronosporales, and that they have similar fates during encystment.

The development and characteristics of periderm and rhytidome in Eucalyptus marginata

To understand the pathway used by Phytophthora cinnamomi Rands to penetrate the bark of jarrah, the present study describes unwounded periderm and rhytidome development. Periderm formation is described from its initiation in 4-week-old seedlings to the formation of rhytidome in saplings. Periderm in young seedlings consists of a single type of phellem, namely thin-walled suberised cells. In older seedlings where multiple layers of periderm have formed, layers of thick-walled lignified phellem cells in compacted bands alternate with thin-walled suberised cells. Rhytidome formation in older lignotuberous seedlings and in sapling jarrah occurs through the isolation of secondary phloem by periderm. The rhytidome consists of expanded and partially disintegrated secondary phloem tissue sandwiched between layers of phellem cells. Localised periderm formation beneath stomata results in the formation of lenticels, which are ephemeral features. Superficial periderms occur at sites of leaf and shoot abscission, and of lateral shoot emergence. Concealed axillary shoots lack cuticle on emergence. As the trees age, the internal production of lignified and suberised periderm and rhytidome results in an impenetrable barrier to invasion by P. cinnamomi. However, external sites including lenticels and leaf and shoot abscission and emergence areas, all provide points of ingress in unwounded stems.