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Histopathology of Pelargonium Species Infected with Xanthomonas pelargonii. S. H. Wainwright, Former Graduate Assistant, The Pennsylvania State University, University Park 16802; Paul E. Nelson, Professor of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 62:1337-1347. Accepted for publication 29 May 1972. DOI: 10.1094/Phyto-62-1337.

The histology of Pelargonium spp. susceptible, moderately resistant, and resistant to Xanthomonas pelargonii was studied. The mode of spread of the pathogen was similar in all species, initially involving movement of the pathogen throughout the plant in the xylem vessel elements and subsequent movement laterally into adjoining parenchyma cells. The relative numbers of the pathogen and the numbers of fascicles initially invaded were low in resistant species and high in susceptible species. In susceptible species, bacterial pockets formed around affected protoxylem vessel elements, enlarging to encompass all xylem cells in the fascicle, and finally portions of the cambium, phloem, cortex, and epidermis. Pelargonium spp. responded to infection by proliferation of a ring of cells around affected portions of fascicles, with cells immediately inside this ring having a suberinlike material formed on their walls. This appears to be a secondary defense reaction, restricting the lateral spread of the pathogen. Tanninlike materials were found in the tissues of all Pelargonium spp. tested, but appear to be of different types in the susceptible and resistant species. It is suggested that tanninlike substances may be, in part, responsible for disease resistance through their actions as bacteriostatic agents and enzyme inhibitors.