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Histology of Onion Leaves Infected with Pseudomonas cepacia. Stanley O. Kawamoto, Former Research Assistant, Department of Plant Pathology, Cornell University, Ithaca, New York 14850, Senior author is now Research Technician II; James W. Lorbeer, Professor, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 62:1266-1271. Accepted for publication 16 May 1972. DOI: 10.1094/Phyto-62-1266.

Pseudomonas cepacia moved into the leaf sheaths through the intercellular spaces when stab-inoculated into the base of onion leaf blades. Although large bacterial masses developed, separate small groups of bacteria frequently were formed in the intercellular spaces of both blade and sheath. The small group type of distribution was observed when the lesions were periodically sprinkled with water. Except for the xylem vessels and epidermal cells, host cells were compressed and there were large bacterial masses in the intercellular spaces in close association with these collapsed cells. In the blade, the large parenchyma cells were the first to collapse; the smaller, more compactly packed parenchymatous cells near the periphery of the blade collapsed later. Tissues composed of these cells eventually were macerated. In the sheath, bacteria were widespread among the loosely organized parenchyma cells adjacent to the adaxial epidermis. Bacterial masses and scattered bacteria in close association with host cell walls were commonly observed in this area. Bacteria in the intercellular spaces of the closely packed parenchyma cells beneath the abaxial epidermis usually were restricted to dense, compact masses.