Abstract
Abstract Necrotic and chlorotic leaf spots observed on nursery grown poplars during moist spring conditions were caused by Pseudomonas syringae pv. syringae. Infected tissue was necrotic, collapsed and desiccated, and contained no viable bacteria at a late stage. Organelles in cells of chlorotic tissue were not moribund and cell wall proliferations were absent. Tissues were negative-imaged and cytoplasm was condensed and granulated. Tobacco leaves infiltrated with >109 cells/ml of P. syringae pv. syringae exhibited ultrastructural changes one hour following infiltration. The plasmalemma invaginated and proliferations formed in cell walls. Vesiculation of boundary and internal membranes occurred in most organelles. After 3 and 6 hours, vesiculation increased, plasma membranes ruptured and organelles degenerated. Small groups of bacteria were observed clustered in depressions in cell walls and in pockets between adjacent cells. Bacteria were covered by a fibrillar network and caused no degradation of the outer wall. Twelve hours following infiltration, most spongy and palisade mesophyll cells had collapsed and intercellular spaces were filled with actively dividing bacteria.
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