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Ecology and Epidemiology

Systemic Invasion of Plum Leaves and Shoots by Pseudomonas syringae pv. syringae Introduced into Petioles. Isabel M. M. Roos, Fruit and Fruit Technology Research Institute, Private Bag X5013, Stellenbosch 7600; M. J. Hattingh, Department of Plant Pathology, University of Stellenbosch, Stellenbosch 7600, South Africa. Phytopathology 77:1253-1257. Accepted for publication 18 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1253.

Systemic spread of an antibiotic-resistant strain of Pseudomonas syringae pv. syringae inoculated into plum petioles was investigated by scanning electron microscopy. The pathogen colonized xylem elements of the petioles and then spread to the xylem of shoots and to the xylem and other elements of the leaf veins. It was also seen in axillary buds of invaded leaves. Lateral spread from major xylem vessels into adjacent tissue was more pronounced in leaf veins than in petioles and shoots. The presence of microcolonies on leaf surfaces indicates that the pathogen is extruded from invaded tissue, most likely through stomata. The study suggests that epiphytic populations of the pathogen are constantly replenished from systemically invaded, symptomless plum trees.