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Ecology and Epidemiology

Systemic Invasion of Cherry Leaves and Petioles by Pseudomonas syringae pv. morsprunorum. 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 77:1246-1252. Accepted for publication 18 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1246.

Fully expanded cherry leaves were inoculated in spring with 10⁴, 10⁵, or 10⁶ colony-forming units (cfu) per milliliter of a virulent strain of Pseudomonas syringae pv. morsprunorum. Appearance of symptoms depended on the inoculation method. Water-soaked veins and lesions developed on leaves wounded by rubbing during inoculation. At higher inoculum concentrations, more lesions developed. Spraying pathogen suspensions onto leaves until tissue became water soaked was less effective. No lesions developed on leaves sprayed until runoff without water soaking. Systemic invasion by P. s. pv. morsprunorum of symptomless leaf and petiole tissue was confirmed by isolation of the pathogen and scanning electron microscopy (SEM). Most shoots and axillary buds of trees inoculated by applying 10⁶ cfu/ml of P. s. pv. morsprunorum to leaves during wounding had died back by the next spring, implying further spread of the pathogen. SEM indicated that P. s. pv. morsprunorum applied by spraying probably gained entry through stomata and then spread intercellularly from the mesophyll through the parenchyma of the bundle sheath into the vascular system of a minor vein. Once a vein had been invaded, migration occurred to other regions in the leaf blade and petiole.