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Histopathology of Xanthomonas campestris pv. citri from Florida and Mexico in Wound-Inoculated Detached Leaves of Citrus aurantifolia: Transmission Electron Microscopy. M. M. Dienelt, United States Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705; R. H. Lawson, United States Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705. Phytopathology 79:336-348. Accepted for publication 26 September 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/Phyto-79-336.

Detached leaves of Citrus aurantifolia were wound-inoculated with pathogens Xanthomonas campestris pv. citri XC90, a Mexican strain inducing raised blisters; X. c. citri F1, a Florida strain inducing water-soaking; X. c. citri F20, a Florida strain inducing small necrotic lesions; non-citrus pathogen X. c. pruni XP1; saprophyte Erwinia herbicola EH1; or water. Cytological changes were traced from the wound to bacterial localization and were divided into six zones. In water, XP1 and EH1 inoculations, the wound is bordered by disrupted cells (zone one), followed by modified preexisting cells similar to impervious tissue (zone two) and a periderm (zone three). Zone two modifications include suberin wax lamellae on inner cell walls, outer wall surface lamellae, wall projections, thick layered cell walls, and intercellular accumulations of osmophilic granules. Zones one through three develop abnormally and are followed by regions of bacterial invasion (zone four) and localization (zones five and six) in leaves wound-inoculated with strains of X. c. citri. Zones five and six resemble wound repair zones two and three, respectively, with zone five osmophilic granule production followed by zone six host cell division. When surrounded by osmophilic granules at the wound, bacteria of all strains are immobilized; however, localization of X. c. citri is incomplete until zone five. The three isolates of X. c. citri can be distinguished in zones one through four. Strain XC90 stimulates, but strain F1 suppresses osmophilic granule production, wall modifications, and cell division at the wound. In zone four, strains F1 and F20 are present primarily in intercellular spaces, but strain XC90 invades along middle lamellae of proliferating cells. Both Florida strains are associated with a fibrillar matrix that does not occur in XC90 infections. Wound repair abnormalities, patterns of invasion, and the fibrillar matrix provide the major cytological points of strain differentiation.