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Histopathology of Xanthomonas campestris pv. citri from Florida and Mexico in Wound-Inoculated Detached Leaves of Citrus aurantifolia: Light and Scanning Electron Microscopy. R. H. Lawson, United States Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705; M. M. Dienelt(2), and E. L. Civerolo(3). (2)United States Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705; (3)United States Department of Agriculture, Agricultural Research Service, Fruit Laboratory, Beltsville, MD 20705. Phytopathology 79:329-335. 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-79-329.

Detached leaves of Citrus aurantifolia were wound-inoculated with three strains of the citrus pathogen Xanthomonas campestris pv. citri (strains F1 and F20 from Florida and XC90 from Mexico), a noncitrus pathogen X. campestris pv. pruni (strain XP1), a saprophyte Erwinia herbicola (strain EH1), or water. The wound response in leaves inoculated with water, strain XP1, or strain EH1 appears similar, consisting of three distinct zones of cytological change. Zones develop abnormally in leaves inoculated with X. c. citri. A zonal terminology based on host anatomical changes associated with normal wound repair was adopted that can also be used in infected tissue where normal host anatomy is modified. In water, strain EH1 and strain XP1 inoculations, zone one is closest to the wound and composed of disrupted preexisting cells, zone two contains preexisting cells with thickened cell walls and occluded intercellular spaces and zone three is a periderm. Strain XC90 induces large raised foliar lesions that occur when stimulation of cell division in zone three or bundle sheath, vascular parenthyma, and procambium cells results in protrusion of proliferating host cells through the epidermal surface. Strain F1, which produces large water-soaked lesions, inhibits development of zones two and three and is associated with a faintly stained matrix. Strain F20, which produces small necrotic lesions without water-soaking, causes initial slight suppression of cell division and subsequent disruption of zone three. Like strain F1, strain F20 was associated with matrix formation in intercellular spaces. Two characteristics, the intercellular matrix and the ability to trigger and/or prolong meristimatic activity, differentiate the Mexican and the Florida strains of X. c. citri.