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Structural and Morphological Changes Associated with the Pierce's Disease Bacterium in Bunch and Muscadine Grape Tissues. Pi- Yu Huang, Research cooperator, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; R. D. Milholland(2), and M. E. Daykin(3). (2)(3)Professor, and research technician, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 76:1232-1238. Accepted for publication 20 June 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1232.

Bacteria with rippled, intermediate, and smooth cell walls occurred consistently in the lumen of tracheary elements of naturally infected and artificially inoculated grape leaves showing Pierce's disease (PD) symptoms from the field and greenhouse, respectively. Bacteria did not occur in tissues prepared from healthy controls. Bacteria were rod-shaped, averaging 0.5 x 1.5 μm. Bacteria isolated from bunch grapevines exhibiting PD symptoms also had rippled, intermediate, and smooth cell walls among the cells from a single-colony culture. Morphological change of the PD bacterium from rippled to smooth cell walls developed faster in tolerant muscadine tissues than in susceptible bunch grape tissues. An electron-dense substance was associated with smooth cell-walled bacteria but not with bacteria with the rippled cell walls. The reason for the differential association of the electron-dense substance with bacteria with smooth and not with rippled cell-walls is not known. However, the encapsulation of smooth cell-walled bacteria by the electron-dense substance occurred more frequently in muscadine grapevines than in the bunch grapes. This substance may be an important mechanism whereby muscadine grapevines defend themselves against the PD bacterium. Based on histochemical tests, the vascular plugs associated with PD are primarily pectic substances with some gums and tannins. Therefore, the electron-dense substances surrounding the smooth cell-walled bacteria are of host origin.