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The Ultrastructure of the Almond Leaf Scorch Bacterium with Special Reference to Topography of the Cell Wall. S. K. Lowe, Research Associate, Department of Plant Pathology, University of California, Davis, California 95616; G. Nyland(2), and S. M. Mircetich(3). (2)Professor, Department of Plant Pathology, University of California, Davis, California 95616; (3)Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, University of California, Davis, California 95616. Phytopathology 66:147-151. Accepted for publication 5 August 1975. DOI: 10.1094/Phyto-66-147.

The ultrastructure of a bacterium occurring in xylem tissue of leaves of almond (Prunus amygdalus) displaying typical leaf-scorch (ALS) symptoms were studied in situ by electron microscopy. Intracytoplasmic organelles were observed: ribosomes, DNA-like strands, a nuclear region; and occasionally, electron-dense granules and inclusion bodies. The bacterial wall profile consisted of a markedly rippled or ridged outer trilaminar (dense-light-dense) membrane (OM) and a cytoplasmic membrane (CM). Of the two opaque layers of the OM, the inner appeared more electron-dense than the outer. No distinguishable dense intermediate (peptidoglycan) or R layer was present in the periplasmic space. The ALS organism apparently multiplies by binary fission. Extracellular strands and spherical particles were seen in a matrix of low electron-density among the bacteria. A tuft of microfibrils or lotussy was occasionally noted on a few cells. Prominent bands or ridges on the surface of the OM account for the rippled appearance of the bacterial cell wall. The ridges, separated by furrows or depressions, are randomly or somewhat annularly oriented and appear thicker, wavier in outline, and more closely spaced near the poles. A reconstructed general view of the surface of the cell is shown in sequential sections. Similarity in topography of the cell wall may be supportive evidence for relatedness of the bacteria of grape Pierce’s disease, phony peach disease, and ALS.

Additional keywords: xylem pathogen, Draeculacephala minerva, grape Pierce’s disease, phony peach disease.