ELECTRON-MICROSCOPY STUDIES OF A XYLEM-LIMITED BACTERIUM IN SWEET ORANGE AFFECTED WITH CITRUS VARIEGATED CHLOROSIS DISEASE IN BRAZIL

Citrus variegated chlorosis (CVC) a serious discase affecting sweet orange (Citrus sinensis Osb.) groves in Sao Paulo and Minas Gerais States, Brazil, was first observed in 1987 and found to be associated with a Gram-negative, xylem-limited bacterium (CVC-B). Electron microscopic studies revealed that the bacterium may appear in situ evenly distributed in the lumen of the tracheary elements and, occasionally, embedded in a lucent matrix adhering to the inner surface of the cell wall elements. CVC-B measured 1-3.5-mu-m X 0.3-0.5-mu-m and appeared to be at different developmental stages; fibril-like structures were seen attached to bacteria. CVC-B rippled wall showed a periodical or annulated structural arrangement with thickness from 25-35 nm in the furrows and 45-55 nm in the ridges. The wall consisted of 3 layers, i.e., an outer, an inner (each comprised of 3-layered unit membrane structure) and a middle peptidoglycan layer. DNA-like threads and ribosome-like granules were present in the cytoplasm. CVC-B is structurally and morphologically similar to Xylella fastidiosa of which it could be a citrus strain or a strain from other hosts adapted to citrus.