Distribution of Clavibacter xyli subsp. xyli in Stalks of Sugarcane Cultivars Differing in Resistance to Ratoon Stunting Disease. M. J. Davis, University of Florida, Institute of Food and Agricultural Sciences, Ft. Lauderdale Research and Education Center, 3205 College Avenue, Ft. Lauderdale, FL 33314. J. L. Dean, and N. A. Harrison. USDA-ARS, U.S. Sugarcane Field Station, Canal Point, FL 33438; and University of Florida, Institute of Food and Agricultural Sciences, Ft. Lauderdale Research and Education Center, 3205 College Avenue, Ft. Lauderdale, FL 33314. Plant Dis. 72:443-448. Accepted for publication 2 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0443.

The spatial and temporal distributions of Clavibacter xyli subsp. xyli populations in stalks of sugarcane cultivars were studied. Sap containing cells of the pathogen was extracted by centrifugation from stalk sections, and the cells were enumerated using a fluorescent-antibody direct-count procedure. Small but significant differences in mean bacterial populations were detected among sap extracts from nodal and internodal sections of cultivar CP 53-1, depending upon the direction of extraction. Sap extracted in a basipetal direction from nodal tissues excised immediately below leaf scars contained the greatest numbers, otherwise populations in extracts from internodal tissues equalled or exceeded those from nodal tissues. No effect due to direction of extraction was found among populations in sap extracted from internodal tissues. Pathogen populations developed at a faster rate and to a greater extent in internodal tissues of the more susceptible of nine cultivars examined on three successive sampling dates. Larger populations were observed in sap from mature as compared with less mature internodes. As tissues matured, population densities at different internode locations became correlated with yield reduction estimates for the cultivars.