VIEW ARTICLE

Resistance

Quantitative Variability of Clavibacter xyli subsp. xyli Populations in 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, Ft. Lauderdale, FL 33314; J. L. Dean(2), and N. A. Harrison(3). (2)USDA, ARS, U.S. Sugarcane Field Station, Canal Point, FL 33438; (3)University of Florida, Institute of Food and Agricultural Sciences, Ft. Lauderdale Research and Education Center, Ft. Lauderdale, FL 33314. Phytopathology 78:462-468. Accepted for publication 13 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-462.

Cells of Clavibacter xyli subsp. xyli, the bacterium that causes ratoon stunting disease, were extracted by centrifugation from a basal internode of sugarcane stalks and enumerated using a fluorescent-antibody direct-count procedure. When pathogen populations in 11 cultivars were examined on different dates or when grown at two different locations, the distributions of density estimates for individual populations were frequently not normally distributed about the arithmetic mean. Furthermore, the means and variances of the different populations were positively correlated, but the coefficient of variation decreased with greater densities. Generally, a quartic-root transformation normalized the data for individual populations and eliminated the relationship between means and variances, thus validating the use of parametric statistics. Pathogen densities were different among sugarcane cultivars but increased at approximately the same rate within each crop during the time period sampled. A significant correlation was observed between mean population density estimates for cultivars when crops or geographic locations were compared. When yield reduction in cultivars due to ratoon stunting disease was determined at one location, it was significantly correlated with corresponding estimates of pathogen density.