Quantitative variability of Clavibacter xyli subsp. xyli populations in sugarcane cultivars differing in resistance to ratoon stunting disease.
Cells of C. xyli subsp. xyli, causal agent of 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 2 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 approx. 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.