VIEW ARTICLE

Quantitative Aspects of Pathogenesis of Xanthomonas pruni in Peach Leaves. E. L. Civerolo, Research Plant Pathologist, Fruit Laboratory, Agricultural Research Service, Beltsville, Maryland 20705; Phytopathology 65:258-264. Accepted for publication 26 September 1974. DOI: 10.1094/Phyto-65-258.

An assay system for quantitatively defining the virulence of Xanthomonas pruni in its natural host is described. Approximately 10µl of inoculum containing approximately 10² to 10⁴ colony-forming units (CFU)/ml was introduced, by spraying at 1.25 × 10^–1 Kg/cm², into the intercellular spaces at selected circular areas of young Sunhigh peach seedling leaves. The assay can be completed in 14 days. The number of lesions induced by X. pruni in this system is directly proportional to the number of CFU in the inoculum. Approximately 16 to 18 CFU are required to cause a single lesion, indicating that the efficiency of this system is relatively high. Comparative analyses of log-dose/probit-response data of four X. pruni isolates are presented. In infectivity titrations of X. pruni, quantal (all-or-none) responses are conveniently recorded as the presence or absence of a lesion at an inoculation site. Determination of slope and median effective dose (ED₅₀) values from the dose-response curves allows comparison of the virulence of X. pruni isolates and of host susceptibility, and permits conclusions about how the bacterium produces the response. Since the slopes of the dose-response curves are less than two, X. pruni cells probably act independently during growth in vivo to cause infection and a single X. pruni cell is probably capable of causing infection if the proper environment is available. Based on comparative ED₅₀ values, a sweet cherry isolate of X. pruni is less virulent on Sunhigh peach seedling leaves than peach, apricot, and plum isolates.