Multiplication and Translocation of Xanthomonas campestris pv. poae (JT-P482) in Annual Bluegrass (Poa annua L.).

Abstract

Growth of a Rifampin-resistant strain (Rif-482) of Xanthomonas campestris pv. poae (JT-P482) in annual bluegrass monitored over several weeks indicates its potential as a tool for the study of interaction between X. campestris pv. poae (JT-P482, wild type) and annual bluegrass. After inoculation with 109cfu/ml of bacterial suspension, populations of both Rif-482 and JT-P482 in plant sections increased during the period from 3 days (1.0×108cfu/g FW) to 3 weeks after inoculation, when they reached a maximum (1.5×1010cfu/g FW). Subsequently, Rif-482 decreased to 5.4×108cfu/g FW 9 weeks after treatment (WAT). Efficacy (=% control) increased up to 4 WAT (86% control and 88% control of plants inoculated with Rif-482 and JT-P482, respectively) with plants showing the blighted and dried symptoms characteristic of heavy wilting. These results showed that the multiplication rate and efficacy of Rif-482 in plants matched those of JT-P482. The time lag between the peak of bacterial population (>1010cfu/g FW) and the peak of disease development (>80% control) was between one and two weeks. Tests of various initial inoculum concentrations of both isolates from 103 to 1010cfu/ml showed that the two followed a similar pattern of cell population increase and efficacy 3 WAT. The multiplication pattern of Rif-482 in the plant during the vegetative to early reproductive phases of plant growth was traced in different plant parts over time. Our findings show that Rif-482 bacteria inoculated to a leaf translocate systemically from the inoculation site through the stem to the root, then to all plant-parts. They also show a correlation between the development of disease and a decrease in the water content of plants, which appears to be the mechanism by which X. campestris pv. poae injures annual bluegrass.