Effect of Temperature on the Control of Annual Bluegrass (Poa annua L.) with Xanthomonas campestris pv. poae (JT-P482)

Abstract A host-specific pathogen, Xanthomonas campestris pv. poae (JT-P482), which causes lethal wilting of annual bluegrass ( Poa annua ), was isolated in 1993 and has since been developed and commercialized as a bioherbicide in Japan. The effectiveness of X. campestris pv. poae against annual bluegrass is significantly affected by temperature. To determine the optimum temperature for maximum control, the relationship between the levels of annual bluegrass control and the rates of bacterial multiplication in whole plants was investigated. The doubling times of the bacterium in planta and in vitro were also compared. The optimum air temperature was 30°C to 35°C, with corresponding temperatures of 27.9 to 29.6°C in plants, and 30°C in culture liquid. At 17°C, the rate of multiplication of bacteria and the rate of plant growth reached an equilibrium and this appeared to be the threshold temperature for annual bluegrass control. No bacterial multiplication was observed in either plants or culture around 6°C. At higher temperatures (>25°C air temperature, > 24.8°C plant temperature), the bacteria quickly multiplied to 10 10 colony-forming units (CFU) per gram of fresh weight (FW) of annual bluegrass within 1 week after treatment and had the potential to cause satisfactory plant wilting (>75% control). The doubling time during the exponential growth phase had a mostly constant time range (7.7–8.5 h) at temperatures of between 24.8 and 34.3°C in plant tissue (25–40°C air temperature). It appears that xylem occlusion occurs as a result of bacterial growth reaching a maximum of 10 10 CFU/g of FW, combined with an increase in polysaccharide production at higher temperatures.