SURVIVAL OF XANTHOMONAS-CAMPESTRIS PV TRANSLUCENS BETWEEN SUCCESSIVE WHEAT CROPS IN ARKANSAS

Reducing the level of initial inoculum may be a feasible means of controlling bacterial streak and black chaff of wheat caused by Xanthomonas campestris pv. translucens. The objective of this research was to determine the principal source of inoculum under Arkansas conditions. Using artificially infested seeds, a rifampicin-resistant mutant of the pathogen (strain 88-14(Rif)) was established in field plots of wheat cultivars Florida 302 (susceptible) and Terral 101 (moderately resistant) at two locations and in a plot of Florida 302 at a third location. Bacterial streak caused by strain 88-14(Rif) developed in all plots. Strain 88-14(Rif) was not detected in crop debris, soil, or possible alternative host plants in the field 3 mo after harvest at any location. No bacterial streak symptoms were observed at any location on Florida 302 planted with disinfested seed 4 mo after harvest. However, strain 88-14(Rif) was isolated from one of 480 Florida 302 leaves assayed from one location at one sampling time. The percentage of harvested seed infested with strain 88-14(Rif) 2 mo after harvest ranged from 9.8 to 37.7% for Florida 302 and from 0.3 to 5.3% for Terral 101. Under growth chamber conditions, transmission of seedborne strain 88-14(Rif) to seedlings ranged from 4.0 to 24.5% for Florida 302 and from 0.0 to 0.2% for Terral 101. Under field conditions, however, strain 88-14(Rif) was isolated only from one seedling of one Florida 302 seed lot. Based on the poor survival of the pathogen in the field and the relatively high percentage of infested seed and transmission to seedlings under growth chamber conditions for the susceptible cultivar, infested seed is suspected to be the principal source of inoculum.