Abstract: Field studies were conducted with a biocontrol agent, Pseudomonas syringae pv. tagetis, to determine the optimal application parameters for controlling woollyleaf bursage, a pernicious perennial weed on the Texas Southern High Plains. Parameters investigated were bacterial concentration, time, season, and frequency of application. Pseudomonas syringae pv. tagetis concentration ranging from 104 to 108 colony forming units (CFU)/ml produced similar incidence of disease on woollyleaf bursage. Weekly, biweekly, and triweekly applications were found to be equally effective. Optimal time for application of P. syringae pv. tagetis was at midday. A single application in April, May, or June resulted in higher incidence of disease for 5 wk than the noninoculated plots, whereas single applications in July and August were not consistently different from the noninoculated plots. Weed density was negatively correlated with disease incidence for April, July, and August applications, particularly when correlations were based on disease incidence from 1 to 4 wk prior to weed density measurement. Weed densities during 1997 were lower at the end of the year when bacteria applications were made in April, May, or June, as opposed to July or August, or plots were not inoculated. In 1998, weed density was lower when bacteria applications were made in April as opposed to May or June. These investigations indicate that P. syringae pv. tagetis does infect woollyleaf bursage on the Texas southern High Plains.
Nomenclature: Woollyleaf bursage; Ambrosia grayi (A. Nels.) Shinners #3 AMBGR; Pseudomonas syringae pv. tagetis; biological control.
Additional index words: Biological control agent, biopesticide.
Abbreviations: AT, application time; B, bacteria; CFU, colony forming units; EP, evaluation period; IOD, incidence of disease.