Growth chamber and field experiments were conducted to assess the potential of Pseudomonas syringae pv. tagetis (Pst) as a biocontrol agent for Canada thistle. Silwet L-77, an organosilicone surfactant, was required to facilitate Pst penetration into Canada thistle leaves. Growth chamber experiments indicated that maximum Pst populations inside leaves were obtained with a Silwet L-77 concentration of 0.3% (v/v) or greater. High Pst populations (10⁹ colony-forming units [cfu] per gram fresh weight) were found in leaves 48 h after treatment with 10⁸ or 10⁹ cfu ml⁻¹ Pst plus Silwet L-77 (0.3%, v/v). In growth chamber experiments, foliar application of Pst (10⁹ cfu ml⁻¹) plus Silwet L-77 (0.3%, v/v) on 4- to 5-wk-old Canada thistle reduced shoot dry weight by 52% (measured 14 d after treatment) and chlorophyll content of emerging leaves by 92% (measured 10 d after treatment). In field trials conducted in 1999 and 2000, Pst (10⁹ cfu ml⁻¹) plus Silwet L-77 (0.3%, v/v) were applied at 700 L ha⁻¹, and the method of application (paint gun, backpack sprayer, boom) and the number of applications (one or two separated by 14 d) were examined. Averaged over 2 yr, two applications with a backpack sprayer resulted in 67% disease incidence (apical chlorosis) of treated plants measured 4 wk after the initial treatment (WAIT). At the time of flower bud formation (8 WAIT), there was little or no disease incidence, 31% reduction in plant height, 81% reduction in number of flower buds, and 20% reduction in shoot survival during 1999 but no effect on survival in 2000.

Nomenclature: Canada thistle, Cirsium arvense (L.) Scop. CIRAR; soybean, Glycine max L. ‘Lambert’, ‘Kato’.