|Plaisance, K - UNIVERSITY OF MINNESOTA|
|Johnson, D - ENCORE TECHNOLOGIES INC|
Submitted to: International Symposium on Biological Control of Weeds
Publication Type: Abstract Only
Publication Acceptance Date: July 9, 1999
Publication Date: N/A
Technical Abstract: Pseudomonas syringae pv. tagetis (Pst) causes apical chlorosis in Canada thistle (Cirsium arvense) and other composite weeds due to the production of tagetitoxin, a RNA polymerase III inhibitor that blocks chloroplast biogenesis. Previous research demonstrated the potential of Pst as a biological agent for controlling Canada thistle in soybean. Multiple inundative foliar applications of Pst in an aqueous suspension containing an organosilicone surfactant (Silwet L-77) provided good control of Canada thistle without causing injury to soybean. A better understanding of factors regulating the efficacy of inundative Pst applications on Canada thistle is needed to develop a formulation that will control this weed with a single application. The dynamics of the endophytic Pst population and tagetitoxin production in Canada thistle leaves were measured following inundative application. The optimum concentration of Silwet L-77 for obtaining maximum foliar penetration of Pst was approximately 0.3% (v/v). Initial endophytic populations in mature leaves were directly related to the concentration of Pst applied (10**6 -10**9 cfu/ml). Within 48 h after foliar application of 10**9 cfu/ml Pst, endophytic populations reached a plateau of approximately 10**8 cfu/gram fresh weight. During a two week period following foliar application, endophytic populations declined to levels approximating those measured at the time of application. Tagetitoxin production, as measured by chlorosis of newly developed leaves 10 days after Pst application, was variable. Pst concentrations greater than 10**6 cfu/ml were required to get good symptom development. Approximately 60% of plants treated with 10**9 cfu/ml Pst exhibited chlorosis in newly developed leaves.