|Plaisance, K - UNIVERSITY OF MINNESOTA|
|Johnson, D - ENCORE TECHNOLOGIES|
Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: February 10, 1999
Publication Date: N/A
Technical Abstract: Previous research demonstrated the potential of Pseudomonas syringae pv. tagetis (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) provides good control of Canada thistle without causing injury to soybean. A better understanding of the factors regulating the efficacy of Pst on Canada thistle is needed to develop a formulation that will control Canada thistle with one application. The dynamics of the endophytic Pst population and tagetitoxin production in Canada thistle leaves following foliar application were investigated. Optimum concentration of Silwet L-77 for obtaining maximum penetration of Pst into leaves was about 0.3% (v/v). When Pst (10*6 to 10*9 cfu/ml) was sprayed onto mature leaves, initial endophytic populations were directly related to the concentration of bacteria applied. Following foliar application of 10*9 cfu/ml, endophytic populations reached a plateau of about 10*8 cfu/gram fresh weight after 48 h. Endophytic populations were an order of magnitude higher along leaf edges (outer 2 mm) versus the leaf midsection. During a 2 week period following foliar application, endophytic populations declined to levels approximating those at the time of application. Pst was able to move from mature, treated leaves to newly developed leaves via the vascular system. Endophytic Pst populations in newly developed leaves, measured 1 week after foliar application of 10*9 cfu/ml, ranged from 10*3 to 10*5 cfu/gram fresh weight. Tagetitoxin production, as measured by chlorosis of newly developed leaves 10 days after Pst application, was variable. About 60% of plants treated with 10*9 cfu/ml Pst exhibited chlorosis in newly developed leaves.