2.10.21
PSEUDOMONAS SYRINGAE PV. TAGETIS AS A BIOLOGICAL CONTROL AGENT FOR WEEDS: POPULATION DYNAMICS IN LEAVES OF HOST AND NONHOST SPECIES

JW GRONWALD1, KL PLAISANCE2, DR JOHNSON2 and DL WYSE2

1USDA-ARS, Plant Science Research, St Paul, Minnesota, USA; 2Department of Agronomy and Plant Genetics, University of Minnesota, St Paul, Minnesota, USA

Background and objectives
Pseudomonas syringae pv. tagetis(Pst) offers the potential for biological control of composite weeds in soybeans [1]. Innundative foliar application of Pst in an aqueous suspension containing an organosilicone surfactant (Silwet L-77,0.3%) provides good control of sunflower, variable control of Canada thistle (Cirsium arvense), but has no effect on soybeans. Chlorosis in developing leaves of sunflower and Canada thistle results from the production of tagetitoxin by Pst. Population dynamics of endophytic Pst following foliar application and plant response to injected tagetitoxin were evaluated in host (sunflower, Canada thistle) and nonhost (soybean) plants.

Materials and methods
Plants were grown in a growth chamber with a 16-h photoperiod and day/night temperatures of 25/20C, respectively. Pst (106 to 109 ;cfu/ml) was sprayed (approximately 1 ;ml/plant) on leaves of 8-day-old sunflower and soybean plants and 15-day-old Canada thistle. At selected intervals, epiphytic Pst populations were removed from the leaf surface by rinsing with 70% ethanol and sterile water. Discs were removed from mature leaves, homogenized in sterile water and endophytic populations determined by serial dilution and plating. To remove trichomes from soybean leaves, the leaf surface was rubbed with 70% ethanol using sterile gauze. Tagetitoxin (500 ng) was injected into the cotyledons of 8-day-old sunflower and soybean using a syringe. The chlorophyll content of developing leaves was determined.

Results and conclusions
The endophytic Pst population in sunflower leaves, measured 60 ;min after spraying with 109 ;cfu/ml, was 101 ;cfu/g fresh wt. The endophytic population increased rapidly during the subsequent 24 ;h, reaching a peak of 109 ;cfu/g fresh wt, which declined slightly during the subsequent 48 ;h. Chlorophyll content of sunflower leaves that emerged following application of 109 cfu/ml Pst was reduced by approximately 90%. When sunflower was sprayed with 101 ;cfu/ml, the endophytic population increased to a maximum of approximately 1011 ;cfu/g fresh weight after 48 ;h. In most cases, spraying sunflower plants with 106 ;cfu/ml Pst had little effect on chlorophyll content of developing leaves. Spraying 101 ;cfu/ml Pst on soybean leaves resulted in an initial endophytic population of 105 ;cfu/g fresh wt. The low endophytic population was apparently due to entrapment of the spray droplets by leaf trichomes. Removing leaf trichomes prior to spraying by rubbing the leaf surface with ethanol resulted in an endophytic population of 101 ;cfu/g fresh wt. In contrast to sunflower, the endophytic Pst population in soybean leaves did not increase during a 72-h interval after application nor was the chlorophyll content of developing leaves reduced. Canada thistle plants sprayed with 109 ;cfu/ml Pst exhibited variable endophytic populations and injury, as measured by chlorosis of developing leaves. Injecting tagetitoxin (500 ;ng) into sunflower cotyledons reduced chlorophyll content of developing leaves by approximately 90%. In contrast, injecting tagetitoxin (500 ;ng) into soybean cotyledons had no effect on chlorophyll content of the developing trifoliate. The results suggest that soybean may not be injured by innundative foliar applications of Pst owing to multiple mechanisms, including restricted entry of the bacterium into leaves and reduced susceptibility to tagetitoxin.

References
1. Johnson DR, Wyse DL, Jones KJ, 1996. Weed Technology 10, 621-624.