Submitted to: International Congress of Plant Pathology Abstracts and Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 8/14/1998
Publication Date: N/A
Technical Abstract: Pseudomonas syringae pv. tagetis (Pst) offers the potential for biological control of composite weeds in soybeans. Inundative 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, but has no effect on soybeans. Chlorosis in developing leaves of sunflower and Canada thistle is due to the production of tagetitoxin by Pst. Population dynamics of endophytic Pst following foliar application were evaluated in host (sunflower, Canada thistle) and nonhost (soybean) plants. The endophytic Pst population in sunflower leaves, measured 60 min after spraying with 10**9 cfu/ml, was 10**7 cfu/g fresh wt. The endophytic population increased rapidly during the subsequent 24 h, reaching a peak of 10**9 cfu/g fresh wt which declined slightly during the subsequent 48 h. Chlorophyll content of sunflower leaves that emerged following application of 10**9 cfu/ml Pst was reduced by about 90%. When sunflower was sprayed with 10**6 cfu/ml, the initial endophytic population was 10**5 cfu/g fresh wt which increased to a maximum of 10**8 cfu/g fresh weight after 48 h. In most cases, spraying sunflower plants with 10**6 cfu/ml Pst had little effect on chlorophyll content of developing leaves. Spraying 10**9 cfu/ml Pst on soybean leaves resulted in a low endophytic population (10**5 cfu/g fresh wt) due to entrapment of the spray droplets by leaf trichomes. In contrast to sunflower, the endophytic Pst population in soybean leaves did not increase during a 72 h interval after application nor was chlorophyll content of developing leaves reduced. Canada thistle plants sprayed with 10**9 cfu/ml Pst exhibited variable endophytic populations and injury as measured by chlorosis of developing leaves.