Effects of three soil-applied herbicides on populations of plant disease suppressing bacteria in the cotton rhizosphere

Abstract

The potential impact of three widely used herbicides, pendimethalin, prometryn, and trifluralin, on populations of five plant disease suppressing bacterial isolates (three isolates of Pseudomonas fluorescens and two isolates of Burkholderia cepacia) in the rhizosphere of cotton seedlings was investigated. All isolates are efficient cotton root colonizers and each is capable of suppressing a plant disease. In microcosm experiments, application of each of the test herbicides at the rates of 1, 2, and 4 µg active ingredient (a.i.) g^-1 soil caused significant (p<0.05) reductions in populations of most of the isolates in the rhizosphere, 14 days after the release of bacteria into the soil by seed coating. The responses of the isolates to the herbicides varied depending on the isolate and the type and concentration of the herbicides. In microcosm experiments the impact of pendimethalin, prometryn, and trifluralin at the respective concentrations of 2.4, 3.6, and 1.8 μg a.i. g^-1 soil on the population of isolate D1 in the cotton rhizosphere declined with time during a four week period of monitoring following the release of the isolate into the soil by seed coating. The impact of soil applied test herbicides on the population sizes of D1 in cotton rhizosphere was also studied in two field experiments (Safford and Tucson, Arizona) where the bacteria were added as a soil drench. In the Safford experiment pendimethalin and prometryn, but not trifluralin, caused significant (p <0.05) reductions in the population of the bacterium 15 days after sowing. In the Tucson experiment a significant (p < 0.05) reduction in the population of the bacterium was observed 15 and 25 days after sowing in soils treated with pendimethalin and prometryn and 25 days after sowing in soils treated with trifluralin.