The effect of olive oil mill wastewater (OMW) on soil microbial communities and suppressiveness against Rhizoctonia solani

The effect of olive oil mill wastewater (OMW), an important pollutant of agricultural origin, on soil suppressiveness against the plant pathogenic fungus Rhizoctonia solani was examined. Soil was treated with OMW and the pathogen was introduced into the soil 45 days after the last treatment, while lettuce seeds were planted at four successive time periods after pathogen introduction. The damping-off due to R. solani in soil that has been previously treated with OMW was significantly reduced compared to the control (soil treated with water). The effect on soil suppressiveness of the aerobically treated olive mill wastewater (TOMW) was also examined, but was not found to be significantly different from the control. The OMW treated soil exhibited significantly higher respiration compared to the other treatments during the period of waste addition, which immediately decreased when the additions stopped. The bacterial population of r-strategists in the OMW treated soil was significantly higher compared to the soil that received TOMW or water. A transitory phytotoxic effect of the wastes was also observed, but this did not correlate with soil suppressiveness. Results suggest that addition of OMW and initially TOMW to soil creates a nutrient rich environment that is dominated by r-strategists. Such an environment provides unfavourable growth conditions for R. solani. (C) 2003 Elsevier B.V. All rights reserved.