Abiotic soil factors influencing the deleterious effect of atrazine on ungerminated conidia of Cochliobolus sativus

Abstract

To determine why viability of conidia of Cochliobolus sativus declines in some soils treated with atrazine and not in others, the influence of soil organic matter, texture and pH on the lethal effect of atrazine was examined. Viability of conidia on Boyer sandy loam (SL) (−1 kPa matric potential) containing 25μg atrazine g −1 was 7% after 3 weeks, as compared with 99% in the control. Decreasing the organic carbon of Boyer SL from 0.73 to 0.02% by H 2O 2 digestion, or to 0.04% by NaOH extraction, nullified the lethal effect of atrazine. The addition of 4mg humic acid g −1 to NaOH-extracted Boyer SL containing atrazine partially restored the lethal effect. Increasing the pH of Boyer SL from 5.2 to 7.5 nullified the lethal effect of atrazine. Viability of conidia on Spinks SL (pH 6.6) containing atrazine remained at 99% after 3 weeks. The addition of 4mg humic acid g −1 from Boyer SL to atrarine-treated Spinks SL reduced viability to 86%. Viability of conidia in atrazine-treated acidified Spinks SL (pH 5.4) was 65%. The response of conidia to atrazine in soils supplemented with 4% bentonite clay, or in separated sand or silt and clay fractions of soils was not affected except when the soil pH was altered. Thus, a low pH and the presence of humic acid increased the toxicity of atrazine to conidia of C. sativus.