Soil factors influencing the efficacy of liquid swine manure added to soil to kill Verticillium dahliae

Addition of liquid swine manure (SwM) to field soils killed Verticillium dahliae microsclerotia (MS) and reduced verticillium wilt of potato, but only at one (site B) of several fields tested. This study examined what factors in soil influence the capability of SwM to kill V. dahliae MS. When added to soil from site B in a microcosm assay, the SwM used in the field study killed MS within I day after application. Also, efficacy increased as the concentration of SwM increased, indicating that one or more components were directly toxic to MS. Toxicity of SwM to MS was reduced with increasing soil moisture, indicating that the active product(s) was (were) undergoing dilution. Adjusting the pH of site B soil from 5.0 to 6.5 eliminated the toxicity of the SwM. Conversely, when soil from a location where SwM had no effect was reduced from its initial value of 7.5 to below 6, MS mortality occurred. Adding increasing concentrations of SwM at pH 7.7 to soils commonly raised the pH of the mixture to levels where efficacy of kill became progressively less effective. In soils with high buffering capacity, the pH did not immediately rise and MS kill increased with higher rates. Liquid swine manure killed MS to the same extent in soils ranging from sand to loam when these were made equal with respect to pH and SwM concentration. It was also equally effective in soils with organic carbon contents ranging from 1.4 to 6% when soil pH and moisture levels were made equal. Increasing soil temperature slightly improved the toxicity of the SwM. The results demonstrate that the efficacy of SwM in soil to kill V. dahliae MS is most influenced by soil pH and, to a lesser extent, by soil moisture level, buffering capacity, and temperature.