Soil properties associated with the variable effectiveness of meat and bone meal to kill microsclerotia of Verticillium dahliae

In previous studies we showed that adding meat and bone meal (MBM) to a sandy soil resulted in the death of microsclerotia (MS) of the fungal wilt pathogen Verticillium dahliae. No effect on MS was found when the same quantity of MBM was added to a loam soil. MS mortality in the sandy soil was attributed to the accumulation of ammonia (NH3) or nitrous acid (HNO2), products that did not accumulate to lethal levels in the loam soil. The objective of this study was to identify soil properties associated with the accumulation of NH3 and HNO2. Using a microcosm system we tracked the survival of NIS and changes in pH and concentrations of NH3 and HNO2 over a 29-day incubation period in 12 soils from diverse regions and amended with MBM (0 or 2% (w/w) dry soil). Prior to the initiation of the study, and at periodic intervals after amendment, we also determined numerous biological, chemical, and physical soil factors possibly influencing NH3 and HNO2 accumulation. The results showed that NIS died within 7 days in four soils due to NH3 accumulation. In six soils where NH3 levels were < 20 mM, MS germination was reduced, but the effects were only fungistatic as germination returned when NH3 dissipated. In two acid soils, HNO2 was lethal at levels exceeding 0.02 mM but not in four soils of basic pH which also accumulated similar levels of this gas. The influence of acidity in enhancing the lethal effect of HNO2 was confirmed in an aqueous exposure assay. Germination of MS was unaffected in four soils where neither NH3 nor HNO2 accumulated. Univariate and multivariate statistical analyses corroborated that NH3 toxicity occurred in soils with low rates of nitrification, low levels of cation exchange capacity, moisture levels, organic carbon contents, but high bulk density and sand contents. The results indicate that soil properties are related to the effectiveness of MBM and likely other nitrogenous amendments in reducing the viability of soilborne pathogens by influencing the accumulation and lethal effect of NH3 or HNO2. The results of this and other studies in our laboratories examining the effect of individual soil properties indicate simply measuring such properties as organic carbon content and pH may be sufficient to predict where MBM will be effective to control Verticillium wilt. (C) 2003 Elsevier B.V. All rights reserved.