Microbial aspects of accelerated degradation of metam sodium in soil.

Preplant soil fumigation with metam sodium is used worldwide to control soilborne diseases. The development of accelerated degradation of pesticides in soil, including metam sodium, results in reduced pesticide efficacy. Therefore, we studied microbial involvement in accelerated degradation of methyl isothiocyanate (MITC) following repeated soil applications of the parent compound, metam sodium. MITC degradation was reduced in soil with a history of metam sodium applications following sterilization, indicating the key role of microorganisms in accelerated degradation. Accelerated degradation of MITC was induced by inoculation of soil with no previous application of metam sodium with soil with a history of metam sodium applications. We developed a method to extract the active microbial fraction responsible for MITC degradation from soil with a history of metam sodium applications. This concentrated soil extract induced accelerated degradation of MITC when added to two different soils with no previous application of metam sodium. An extensive shift in total bacterial community composition in concentrated soil extracts occurred after a single metam sodium application. Two Oxalobacteraceae strains, MDB3 and MDB10, isolated from Rehovot soil following triple application of metam sodium rapidly degraded MITC in soil with no previous application of metam sodium. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis of bacterial community composition showed relative enrichment of MDB3 following metam sodium application, suggesting its potential in situ involvement in accelerated degradation development in Rehovot soil. Responses of resident Oxalobacteraceae community members to metam sodium applications differed between Rehovot and En Tamar soils. Isolate MDB10 did not induce accelerated degradation of MITC in En Tamar soil and, with the slow dissipation of MITC, soil suppressiveness of accelerated degradation is suggested. The isolation and identification of MITC-degrading bacteria might be helpful in developing tools for managing accelerated degradation.

Accelerated degradation of metam-sodium in soil and consequences for root-disease management.

We studied the development of accelerated degradation (AD) of methyl isothiocyanate (MITC) following repeated applications of its parent compound, metam-sodium (MS). Laboratory studies and four sets of field experiments were conducted during 2002-04 in three commercial fields in Israel. Repeated applications of MS to the three soils in the laboratory under controlled conditions demonstrated AD of MITC in some soils. In a peanut field, MS significantly reduced the incidence of Pythium pod rot and improved pod quality after a single application but its effectiveness was greatly reduced after two applications. In a second experiment, MS was significantly effective after a single application in controlling Verticillium wilt in potato but its efficacy diminished after three consecutive applications. In an additional experiment, fumigation with MS following single or double applications was more effective in reducing Verticillium wilt severity of potato compared with triple applications. Soils which did not develop AD of MITC were also recorded. Preplant MS fumigation of melon fields was effective at reducing sudden wilt following a single and two consecutive applications. Our study shows that development of AD of MITC might occur following repeated applications of MS in commercial fields. The data on MITC dissipation in soil following repeated MS applications under controlled conditions indicate the chemical's potential loss of activity under regular agricultural practices and the need for a management strategy to prevent such a development.

Generation and Dissipation of Methyl Isothiocyanate in Soils Following Metam Sodium Fumigation: Impact on Verticillium Control and Potato Yield.

The fate of methyl isothiocyanate (MITC) was studied in agricultural soils following metam sodium (MS) application in a controlled system and under field conditions as it was related to disease control. Soil samples were collected from 34 field sites in Israel with no history of MS application. The generation and dissipation curves of MITC in these soils, under controlled conditions, varied significantly among the soils, as reflected by the concentration by time (C × T) product. This value was significantly related with the mortality level of Fusarium oxysporum f. sp. radicis-lycopersici as a test organism and sand content of the soils. Seven field experiments were conducted in potato fields from 2001 to 2004. The MS treatments significantly reduced Verticillium wilt incidence and severity in five and four experiments, respectively, out of seven. Combining MS with formalin was more effective for controlling disease than MS alone in most cases. A significant relationship was found between mortality of F. oxysporum f. sp. radicis-lycopersici in soil samples to which MS was applied under controlled conditions and the incidence of Verticillium wilt disease in the field, and between CMITC × T products and the incidence of Verticillium wilt disease in the field. These tests can be used for preplant assessment of potential MS efficacy.

Development of crown and root rot disease of tomato under irrigation with saline water.

ABSTRACT We studied the effect of water salinity on the incidence and severity of crown and root rot disease of tomato, as well as on the pathogen and on the plant's response to the pathogen. Irrigation with saline water significantly increased disease severity in tomato transplants inoculated with Fusarium oxysporum f. sp. radicis-lycopersici, and mineral fertilization further increased it. In one field experiment, disease incidence in plots irrigated with saline water (electrical conductivity [EC] = 3.2 +/- 0.1 dS m(-1)) and in those irrigated with fresh water (EC = 0.4 +/- 0.1 dS m(-1)) was 75 and 38%, respectively. Disease onset was earlier and yield was lower in plots irrigated with saline water. In a second field experiment, final disease incidence 250 days after planting, was 12% in plants which had been irrigated with saline water (EC = 4.6 +/- 0.1 dS m(-1)) and 4% in those irrigated with fresh water (EC = 1.2 +/- 0.1 dS m(-1)). Irrigation of tomato transplants with 20 mM NaCl did not inhibit plant development, but partial inhibition was observed at higher NaCl concentrations. Growth of the pathogen in culture or survival of conidia added to soil were not affected by saline water. Plants which were preirrigated with saline water were more severely diseased than those preirrigated with tap water. It was concluded that disease increases effected by saline water are associated with the latter's effect on plant response.

Dissipation of soil fumigants from soil following repeated applications.

BACKGROUND: The dissipation of pesticides in soil and the occurrence of accelerated degradation following repeated applications are well-known phenomena with many pesticides, but much less so with soil fumigants. The fate of various soil fumigants was studied in different agricultural soils following repeated applications of chloropicrin. RESULTS: Fumigant dissipation reflected by Σconcentration × time (ΣC × T) and half-life values varied widely among the tested soils. Methyl iodide (MI) had the slowest dissipation rate compared with other fumigants in all tested soils. Elimination of biotic agents by soil sterilization prior to MI application did not affect MI concentration in Oxnard soil. Clay content and fumigant dose (ΣC × T values) of chloropicrin, 1,3-dichloropropene and MI were significantly correlated. No significant correlations were found between soil properties and ΣC × T values following metam sodium and methyl bromide (MBr) application. CONCLUSION: The fate of the tested soil fumigants is highly dependent on and specific to the fumigant, previous fumigant application and soil type. This study suggests that biotic factors are more essential in the dissipation of metam sodium and MBr than abiotic factors. By contrast, MI dissipation from the tested soils is affected more by abiotic factors than by biotic activities.