140
Hugh D. Sisler
biological involvement. Nabam, for example, is extremely unstable in aqueous
solutions and is transformed to the following products: ethylenethiourea (ETU),
5,6-dihydro-3H-imidazo(2,I-c)-1,2,4-dithiazole-3-thione (DIDT) , polymeric
ethylenethiuram disulfide, ethylenediamine, ethylenediisothiocyanate, elemental
sulfur, CS2 , and H2S (Kaars Sijpesteijn et al., 1977). Zineb and maneb yield
the same decomposition products in aqueous solution as nabam, but at a slower
rate (Kaars Sijpesteijn et al., 1977).
ETU (Fig. 6.3), an impurity found in technical EBDC fungicides (Woodcock, 1977), is also a major degradation product of these fungicides in various
environments (Anomymous, 1977) and is a product about which there has been
much concern because of its potential hazard to man (Fishbein, 1977). Although
ETU forms by chemical degradation of EDBC fungicides, microorganisms accelerate formation from the intermediate breakdown product DIDT. Bacteria and
fungi as well as crude enzyme preparations from these organisms convert DIDT
to ETU; however, these microorganisms do not degrade ETU (Vonk, 1975).
Soil degradation products of nabam include ETU, DIDT, CS2 , H2S, and
COS (Kaufman, 1977; Kaufman and Fletcher, 1973; Moje et al., 1964; Munnecke, 1972). ETU is degraded rapidly in nonsterile soil; 200 ppm is converted
to ethyleneurea in 8 days (Kaufman and Fletcher, 1973). A similar but slower
conversion of ETU occurs in autoclaved soil; however, further degradation of
ethyleneurea with evolution of CO2 occurs only in nonsterile soil (Kaufman and
Fletcher, 1973). Microorganisms therefore are apparently involved not only in
hastening conversion of ETU to ethyleneurea, but also in degrading the latter.
Hydation and Jaffe's base are also metabolites of ETU found in soil (Kaufman
and Fletcher, 1973).
Ethyleneurea and 2-imidazoline are metabolites of ETU in plants (Hoagland
and Frear, 1976; Vonk, 1975). In cucumbers, the conversion of ETU to these
products is rapid; the products are subject to further degradation (Kaars Sijpesteijn
et al., 1977).
Although ETU is subject to rapid degradation by light and microorganisms,
the compound is reasonably stable in the absence of these agents. Formation of
ETU from EBDC residues on food crops may be accentuated by heating during
processing (Anomymous, 1977; Newsome, 1976). Every effort should therefore
be made to reduce these residues to the lowest possible level before processing.
6.4.3.2. Dialkyldithiocarbamates
Agricultural fungicides in this group include the water-insoluble derivatives,
zinc dimethyldithiocarbamate (ziram), ferric dimethyldithiocarbamate (ferbam),
and the oxidation product tetramethylthiuram disulfide (thiram). Sodium dimethyldithiocarbamate (NaDDC), a fungitoxic water-soluble salt, has no practical
agricultural application but is frequently used in mode of action or transformation
Hugh D. Sisler
biological involvement. Nabam, for example, is extremely unstable in aqueous
solutions and is transformed to the following products: ethylenethiourea (ETU),
5,6-dihydro-3H-imidazo(2,I-c)-1,2,4-dithiazole-3-thione (DIDT) , polymeric
ethylenethiuram disulfide, ethylenediamine, ethylenediisothiocyanate, elemental
sulfur, CS2 , and H2S (Kaars Sijpesteijn et al., 1977). Zineb and maneb yield
the same decomposition products in aqueous solution as nabam, but at a slower
rate (Kaars Sijpesteijn et al., 1977).
ETU (Fig. 6.3), an impurity found in technical EBDC fungicides (Woodcock, 1977), is also a major degradation product of these fungicides in various
environments (Anomymous, 1977) and is a product about which there has been
much concern because of its potential hazard to man (Fishbein, 1977). Although
ETU forms by chemical degradation of EDBC fungicides, microorganisms accelerate formation from the intermediate breakdown product DIDT. Bacteria and
fungi as well as crude enzyme preparations from these organisms convert DIDT
to ETU; however, these microorganisms do not degrade ETU (Vonk, 1975).
Soil degradation products of nabam include ETU, DIDT, CS2 , H2S, and
COS (Kaufman, 1977; Kaufman and Fletcher, 1973; Moje et al., 1964; Munnecke, 1972). ETU is degraded rapidly in nonsterile soil; 200 ppm is converted
to ethyleneurea in 8 days (Kaufman and Fletcher, 1973). A similar but slower
conversion of ETU occurs in autoclaved soil; however, further degradation of
ethyleneurea with evolution of CO2 occurs only in nonsterile soil (Kaufman and
Fletcher, 1973). Microorganisms therefore are apparently involved not only in
hastening conversion of ETU to ethyleneurea, but also in degrading the latter.
Hydation and Jaffe's base are also metabolites of ETU found in soil (Kaufman
and Fletcher, 1973).
Ethyleneurea and 2-imidazoline are metabolites of ETU in plants (Hoagland
and Frear, 1976; Vonk, 1975). In cucumbers, the conversion of ETU to these
products is rapid; the products are subject to further degradation (Kaars Sijpesteijn
et al., 1977).
Although ETU is subject to rapid degradation by light and microorganisms,
the compound is reasonably stable in the absence of these agents. Formation of
ETU from EBDC residues on food crops may be accentuated by heating during
processing (Anomymous, 1977; Newsome, 1976). Every effort should therefore
be made to reduce these residues to the lowest possible level before processing.
6.4.3.2. Dialkyldithiocarbamates
Agricultural fungicides in this group include the water-insoluble derivatives,
zinc dimethyldithiocarbamate (ziram), ferric dimethyldithiocarbamate (ferbam),
and the oxidation product tetramethylthiuram disulfide (thiram). Sodium dimethyldithiocarbamate (NaDDC), a fungitoxic water-soluble salt, has no practical
agricultural application but is frequently used in mode of action or transformation