Biodegradation of Agricultural Fungicides
145
product of folpet (Fig. 6.8), while much of the fungicide sulfur is released as
volatile products, including carbonyl sulfide (Siegel and Sisler, 1968), presumed
to arise from the hydrolysis of the intermediate breakdown product thiophosgene
(Somers et ai., 1967). A number of products formed in the reaction of folpet
with cell components contain sulfur derived from the fungicide (Siegel, 1970).
One of these is probably the thiazolidine thione derivative of glutathione (Richmond and Somers, 1968).
TICA is a major metabolite found in the urine of rats fed captan (DeBaun
et ai., 1974). It is apparently formed directly in the reaction of thiophosgene
with cysteine (DeBaun et ai., 1974; Lukens and Sisler, 1958) or in an analogous
reaction with reduced glutathione (Fig. 6.8) followed by the action ofpeptidases.
Other animal metabolites include a salt of dithiobis(methane sulfonic acid) and
the disulfide monoxide derivative of dithiobis(methane sulfonic acid) (Fig. 6.8),
tetrahydrophthalimide, and tetrahydrophthalic acid (DeBaun et ai., 1974; Engst
and Raab, 1973; Lukens and Sisler, 1958).
Although captan and related phthalimide fungicides are easily degraded by
chemical, photochemical, and biological processes, there is little information
concerning the ultimate fate of degradation products such as the phthalimide
moiety or thiazolidinethione derivatives.
6.4.5. Chloroneb
Chloroneb (1,4-dichloro-2,5-dimethoxybenzene) is a narrow-spectrum fungicide used primarily for seed treatment or soil application to control soil-borne
pathogens such as Pythium and Rhizoctonia, It is slightly systemic in plants,
accumulating primarily in the roots and lower portion of the stem (Fielding and
Rhodes, 1967).
A half-life in soil of 3-6 months was determined for chloroneb at an application rate 2.25 kglha (Rhodes et ai., 1971). About 90% of the residue
recovered from soil was chloroneb. The remainder was unidentified but was not
2,5-dichloro-4-methoxyphenol, 2,5-dichlorohydroquinone, or 2,5-dichloroquinone.
Bean plants, (Rhodes et ai., 1971; Thorne, 1973), various fungi (Hock and
Sisler, 1969; Wiese and Vargas, 1973), and animals (Gutenmann and Lisk,
1969; Rhodes and Pease, 1971) demethylate chloroneb to 2,5-dichloro-4-methoxyphenol (Fig. 6.9). Conversion is relatively slow in bean plants. After 12 days
the plants contain about equal amounts of chloroneb and DCMP, which account
for 95% of the chloroneb taken up. More recent work (Thorne, 1973) indicates
that most of the DCMP in bean plants actually exists as the j3-o-g1ucoside (Fig.
6.9). Conjugates of DCMP are also reported as met<: jolites in animals (Gutenmann and Lisk, 1969; Rhodes and Pease, 1971).
Wiese and Vargas (1973) demonstrated that some fungi convert chloroneb
145
product of folpet (Fig. 6.8), while much of the fungicide sulfur is released as
volatile products, including carbonyl sulfide (Siegel and Sisler, 1968), presumed
to arise from the hydrolysis of the intermediate breakdown product thiophosgene
(Somers et ai., 1967). A number of products formed in the reaction of folpet
with cell components contain sulfur derived from the fungicide (Siegel, 1970).
One of these is probably the thiazolidine thione derivative of glutathione (Richmond and Somers, 1968).
TICA is a major metabolite found in the urine of rats fed captan (DeBaun
et ai., 1974). It is apparently formed directly in the reaction of thiophosgene
with cysteine (DeBaun et ai., 1974; Lukens and Sisler, 1958) or in an analogous
reaction with reduced glutathione (Fig. 6.8) followed by the action ofpeptidases.
Other animal metabolites include a salt of dithiobis(methane sulfonic acid) and
the disulfide monoxide derivative of dithiobis(methane sulfonic acid) (Fig. 6.8),
tetrahydrophthalimide, and tetrahydrophthalic acid (DeBaun et ai., 1974; Engst
and Raab, 1973; Lukens and Sisler, 1958).
Although captan and related phthalimide fungicides are easily degraded by
chemical, photochemical, and biological processes, there is little information
concerning the ultimate fate of degradation products such as the phthalimide
moiety or thiazolidinethione derivatives.
6.4.5. Chloroneb
Chloroneb (1,4-dichloro-2,5-dimethoxybenzene) is a narrow-spectrum fungicide used primarily for seed treatment or soil application to control soil-borne
pathogens such as Pythium and Rhizoctonia, It is slightly systemic in plants,
accumulating primarily in the roots and lower portion of the stem (Fielding and
Rhodes, 1967).
A half-life in soil of 3-6 months was determined for chloroneb at an application rate 2.25 kglha (Rhodes et ai., 1971). About 90% of the residue
recovered from soil was chloroneb. The remainder was unidentified but was not
2,5-dichloro-4-methoxyphenol, 2,5-dichlorohydroquinone, or 2,5-dichloroquinone.
Bean plants, (Rhodes et ai., 1971; Thorne, 1973), various fungi (Hock and
Sisler, 1969; Wiese and Vargas, 1973), and animals (Gutenmann and Lisk,
1969; Rhodes and Pease, 1971) demethylate chloroneb to 2,5-dichloro-4-methoxyphenol (Fig. 6.9). Conversion is relatively slow in bean plants. After 12 days
the plants contain about equal amounts of chloroneb and DCMP, which account
for 95% of the chloroneb taken up. More recent work (Thorne, 1973) indicates
that most of the DCMP in bean plants actually exists as the j3-o-g1ucoside (Fig.
6.9). Conjugates of DCMP are also reported as met<: jolites in animals (Gutenmann and Lisk, 1969; Rhodes and Pease, 1971).
Wiese and Vargas (1973) demonstrated that some fungi convert chloroneb