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Hugh D. Sisler
gus Pyricularia oryzae is markedly inhibited by another synergistic fungicide,
diisopropyl S-benzylphosphorothiolate (Kitazin) (Uesugi and Sisler, 1978). The
latter compound apparently inhibits a hydroxylase or N-demethylase involved
in the metabolism of the former.
Several fungicides such as imazalil, triadimefon, triforine, and fenarimol
strongly inhibit sterol C-14 demethylation, possibly by interfering with a cytochrome P-450 mixed-function oxidase (Ragsdale, 1977). Although these fungicides are probably not general inhibitors of this type of activity (Ragsdale,
1977), the possibility that they may interfere with the oxidative metabolism of
other pesticides should not be overlooked.
6.6. SUMMARYAND CONCLUSIONS
With a few minor exceptions, agricultural fungicides do not pose a serious
acute toxicity hazard to mammals and most other forms of animals life. On the
other hand, obscure or delayed toxic effects of chemicals are not always recognized; therefore exposure to these compounds should be held to the minimum
possible levels.
Most organic fungicides do not remain for long periods in the environment,
since they are readily degraded by chemical, photochemical, or biological processes. There are notable exceptions, however, such as PCNB, which may persist
in some soils for 3 or more years. Metabolites of fungicides sometimes persist
for much longer periods than the parent compound. PCNB, for example, is
rapidly reduced in anaerobic soils to PCA, a compound which remains largely
undegraded after 2 or more years. Benomyl is rapidly converted to carbendazim
and other residues in soils which may have a half-life as long as 1 year. The
ultimate fate of many organic fungicide degradation products has not been fully
determined. More information is needed concerning the fate of such residues as
the phthalimide moiety of captan and related compounds, the benzimidazole
moiety of benomyl and related fungicides, and PCA derived from PCNB.
Environmental factors can have a marked effect on the biodegradation of
fungicides. Compounds which readily degrade in soils under favorable conditions
for microbiological activity may persist for very long periods in unfavorable
environments such as extreme pH, very low temperatures, or anaerobic conditions.
Interaction of a fungicide or its breakdown products with other substances
under certain conditions can lead to the formation of hazardous or otherwise
undesirable products. An example is the formation of dimethylnitrosomine from
thiram or its breakdown products in acidic soils under flooded condtions.
Hugh D. Sisler
gus Pyricularia oryzae is markedly inhibited by another synergistic fungicide,
diisopropyl S-benzylphosphorothiolate (Kitazin) (Uesugi and Sisler, 1978). The
latter compound apparently inhibits a hydroxylase or N-demethylase involved
in the metabolism of the former.
Several fungicides such as imazalil, triadimefon, triforine, and fenarimol
strongly inhibit sterol C-14 demethylation, possibly by interfering with a cytochrome P-450 mixed-function oxidase (Ragsdale, 1977). Although these fungicides are probably not general inhibitors of this type of activity (Ragsdale,
1977), the possibility that they may interfere with the oxidative metabolism of
other pesticides should not be overlooked.
6.6. SUMMARYAND CONCLUSIONS
With a few minor exceptions, agricultural fungicides do not pose a serious
acute toxicity hazard to mammals and most other forms of animals life. On the
other hand, obscure or delayed toxic effects of chemicals are not always recognized; therefore exposure to these compounds should be held to the minimum
possible levels.
Most organic fungicides do not remain for long periods in the environment,
since they are readily degraded by chemical, photochemical, or biological processes. There are notable exceptions, however, such as PCNB, which may persist
in some soils for 3 or more years. Metabolites of fungicides sometimes persist
for much longer periods than the parent compound. PCNB, for example, is
rapidly reduced in anaerobic soils to PCA, a compound which remains largely
undegraded after 2 or more years. Benomyl is rapidly converted to carbendazim
and other residues in soils which may have a half-life as long as 1 year. The
ultimate fate of many organic fungicide degradation products has not been fully
determined. More information is needed concerning the fate of such residues as
the phthalimide moiety of captan and related compounds, the benzimidazole
moiety of benomyl and related fungicides, and PCA derived from PCNB.
Environmental factors can have a marked effect on the biodegradation of
fungicides. Compounds which readily degrade in soils under favorable conditions
for microbiological activity may persist for very long periods in unfavorable
environments such as extreme pH, very low temperatures, or anaerobic conditions.
Interaction of a fungicide or its breakdown products with other substances
under certain conditions can lead to the formation of hazardous or otherwise
undesirable products. An example is the formation of dimethylnitrosomine from
thiram or its breakdown products in acidic soils under flooded condtions.