138
Hugh D. Sisler
hances microbial degradation of PCNB (Ko and Farley, 1969; Wang and Broadbent, 1973). In moist, nonsterilized soil, 82% of the PCNB persists after 3 weeks
whereas less than 1% remains in submerged soil (Ko and Farley, 1969). The
major metabolite of PCNB in soil is pentachloroaniline (PCA) (Fig. 6.2), the
formation of which is favored by reducing conditions (Ko and Farley 1969).
Murthy and Kaufman (1978) demonstrated that [14C]-PCNB is converted to [14(:]PCA and [14C]pentachlorothioanisole ([14C]-PCfA) in flooded soils. After 40
days the products recovered in hexane consisted of 13% PCNB, 86% PCA, and
0.5% PCfA. Although pentachlorophenol (PCP) was detected, no 14C02 was
evolved, indicating a high resistance of the metabolites to complete degradation.
PCA appears to be a very stable compound in soils (Ko and Farley, 1969). Little
loss of this metabolite from aerobic soils occurred even after a period of nearly
2 years (D. D. Kaufman, personal communication).
A variety of fungi and actinomycetes in pure culture convert PCNB to PCA
and PCTA (Chacko et a/., 1966; Nakanishi and Oku, 1969). On the other hand,
some microorganisms oxidize PCA to PCNB (Kaufman, 1977), a factor which
may account for the long persistence of PCNB in aerated soils (Kaars Sijpesteijn,
et a/., 1977; Kaufman, 1977). Plants take up PCNB from the soil. The metabolites PCA and PCTA are found in the tissues (Gorbach and Wagner, 1967;
Kuchar et a/., 1969), but whether these metabolites are formed from PCNB in
the tissue or are taken up from the soil, or both, remains an interesting question.
The ultimate fate of PCA and PCTA is of interest. Murthy and Kaufman
(1978) regard them as end products of PCNB degradation in the soil and assume
that they remain as residues unless absorbed by plants. Conversion to PCP and
further breakdown of this metabolite would seem to be the only pathway for
further degradation (Murthy and Kaufman, 1978).
Soils treated with technical PCNB, which contains impurities of pentachlorobenzene, hexachlorobenzene, and tetrachloronitrobenzene, can be expected
to contain appreciable residues of the parent compound and of these impurities
as well as PCA and PCTA for at least 2-3 years (Beck and Hansen, 1974). It
would seem desirable, therefore, to have substitutes for PCNB which are more
readily degradable.
6.4.3. Dithiocarbamates
Dithiocarbamates constitute the most important class of organic fungicides
used for plant disease control on a worldwide basis (EBDC, Fungicide Assessment Team, 1977). Two major groups within this class, the monoalkyl and the
dialkyl derivatives, are recognized in accordance with differing modes of action
and degradation pathways (Kaars Sijpesteijn et a/., 1977; Kaars Sijpesteijn and
van der Kerk, 1954). The former group, drived from primary amines, possess
a reactive hydrogen on the nitrogen atom(s) and as a consequence yield con-
Hugh D. Sisler
hances microbial degradation of PCNB (Ko and Farley, 1969; Wang and Broadbent, 1973). In moist, nonsterilized soil, 82% of the PCNB persists after 3 weeks
whereas less than 1% remains in submerged soil (Ko and Farley, 1969). The
major metabolite of PCNB in soil is pentachloroaniline (PCA) (Fig. 6.2), the
formation of which is favored by reducing conditions (Ko and Farley 1969).
Murthy and Kaufman (1978) demonstrated that [14C]-PCNB is converted to [14(:]PCA and [14C]pentachlorothioanisole ([14C]-PCfA) in flooded soils. After 40
days the products recovered in hexane consisted of 13% PCNB, 86% PCA, and
0.5% PCfA. Although pentachlorophenol (PCP) was detected, no 14C02 was
evolved, indicating a high resistance of the metabolites to complete degradation.
PCA appears to be a very stable compound in soils (Ko and Farley, 1969). Little
loss of this metabolite from aerobic soils occurred even after a period of nearly
2 years (D. D. Kaufman, personal communication).
A variety of fungi and actinomycetes in pure culture convert PCNB to PCA
and PCTA (Chacko et a/., 1966; Nakanishi and Oku, 1969). On the other hand,
some microorganisms oxidize PCA to PCNB (Kaufman, 1977), a factor which
may account for the long persistence of PCNB in aerated soils (Kaars Sijpesteijn,
et a/., 1977; Kaufman, 1977). Plants take up PCNB from the soil. The metabolites PCA and PCTA are found in the tissues (Gorbach and Wagner, 1967;
Kuchar et a/., 1969), but whether these metabolites are formed from PCNB in
the tissue or are taken up from the soil, or both, remains an interesting question.
The ultimate fate of PCA and PCTA is of interest. Murthy and Kaufman
(1978) regard them as end products of PCNB degradation in the soil and assume
that they remain as residues unless absorbed by plants. Conversion to PCP and
further breakdown of this metabolite would seem to be the only pathway for
further degradation (Murthy and Kaufman, 1978).
Soils treated with technical PCNB, which contains impurities of pentachlorobenzene, hexachlorobenzene, and tetrachloronitrobenzene, can be expected
to contain appreciable residues of the parent compound and of these impurities
as well as PCA and PCTA for at least 2-3 years (Beck and Hansen, 1974). It
would seem desirable, therefore, to have substitutes for PCNB which are more
readily degradable.
6.4.3. Dithiocarbamates
Dithiocarbamates constitute the most important class of organic fungicides
used for plant disease control on a worldwide basis (EBDC, Fungicide Assessment Team, 1977). Two major groups within this class, the monoalkyl and the
dialkyl derivatives, are recognized in accordance with differing modes of action
and degradation pathways (Kaars Sijpesteijn et a/., 1977; Kaars Sijpesteijn and
van der Kerk, 1954). The former group, drived from primary amines, possess
a reactive hydrogen on the nitrogen atom(s) and as a consequence yield con-