146
Hugh D. Sisler
Figure 6.9. Pathway of biotransfonnation of chloroneb.
to DCMP whereas others convert DCMP to chloroneb. Certain fungi were capable
of carrying out the conversion in either direction (Fig. 6.9). Thus, microbial
resynthesis of chloroneb from the degradation product DCMP may account in
part for the long-term effectiveness of chloroneb in soil.
6.4.6. Benzimidazoles
Benzimidazole fungicides have assumed major significance as crop protectants within the past decade. The most important compounds of this group are
benomyl (Benlate), methyl-l-(butylcarbamoyl)-2-benzimidazole carbamate; carbendazim (MBC), methyl-2-benzimidazole carbamate; Mertec), 2-(4'thiazolyl)benzimidazole; and thiophanate-methyl, 1,2 bis(3-methoxycarbonylthioureido)
benzene (Fig. 6.10). Although the latter compound is not a benzimidazole, it
undergoes conversion to MBC, through which antifungal activity is mediated
(Selling et at., 1970). MBC is also a conversion product of benomyl and is
regarded as the component responsible for fungitoxicity (Clemons and Sisler,
O:""'NHCOOCH 3
I
c=o
I
HN-C 4 Hg
Benomyl
(X
N
N
""/ 4~
~
N~S
H
Thiabendazole
~N
~N>- NHCOOCH 3
H
MBC (Carbendazim)
5
0
H 1\ H II
~N-C-N-C-OCH3
0-- N-C-N-C- OCH 3
H II H 1\
5
0
Thi ophanate -methyl
Figure 6.10. Benzimidazole fungicides and thiophanate-methyl.