Tolclofos-methyl

Uses

Tolclofos-methyl is a non-systemic organophosphorus fungicide with both protective and curative activities that control soil-borne diseases caused by Rhizoctonia solani, Corticium rolfsii, Tphula incamata and Typhula ishikariensis in/on potatoes, sugar beet, cotton and peanuts.

Uses

Agricultural fungicide.

Definition

ChEBI: An organic thiophosphate that is 2,6-dichloro-4-methylphenol in which the hydrogen of the hydroxy group group has been replaced by a dimethoxyphosphorothioyl group. Tolclofos-methyl is a phospholipid biosynthesis inhibitor and fungicide that is used for co trolling soil-borne diseases caused by Typhula incarnata, Corticium rolfsii, Typhula ishikariensis, and Rhizoctonia solani.

Metabolic pathway

Tolclofos-methyl underwent common degradation and metabolic pathways in water, soil, plants and animals. These reactions are well documented for most organophosphorus compounds and include oxidative desulfuration, hydroxylation/oxidation of the 4-methyl group to the alcohol and carboxylic acid, cleavage of the P-O-aryl linkage, O-demethylation and conjugation. In addition to the above reactions, photolytic isomerisation to the thionate (P=S) was also observed. A schematic presentation of the primary metabolic pathways for tolclofosmethyl is illustrated in Scheme 1.

Degradation

[¹⁴C-phenyl]Tolclofos-methyl(1) was stable to hydrolytic degradation at pH 5, 7 and 9 at 22°C with DT₅₀ values of 139, 417 and 238 days, respectively. Higher temperature led to a more rapid hydrolysis and two hydrolysis products were detected. O-Demethylation and oxidative desulfuration were the major reactions to yield 2,6-dichloro-p-tolyl methyl hydrogen phosphorothioate (2) and toclofos-methyl oxon (O-2,6-dichlorop- tolyl O,O-dimethyl phosphate, 3), respectively. Cleavage of the P-O-aryl linkage yielded 2,6-dichloro-4-methylphenol(4) as a minor degradation product (WHO, 1994).
Tolclofos-methyl degraded in water under natural sunlight irradiation with DT₅₀ values of 44 days (in distilled water), 15-28 days (in natural river and pond water) and less than 2 days in 2% acetone/water. The major degradation reactions included oxidative desulfuration to yield compound 3 and O-demethylation to yield compound 2. The major photodegradation products in river and pond waters and soil thinlayer surfaces were compounds 2, 3, 4 and the O-demethylated 3 (2,6- dichloro-p-tolyl methyl hydrogen phosphate, 5) (Mikami et al., 1984). In acetone solution, demethylation of the isomerisation product [2,6- dichloro-p-tolyl O,S-dimethyl phosphorothioate (6)] yielded 2,6-dichlorop- tolyl S-methyl hydrogen phosphorothioate (7) and 2,6-dichloro-p-tolyl dihydrogen phosphate (8) as the major photodegradates (Mikami et al., 1984).

Toxicity evaluation

Its acute oral toxicity is very low in comparison with a thio-ester type of other organophosphorus fungicides (edifenphos, iprobenfos, and pyrazophos).