Mode metalaxyl

The need to develop and use chiral chromatographic techniques to resolve racemates in pesticide residues will be driven by new hazard and risk assessments undertaken using data from differential metabolism studies. The molecular structures of many pesticides incorporate chiral centers and, in some cases, the activity differs between enantiomers. Consequently, in recent years manufacturers have introduced resolved enantiomers to provide pesticides of higher activity per unit mass applied. For example, the fungicide metalaxyl is a racemic mix of R- and 5-enantiomers, both having the same mode of action but differing considerably in effectiveness. The -enantiomer is the most effective and is marketed as a separate product metalaxyl-M. In future, it will not be satisfactory to rely on hazard/risk assessments based on data from metabolism studies of racemic mixes. The metabolism studies will need to be undertaken on one, or more, of the resolved enantiomers. 

Despite the act that the new compounds have been known and available for years, relatively little is known about their mode of action. Table VI reflects the state of the art. The lack of information is particularly surprising in the case of fosetyl and metalaxyl as far as their indirect effect is concerned, that is, the stimulation of the host plant s defense reactions (e.g. formation of phytoalexins). It is for the first time in the history of fungicides that such effects have been reported. 

The biochemical mode of action has been studied by several authors (16, 18). It appears that metalaxyl inhibits RNA synthesis by Interference with template-bound and a -amanitin-insensitive RNA polymerase action (15). 

Inhibition of the incorporation of uridine into RNA is caused by the herbicides referred to as the chloroacetanilides (e.g., acetochlor, alachlor, butachlor, and several others) and a group of fungicides referred to as phe-nylamides (metalaxyl, ofurace, and oxadixyl). They have similar structure and mode of action ... 

As concerning the mode of action of these fungicides, a recent study of Kerkenaar and Kaars Sijpesteijn (1981) describes the in vitro selectivity of metalaxyl and furalaxyl for the Peronosporales as well as several other effects. Since respiration of Pythium splendens was not inhibited it was supposed that the compounds act on biosynthesis or interfere directly with membranes. 

Since zoxamide has a different mode of action from other products used in the Oomycete market, there is no likelihood of cross-resistance to existing products such as metalaxyl, dimethomorph, cymoxanil or strobilurins. Consequently, zoxamide provides a unique tool for resistance management in the Oomycete fungicide market. 

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