In the May 6 issue of the ICM newsletter, I had an article  on resistance of Phytophthora to the fungicide metalaxyl. I discussed a possible treatment option if resistance occurs, which involved switching to a different seed treatment, such as mefenoxam. Alternating different fungicides is a common strategy in fungicide resistance management; however, experts in fungicide chemistry pointed out that the active ingredients of mefenoxam and metalaxyl are isomers. Isomers are compounds that have the same number of atoms but differ in their structural arrangement. Because mefenoxam and metalaxyl are isomers, they probably have the same mode of action against Phytophthora fungi. Therefore, alternating these two chemicals would not be effective in managing fungicide resistance.
This article originally appeared on page 84 of the IC-488(10) -- May 27, 2002 issue.