Newest Aspects of Nucleic Acid Synthesis Inhibitors – Metalaxyl‐M

Phenylamide fungicides (PAs) were introduces to the market in the 1980s and are still an important element of disease control when used in a mixture with non-cross resistant compounds and combined with other products in a spray program to reduce the selection of resistant subpopulations. The most important member of PAs is metalaxyl-M, the active enantiomer of metalaxyl, its spectrum of activity is restricted to pathogens of the Peronosporales and Pythiales within Oomycetes. It is a highly systemic compound in important crop plants such as grapes, potatoes, vegetables, fruit trees and related perennials mainly used as spray application and seed treatment. The biochemical mode of action was described as inhibition of RNA polymerization, the molecular mechanism of resistance being associated with two MEX loci of the RNApolI gene. In sexual crosses, sensitivity segregated in a semidominant, Mendelian manner yielding resistant, intermediate, and sensitive offsprings in the F2 generation. A single SNP in the RNApolI gene was associated with many but not all resistant phenotypes of Phytophthora infestans field populations. Thus, the mechanism of resistance remains speculative, although many investigations have been made over the past 40 years.