Fungicide: Modes of Action and Possible Impact on Nontarget Microorganisms

Fungicides have been used widely in order to control fungal diseases and increase crop production. However, the effects of fungicides on microorganisms other than fungi remain unclear. The modes of action of fungicides were never well classified and presented, making difficult to estimate their possible nontarget effects. In this paper, the action modes and effects of fungicides targeting cell membrane components, protein synthesis, signal transduction, respiration, cell mitosis, and nucleic acid synthesis were classified, and their effects on nontarget microorganisms were reviewed. Modes of action and potential non-target effects on soil microorganisms should be considered in the selection of fungicide in order to protect the biological functions of soil and optimize the benefits derived from fungicide use in agricultural systems. 1. Introduction Soil is arguably the most important resource for food production. It is a very complex system whose functions not only depend on its physical properties, but also on its biological components. In particular, soil microorganisms are essential players in the cycling of several elements essential to life, including C, N, and P [1]. Understanding the effect of fungicides on the beneficial activities of microorganisms is important to assess the hazards associated with fungicide used in agriculture. Crop productivity and economic returns will be maximized with the use of products controlling well fungal pathogens, but preserving beneficial organisms. Different organisms may possess identical or similar mechanisms and constituents, and fungicides targeting nonspecific binding sites can directly affect nontarget organisms. For example, the toxicity of carboxylic acid fungicides is derived from the ability of these chemicals to bind on DNA topoisomerase II, as common enzyme that unwind, and wind, DNA to allow protein synthesis and DNA replication. This enzyme is found in fungi but also in prokaryotic cells [2]. Some glucopyranosyl antibiotic fungicides are toxic to bacteria, in which they may inhibit the synthesis of amino acids [3]. These fungicides are also toxic to certain nonfungal higher eukaryotic organisms [4]. Indirect nontarget effects are also possible. Microorganisms are either functionally or nutritionally connected with each others, and changes in a component of a microbial community may influence the structure of the whole community. This is particularly true for plant-associated microorganisms, which influence on and are influenced by the plant metabolic status [5–7]. In order to establish a proper