Fungicide Classes and Modes of Action

The classification of fungicides according to their mode of action and cross resistance pattern became necessary to facilitate resistance management at the field level under practical agronomic conditions. If fungicides are recommended to be used in alternation or mixtures to achieve robust disease control and delaying resistance evolution, clear information on the cross-resistance behavior for each 

Fungicides that interfere with nucleic acid synthesis are an important group of phenylamides that have kept a strong position in the Oomycete market despite major resistance problems (Table 12.6). Fungicides inhibiting adenosine- 

FRAC code Target site Chemical group Compounds (examples) Comments 

A2 8 Adenosine-deaminase Hydroxy- pyrimidines Ethirimol Specific for powdery mildew 

A3 32 DNA/RNA synthesis (prop.) Hetero-aromatics Hymexazol Broad spectrum 

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Fungicide Classes and Modes of Action 1425 Table 12.7 Group B Fungicides interfering with mitosis and cell division. 

Fungicide Classes and Modes of Action 427 Table 12.9 Group D Fungicides interfering with amino acid and protein synthesis. 

Fungicide Classes and Modes of Action 429 Table 12.13 Group H Fungicides interfering with glucan synthesis. 

The elucidation of the primary site and biochemical mode of action of inhibitors is often difficult and not necessarily associated with the mechanism of resistance in field isolates. Nevertheless, information on the mechanism of resistance can provide evidence to determine the site of action. Therefore, the classification of fungicides is based on crossresistance reactions rather than chemical similarities of structures or proposed modes of action (Table 1). Based on available information in the literature, three categories of inhibitor classes can be made Classes with known mode of action and known mechanism of resistance, classes with proposed mode of action and unknown mode of resistance but wide-spread field resistance, and classes in which resistance is claimed to occur in the field but both mode of action and resistance are not known. 

However, when classifying the presently available crop protection products of substantial commercial importance, one hnds only a comparatively low number of biochemical modes of action for which compounds have been commercialized. Altogether, four modes of action account for more than 75% of the current insecticide sales (Figure 1). In the helds of herbicides and fungicides, the situation is similar. Here six different modes of action dominate each market. Of these, several represent modes of action that have been commercialized during the last decade. Further classes of compounds demonstrating other modes of action which have been identihed in the past in all three indications have not gained major market shares from an economical point of view. 

Nucleoside chemistry in crop protection (s5mthetic issues, mode of action, and biological efficacy of nucleoside classes active as herbicides, fungicides, and insecticides are presented) 05H(65)667. 

A series of novel 4-phenoxyquinolines has been developed which show potent control of fungicide-sensitive and resistant strains of powdery mildew in grape and cereal crops. Protective, curative, and systemic activity has been observed for these materials, which also control other fungi. Strong activity in field studies has been noted and no crossresistance was encountered when fungicide-resistant strains of mildew were treated with these compounds, thereby suggesting a novel mode of action. The discovery, synthesis and structure-activity relationship of this novel class of fungicides are presented, as well as some data from field and cross-resistance studies. 

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