Phytotoxicity caused by multiple applications of chlorothalonil fungicide.

The way it kills the fungus is termed the fungicide's 'mode of action.' Certain fungicides, usually systemics, have very specific modes of action and are said to be 'at risk' to the development of resistance. An 'at risk' fungicide may bind to a very specific chemical within the fungus or disrupt one specific step required for the reaction. If the fungus has an alternate pathway to complete the reaction, can break down the fungicide or prevents the chemical from acting in the fungus, then the fungus is resistant to that fungicide. It is generally accepted that there is a great deal of variation within populations of fungi and a few individuals in the population naturally are resistant to certain types of chemicals. When the chemical is used, it kills almost all of the sensitive individuals in the population but the survivors are those few individuals resistant to the action of the chemical. The survivors give rise to the next generation and most of that generation is resistant to the chemical.

It is recommended that high risk (1 = low risk; 3 = high risk) chemicals be used sparingly, used in rotation with, or mixed with chemicals with different modes of actions. Fungicide Resistance Action Committees (FRAC) developed a numbering system in which chemicals that have the same mode of action have the same FRAC group number. When selecting chemicals for rotation or mixing, select ones with different FRAC Group numbers (see below) because then you are using ones with different modes of action.

Listed below are some of the commonly used fungicides. Not all fungicides are listed. Chemical, trade names, and availability change frequently. This is not a complete list. It may not be up to date because items change daily.

Group Mode of action

1. Inhibits tubulin formation
2. Inhibits RNA or DNA synthesis
3. DMI (demethylation inhibitor), inhibits sterol synthesis
4. Inhibits RNA synthesis
5. Inhibits isomease in sterol synthesis
6. Disrupts electron transport in mitochondria
7. Disrupts beta-tubulin assembly
8. Quinone outside inhibitors (QOI)
9. Disrupts osmotic signal transduction
10. Disrupts lipid peroxidation
11. Disrupts cell wall synthesis
12. Disrupts C4 demethylation in sterol biosynthesis
13. Disrupts protein synthesis
14. Affects membrane permeability
15. Uncouples oxidative phosphorylation
16. Disrupts phospholipids and cell wall deposition
17. Protein synthesis attachment of tRNA to ribosomal receptor
18. Disrupts proteins involved in stabilizing cell membranes during growth

M Multi-site activity

P Host plant defense induction

U Unknown mode of action

NC Not classified