Protectant (contact) fungicides, such as the inorganics (copper, FRAC group M1) and sulfur (FRAC code M2); the dithiocarbamates (mancozeb, M3), phthalimides (Captan, M4), and chloronitriles (chlorothalonil, M5) are fungicides which have a low chance for fungicide resistance to develop. Protectant fungicides typically offer broad spectrum control for many different pathogens.
Why wouldn’t fungi develop resistance to protectant fungicides? Protectant fungicides are used all the time, often in a weekly manner throughout much of the growing season.
The answer is in their modes-of-action. Protectant fungicides have modes-of-action that prevent fungal development in different manners. In inorganic compounds, sulfur (M2) prevents fungal growth (i.e., spore germination) by disrupting electron transport in the mitochondria. Coppers (M1), on the other hand, cause non-specific denaturation of proteins. Chlorothalonil (M5) inactivates amino acids, proteins and enzymes by combining with thiol (sulfur) groups. In all cases, a protectant fungicide’s chemistry disrupts fungal growth and development either non-specifically or in multiple manners. Because of this, there is a much lower chance for fungi to develop resistance to them.
Protectant fungicides are contact fungicides, meaning they must be present on the leaf surface prior to the arrival of the fungus and must then come into direct contact with the fungus. Protectant fungicides can be redistributed on the leaf surface with rainfall or overhead irrigation, but can also be washed off by too much of either! Remember, that with protectant fungicides, any new growth is unprotected until the next protectant fungicide is applied, in other words, protectant fungicides are not systemic or nor have translaminar activity like some of the newer chemistries.
Protectant fungicides should be tank-mixed with fungicides with higher risks for resistance development. Protectant fungicides used in this manner will help slow (or reduce the chances for) fungicide resistance development on your farm. In any case, it’s best to always follow the label and tank mix protectants with higher risk fungicides when suggested or required to do so.