Chlorothalonils. Strobilurins. Triazoles. Asian soybean rust has thrust an alphabet soup of new names into the mix of products you must understand.
The important thing to know is these fungicides don’t all work alike—a fact that impacts how you use them, says Alison Robertson, Iowa State University extension plant pathologist. “Fungicides kill or inhibit the growth of fungi,” she notes. “But they don’t all achieve that result in the same way.”
Mode of action describes how the chemical affects the fungal pathogen. Some fungicides inhibit a specific enzyme in the metabolic pathway of the fungus; others damage the cell membranes; others react with sulfur-containing enzymes. Fungicides belonging to the same chemical group share the same mode of action. Example: Strobilurins stop energy production in fungi while triazoles prevent production of sterols—key components of fungal cell membranes.
Fungicides are also categorized by their activity. Some fungicides are highly specific and act at a single site. “Triazoles inhibit a certain enzyme at one specific site in the sterol production pathway. The strobilurins bind to a particular protein, thereby blocking energy production,” Robertson says. “Other fungicides are less specific and act at multiple sites, like chlorothalonil.”
Resisting resistance. All of this becomes pertinent as you develop a rust management strategy to avoid resistance. Fungicides with a single-site mode of action, such as triazoles, are a concern since a single genetic change in the pathogen can cause resistance to the fungicide to develop.
To avoid resistance, alternate or tank-mix fungicides like strobilurins and triazoles with other labeled fungicides that have different modes of action.
Robertson says fungicides also differ in how they move within a plant. Contact fungicides tend to be protectant and remain on the leaf surface. “They may spread out slightly from their point of contact, but they do not move inside the leaf tissue,” she says. Leaves or areas of leaves that do not receive fungicide aren’t protected.
Systemic fungicides are absorbed into the leaf tissue. Some are locally systemic (translaminar) and are absorbed where the fungicide is applied and spread within the leaf but not beyond it. In this case, leaves that do not receive fungicide are not protected.
Other fungicides are more mobile and are absorbed into the leaf tissue, redistributed in the leaf and translocated upward in the xylem. “But there’s little evidence to show that these fungicides can be translocated from one leaf to another,” Robertson says. “When growers hear the word systemic, they think of glyphosate, but systemic fungicides are not comparable to systemic herbicides. That’s why it’s important to get good spray coverage of the entire plant because fungicides do not move through the plant like herbicides.”
Fungicides also differ in their protection roles. Protectant or preventative fungicides protect the surface of the leaf by inhibiting spore germination and infection. They can be contact or systemic. Chlorothalonils and strobilurins work preventively.
Curative fungicides inhibit development of disease after infection to some degree and can be used when disease is present at low levels. Triazole fungicides are considered curative fungicides.