"The three major families of fungicides used to combat this disease are the strobilurins, the sterol inhibitors, and the nitriles, which are usually referred to by the principle active ingredient of chlorothalonil," said Matt Montgomery, crop systems educator with University of Illinois Extension. "It is important in combating rust to understand how each of these products work and under what circumstances they should be used."
Montgomery notes that the three fungicide families can generally be classified into one of two categories. Strobilurins and chlorothalonil are both categorized as protectants when used in soybean fields.
"These products inhibit the establishment of the fungus in plant tissue and exhibit limited mobility in beans," Montgomery said. "Recommended use is therefore largely restricted to tissues that lack many soybean rust pustules. Even the best of these materials only works effectively when less than three to five percent of the leaf surface is covered with rust pustules."
The strobilurins include products with a number of different active ingredients, including azoxystrobin which is used in Quadris, trifloxystrobin which is used in a portion of Stratego, and pyraclostrobin which is used in Headline.
"Those active ingredients are currently in various states of registration, and no endorsement is intended," Montgomery said. "All product names are property of their respective companies."
Montgomery points out that the sterol inhibitors, which are also known as triazoles, are deemed as curative products and designed for different uses than the products in the other group.
"The triazoles are more mobile and kill fungal tissue," Montgomery said. "Recommended use is therefore targeted toward those tissues that exhibit clear signs of the disease. This means that growers must shift to triazoles once three to five percent of the leaf surface is covered with rust pustules."
Each of these two major fungicide groups works in distinctly different ways. Strobilurins injure fungi by disrupting the electron transport chain at work within the cells.
"The active ingredient in this group shuts down respiration, which spells death for the fungus," Montgomery said. "Strobilurins affect a fairly narrow site of action causing some real resistance concerns."
Montgomery notes that chlorothalonil also disrupts respiration, but the method by which it does so is very different from the strobilurins.
"The active ingredient in this group inhibits a host of enzymes needed for various cell processes," he said. "Many of those enzymes are needed to advance various parts of the respiration process, which therefore results in inhibited respiration. Fungal tissues therefore die due to energy depletion or starvation."
Most researchers express fewer concerns about the development of resistance compared to the other rust fungicide families because this material affects a fairly broad site of action. Montgomery points out that this does not mean there are no concerns at all, only that there are fewer worries about resistance with this group of products.
"Sterol inhibitors or triazoles also inhibit enzymes," Montgomery said. "However, these active ingredients inhibit enzymes needed to form sterols, which are sometimes termed lipids. Sterol inhibitors affect a fairly narrow site of action causing some resistance concerns."
The sterol inhibitors include such active ingredients as propiconazole, which is used in Tilt and the other portion of Stratego, tebuconazole, which is used in Folicur, tetraconazole, which is used in Domark, and myclobutinil, which is used in Laredo EC.
"No matter what brand name a product goes by, it is essential to understand which mode of action it uses," Montgomery said. "Only then can a grower make an informed decision about how to effectively use it to combat soybean rust that appears his field. The results are beneficial both to both the environment and the bottom line of the farmer."