Producers are sorting through volumes of information about Asian soybean rust as they consider management decisions for what soybean rust fungicide to apply and when, if needed.
While the amount of information available is substantial, the list of fungicide classes comes as a simple trio: triazoles, strobilurins and nitriles. The strobilurins and nitriles provide preventive control only, while the triazoles offer both preventive and curative activity.
“Products should be applied with activity in mind,” says Bob Gordon, fungicide marketing specialist for Dow AgroSciences. “Unless growers are at a point where they need a fungicide with curative activity, the list of fungicides available for preventive applications are broad.”
Gordon, who has 30 years of experience working with fungicides, says a preventive product should be applied before a soybean rust infection occurs. Meanwhile, a product with curative activity should be applied when infection is suspected — even if there are no disease symptoms present.
“You may not see disease symptoms for up to 10 days,” Gordon said. “If a field is even suspected of being infected by soybean rust, applying a fungicide that contains curative activity will be beneficial. At any rate, there should be few or no disease symptoms present when you make your first application.”
Gordon says triazole fungicides are ideal for first applications because they contain both preventive and curative activity, and offer protection whether or not soybeans are infected. Triazoles also offer protection to new growth.
To protect yield potential, triazoles should be applied before 10 percent disease incidence occurs in the lower canopy. With applications made beyond that point, growers can expect some yield loss.
Strobilurins and nitriles are labeled for preventive applications only. These fungicides work by coating the surface of the plant, which protects by blocking the rust fungus from entering the plant. Without thorough coverage, however, a plant's defenses can be compromised.
In a rust management program, Gordon recommends a triazole for the first application, followed by a strobilurin or nitrile as a resistance management tool. The interval between sprays is determined by the extent of existing inoculum and the level to which conditions favor disease development. Triazoles provide up to 21 days of residual activity.