Fungicide Phytotoxicity on Soybean: Triazole Injury Sparks Concern

In a recent study conducted by Tom Allen of Mississippi State University, fungicide phytotoxicity, or triazole injury, has been identified as a potential risk for soybean crops following the application of certain foliar fungicides. The study sheds light on the occurrence of this issue, its potential causes, and the variables that contribute to its severity.

Triazole injury is associated with the use of demethylation inhibitor (DMI) fungicides, classified under the Fungicide Resistance Action Group (FRAC) 3. Active ingredients such as propiconazole, prothioconazole, and tebuconazole, commonly found in fungicides, have been identified as potential culprits in inducing this type of injury. Additionally, fungicides containing thiophanate-methyl, belonging to the methyl benzimidazole carbamates (MBC-FRAC 1), have been linked to minimal injury.

The exact mechanism by which these fungicides cause injury remains unknown. However, symptoms of triazole injury are likely to manifest in the upper canopy approximately 21-28 days after application. Recognizable signs include interveinal chlorosis, characterized by yellowing of the tissue between leaf veins. In severe cases, affected leaf tissue may turn necrotic and brown, ultimately leading to tissue death. Notably, these symptoms can be confused with those of several soybean diseases, including red crown rot and sudden death syndrome (SDS).

The severity of triazole injury is subject to various factors, including adjuvant rate, glyphosate formulation (if tank mixed with a DMI fungicide), environmental conditions, and soybean variety. Furthermore, the specific DMI-active ingredient used influences the extent of phytotoxicity. Propiconazole and tetraconazole-containing products, such as Tilt and Domark, are associated with milder injuries, resulting in limited interveinal chlorosis. Conversely, more severe injuries have been observed with prothioconazole-containing products like Delaro and Stratego YLD.

Fungicide phytotoxicity can affect a significant proportion of leaves in a field, with reports suggesting up to 50% leaf coverage. Despite the severity of symptoms, there have been no reported cases of yield loss attributed to fungicide phytotoxicity.

Farmers and agronomists are urged to be vigilant in monitoring soybean crops post-fungicide application, especially in regions where triazole-containing products are commonly used. As research continues to unravel the mysteries surrounding triazole injury, this information will be invaluable in guiding agricultural practices to minimize the risk of phytotoxicity and ensure the health and productivity of soybean crops. For more information and examples of Triazole injury on soybean visit the new CPN encyclopedia article covering Fungicide Phytotoxicity on Soybean.