University of Arkansas, Fayetteville Division of Agriculture


Cercospora, Corynespora, Soybeans, Fungicide Resistance


Cercospora spp. and Corynespora spp. are two common foliar fungal pathogens in Arkansas and other soybean-producing areas. Two primary diseases caused by Cercospora spp. are Cercospora Leaf Blight (CLB, caused mainly by C. kikuchii) and Frogeye Leaf Spot (C. sojina). Over time, many fungicides used to combat these diseases have become ineffective as the pathogens have developed a resistance to them. The class of the fungicide in question is Triazoles [Demethylation Inhibitors (DMI)–FRAC 3]. Fifteen isolates consisting of Corynespora cassiicola, Cercospora sojina, and Cercospora flagellaris were tested to determine baseline sensitivities using serial dilutions (0, 0.01, 1, 10, 50 mg/L) of the fungicide Tilt (active ingredient: propiconazole). From the three species tested, Corynespora cassiicola (isolate 1601) showed the greatest EC50 value (10 mg/L). Sensitivity levels for Cercospora sojina were close to 1.00 mg/L, and EC50 were evenly distributed across all samples of this isolate. Cercospora flagellaris samples had the lowest EC50 values out of the three species tested as no growth was reported after 1 mg/L. Based on the results of this study, C. cassiicola is at greatest risk of resistance to the DMI fungicide Tilt, followed by Cercospora sojina and C. flagellaris, respectively.