Fungicide sensitivity and spore trapping of Cercospora sojina

Cercospora sojina is the causal agent of frogeye leaf spot disease (FLS) in soybean. FLS management is achieved by resistant varieties, crop rotation and fungicide applications. The use of fungicides may result in the development of fungicide resistance, which has occurred within C. sojina already and in response to the QoI fungicide group. The goal of this project was to investigate any additional loss of fungicide efficacy, using both field and laboratory trials, as well as to develop a threshold for triggering fungicide applications based on inoculum detection using spore traps and molecular tools. The fungicide efficacy of Topguard (Flutriafol), Domark (Tetraconazole), Headline (Pyraclostrobin), and Topsin (Thiophanate-methyl), each applied at the beginning pod development growth stage, were evaluated in field trials at the UT AgResearch and Education Center at Milan, between 2013 through 2019. A mycelium growth assay evaluated the fungicide sensitivity of Tetraconazole, Thiophanate-methyl, and Flutriafol at the rates 0, 0.001, 0.01, 0.1, 1.0 and 10 ppm. The effective concentration to inhibit 50% mycelial growth was calculated. While overtime C. sojina populations were not able to be managed with Pyraclostrobin, there was no loss of sensitivity for Flutriafol, Tetraconazole, or Thiophanate-methyl based on field and laboratory results. However, due to the risk of multiple resistance development, practices that reduce selection pressure, such as fungicide application triggered by the number of spores present in the environment, would be highly beneficial. Therefore, we evaluated (1) the frequency of collection from spore traps: once a week and twice a week; (2) the relationship of detected inoculum and disease development using weekly evaluations across five different locations; and (3) the potential of using the number of spores as a threshold for fungicide application. The results indicated that, once a week was the optimal frequency to collect spores and there was a strong positive correlation between the area under the disease progress curve and the area under the spore progress curve. Additionally, the number of spores was efficient to trigger a fungicide application in environments with high disease severity as well as save an application under low disease pressure.