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Previous studies have shown that foliar fungicides increase the abundance of Diaporthe spp. in mature soybean seeds, thereby reducing seed quality and potentially increasing seedling, stem, and pod disease in subsequent plantings. To mitigate this, we examined a fungicide containing mefentrifluconazole, fluxapyroxad, and pyraclostrobin applied during the R3 growth stage. We compared the seedborne fungal community from soybeans harvested at full maturity and 2 to 3 weeks later to assess fungal colonization. Six trials were conducted in Iowa and South Dakota in 2021 and 2022. Asymptomatic, recently harvested seed was assayed using either an agar-plate or a seed-blotter test, and the results were compared. The agar-plate method involved disinfesting seeds in sodium hypochlorite and ethanol and then transferring them to acidified water agar plates. Fungal isolates were identified using morphology and DNA sequences. This method detected 562 fungal isolates from 25 species and 14 genera. Dry conditions reduced seed infection, but a 2-week delay in harvest increased the likelihood of Alternaria and Cercospora infection in nontreated controls. The seed-blotter test involved disinfesting seeds, placing them on blotter paper, and incubating them. This assay detected higher numbers of fungal infected seed, with Aspergillus as the dominant genus. Storage conditions significantly impacted fungal presence, with more Aspergillus in seeds stored in a shed than in the cooler. Both assay methods showed that the fungicide treatment reduced the number of seeds infected with Alternaria, Cercospora, and Aspergillus. [Figure: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Cercospora leaf blight (CLB) of soybean, caused by Cercospora cf. flagellaris , C. kikuchii , and C. cf. sigesbeckiae , is an economically important disease in the southern United States. Cultivar resistance to CLB is inconsistent; therefore, fungicides in the quinone outside inhibitor (QoI) class have been relied on to manage the disease. Approximately 620 isolates from plants exhibiting CLB were collected between 2018 and 2021 from 19 locations in eight southern states. A novel polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay based on two genes, calmodulin and histone h3 , was developed to differentiate between the dominant species of Cercospora , C. cf. flagellaris , and C. cf. sigesbeckiae . A multilocus phylogenetic analysis of actin, calmodulin, histone h3, ITS rDNA, and transcription elongation factor 1-α was used to confirm PCR-RFLP results and identify remaining isolates. Approximately 80% of the isolates collected were identified as C. cf. flagellaris , while 15% classified as C. cf. sigesbeckiae , 2% as C. kikuchii , and 3% as previously unreported Cercospora species associated with CLB in the United States. PCR-RFLP of cytochrome b ( cytb ) identified QoI-resistance conferred by the G143A substitution. Approximately 64 to 83% of isolates were determined to be QoI-resistant, and all contained the G143A substitution. Results of discriminatory dose assays using azoxystrobin (1 ppm) were 100% consistent with PCR-RFLP results. To our knowledge, this constitutes the first report of QoI resistance in CLB pathogen populations from Alabama, Arkansas, Kentucky, Mississippi, Missouri, Tennessee, and Texas. In areas where high frequencies of resistance have been identified, QoI fungicides should be avoided, and fungicide products with alternative modes-of-action should be utilized in the absence of CLB-resistant soybean cultivars., Competing Interests: The author(s) declare no conflict of interest.