Impact of fungicide applications on sweetpotato roots and slips for managing black rot disease (Ceratocystis fimbriata) and their effect on pesticide residue levels.

North Carolina is the top producer of sweetpotatoes in the United States (US) and accounts for over 50% of the nation's yearly production. The presence of fungal pathogens such as Ceratocystis fimbriata, the causal agent of black rot, remains a major concern for sweetpotato producers and is commonly managed through the application of fungicides. Due to the European Union's (EU) restricted residue tolerances for import products treated with pesticides, the use of fungicides for management of sweetpotato diseases is limited. Identifying fungicides and application practices that ensure disease-free sweetpotatoes while meeting export residue requirements is critical for effective disease management and export marketability. Field experiments were executed in 2022 and 2023 to quantify residue values of three active ingredients (thiabendazole, azoxystrobin, difenoconazole) when applied at either bedding, transplant, or both bedding and transplant when managing sweetpotato black rot (Table 1). Root and vine samples were collected at harvest to analyze the detectable residue levels of the applied active ingredients at different stages of sweetpotato production. High-performance liquid chromatography analyses revealed that the average detected residues for all of the tested active ingredients and application timings fell under the US, Canada, EU, but not UK MRL thresholds. In the field experiments, fungicide treatments were not significantly different from non-treated plots for plot vigor, percent disease incidence, or yield. While the residues from the three tested products in this study were not a concern for US, EU, and Canadian markets when applied during sweetpotato field production, further research is needed to determine their potential as an effective management tool for sweetpotato black rot.