Evaluation of Fungicides Applied Preharvest for Postharvest Diplodia Stem-end Rot Control on Grapefruit

Authors: ZHANG, JIUXU - University Of Florida; YAN, JIAQI - China Agriculture University; MYERS, MONTY - University Of Florida; HU, CUIFENG - University Of Florida; ZHENG, XIAOLIN - China Agriculture University; NOEL, RITA - China Agriculture University; CANO, LILIANA - University Of Florida; DEWDNEY, MEGAN - University Of Florida; WANG, YU - University Of Florida; Zhao, Wei; Bai, Jinhe; Plotto, Anne; RITENOUR, MARK - University Of Florida

Submitted to: Proceedings Florida State Horticultural Science
Publication Type: Proceedings
Publication Acceptance Date: 1/31/2022
Publication Date: 8/31/2022
Citation: Zhang, Jiuxu, Yan, Jiaqi, Myers, Monty, Hu, Cuifeng, Zheng, Xiaolin, Noel, Rita, Cano, Liliana, Dewdney, Megan, Wang, Yu, Zhao, W., Bai, J., Plotto, A., Ritenour, Mark 2022. Evaluation of Fungicides Applied Preharvest for Postharvest Diplodia Stem-end Rot Control on Grapefruit. Proceedings Florida State Horticultural Science. 134/176.

Interpretive Summary: Diplodia stem-end rot (SER, caused by Lasiodiplodia theobromae) has become an increasing issue in citrus grown in Florida since the presence of Huanglongbing (HLB) in citrus groves has increased its prevalence. Improved decay control is essential to sustain Florida’s fresh citrus industry. Pre-harvest fungicides were tested in several grapefruit orchards over the course of two years. Topsin 4.5 FL and Graduate A+ significantly reduced Diplodia SER incidence and severity compared to the control over the two-year study and in all orchards. However, Graduate A+ is currently registered only for postharvest use and the rates used in these studies are higher than what would be allowed for preharvest use. Other fungicides controlled Diplodia SER at various levels and reproducibility. Studies are being continued to evaluate concentration of active ingredients in those fungicides in order to understand their mode of action and optimize their use.

Technical Abstract: Diplodia stem-end rot (SER, caused by Lasiodiplodia theobromae) is an important postharvest disease in Florida citrus. The presence of Huanglongbing (HLB) in citrus groves has increased incidence of Diplodia SER. Implementation of improved decay control methods is essential to sustain Florida’s fresh citrus industry. Evaluation of preharvest fungicides for postharvest Diplodia SER control was conducted during the 2019-20 and 2020-21 seasons on red grapefruit. Compounds and formulations included: Topsin 4.5 FL (thiophanate-methyl), Quadris Top (azoxystrobin + difenoconazole), Graduate A+ (fludioxonil + azoxystrobin), Headline (pyraclostrobin), Mentor EC (propiconazole), Mertect 340F (thiabendazole), Switch 62.5 WG (fludioxonil + cyprodinil), Miravis Prime (fludioxonil + pydiflumetofen), Miravis Top (difenoconazole + pydiflumetofen), Thyme Guard (thyme oil) and Citrus Fix (2,4-Dichlorophenoxyacetic acid or 2,4-D). Control trees were sprayed with water. Topsin 4.5 FL and Graduate A+ significantly reduced Diplodia SER incidence and severity compared to the control over the two-year study and in all orchards. However, Graduate A+ is currently registered only for postharvest use and the rates used in these studies are higher than what would be allowed for preharvest use. Mertect 340F, Miravis Prime, Headline, and Quadris Top significantly reduced Diplodia SER in some tests. Switch 62.5WG, Thyme Guard, Mentor EC, and Citrus Fix did not significantly reduce Diplodia SER. Headline, Quadris Top, and Miravis Prime contain fludioxonil or strobilurin compounds, which are likely the key active ingredients leading to reduced Diplodia SER. Product formulation plays a role in determining decay control efficacy. Further work is in progress to evaluate concentrations of fludioxonil, strobilurins, their combinations, and formulations and new compounds for Diplodia SER control in Florida.