Citrus sour rot management by propiconazole drench application in South Africa

Guazatine was previously used as drench application to control citrus sour rot before degreening. Its use has been restricted to certain export markets, because of maximum residue limits (MRL) on fruit below effective concentrations (i.e., EU: 0.05 mg L^‑1). Propiconazole (PPZ) is a newly registered fungicide for postharvest treatment of sour rot on citrus. The aim of the study was to test the efficacy of the fungicide for the control of sour rot and green mould. Untreated lemon and Clementine fruits were inoculated either with a Galactomyces sp. or a Penicillium digitatum isolate. Drench application of PPZ at increasing time frames after inoculation (6, 14, 18, and 24 h) was studied to determine the extent of PPZ curative action. Additionally, the exposure time (1, 2, and 3 min) required to obtain adequate coverage of the fungicide applied to the fruit was tested. Average residue loaded on lemons and Clementines was measured using liquid chromatography-mass spectrometry (LC-MS). Lemons infected with Galactomyces sp. had infection levels that indicated that PPZ can effectively reduce the incidence of sour rot by 98.7% when treatment was applied at 600 mg L^‑1 within 14 h post inoculation at 1 min exposure time (flow rate approximately 500 L min^‑1). In contrast, treatment had to be applied within 6 h of infection, to reach full efficacy on Clementines. Green mould infection levels also indicated a PPZ treatment window in lemons and Clementines of no more than 14 h. Propiconazole residue on Clementines was within the maximum residue limit (MRL) of 5 mg kg^‑1 when applied at 1 min exposure time. Residue loaded on lemons exceeded the EU-prescribed MRL of 5 ppm even at 1 min exposure time (13 mg kg^‑1).