Propiconazole baseline sensitivity and molecular identification of Galactomyces sp. from citrus sour rot

Sour rot, caused by Galactomyces sp., is an important postharvest disease of citrus. In order to ascertain the specific etiology of citrus sour rot in South Africa and to determine the baseline sensitivity to the demethylation-inhibiting (DMI) fungicide propiconazole, 52 isolates from an organic Satsuma mandarin (Citrus unshiu) orchard in the Western Cape were collected. These were identified as G. citri-aurantii using species specific PCR amplification primers of the beta tubulin gene region. Conversely, phylogenetic analysis using the translation elongation factor one alpha gene (TEF1) identified the pathogen as closely related to Galactomyces reessii. Propiconazole was found to be highly effective and efficient in decreasing in vitro mycelial growth of Galactomyce sp. from sour rot in South Africa. The mean effective concentration for 50% reduction of mycelial growth (EC₅₀ value) for the 52 isolates was 0.31 µg mL^‑1. The discriminatory dose was calculated to be 5.37 µg mL^‑1. Propiconazole is registered for postharvest use on many crops including citrus. The information obtained will be used to monitor fungicide resistance in the packhouse and to create an effective and efficient fungicide application strategies in the future.