Efficacy of Five 14α-Demethylation Inhibitors and Four Biorational Fungicides Against <i>Geotrichum candidum</i> and Sour Rot of Peach

Sour rot, caused by Geotrichum candidum Link (1809), is a postharvest disease of stone fruits, including peaches. The 14α-demethylase inhibitor (DMI) fungicide propiconazole has been used for postharvest sour rot management. However, the recent emergence of reduced-sensitive and resistant G. candidum isolates has compromised its efficacy in South Carolina packinghouses. In this study, the efficacy of four additional DMI fungicides (tebuconazole, difenoconazole, fenbuconazole, and mefentrifluconazole) and four biorational fungicides (natamycin, polyoxin-D, Pseudomonas chlororaphis strain AFS009, and Bacillus subtilis strain AFS032321) was evaluated for mycelium growth (with the exception of the P. chlororaphis and B. subtilis products) and disease inhibition of G. candidum isolates with varying sensitivity to propiconazole. Mycelium growth inhibition assays revealed mean EC 50 values for tebuconazole of 1.7 µg/ml for propiconazole-sensitive, 90.5 µg/ml for propiconazole-reduced-sensitive, and 567.8 µg/ml for propiconazole-resistant isolates. For difenoconazole, the values were 12.4, 558.5, and 587.3 µg/ml, respectively. Fenbuconazole and mefentrifluconazole exhibited no or minimal inhibitory effects in vitro at concentrations up to 1,000 µg/ml for any of the G. candidum isolates tested. The EC 50 values for natamycin for all isolates ranged from 2.63 to 3.66 µg/ml, whereas polyoxin-D had no effect against G. candidum in vitro. Detached fruit assays designed to assess efficacy against sour rot disease development showed that only label rates of formulated propiconazole suppressed sour rot caused by propiconazole-sensitive and propiconazole-reduced-sensitive isolates. Formulated products of all other DMIs and the four biorational fungicides were ineffective. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .