Baseline Sensitivities of Major Citrus, Pome, and Stone Fruits Postharvest Pathogens to Natamycin and Estimation of the Resistance Potential in Penicillium digitatum

Abstract

Natamycin is a biofungicide that was registered in the United States in 2016 and approved in California in 2017 for postharvest use on citrus and stone fruits. It has been used as a food preservative for many decades, with no resistance ever observed to date. The objective of this study was to determine baseline sensitivities for mycelial growth of 43 to 72 isolates of seven postharvest pathogens to natamycin and the resistance potential of Penicillium digitatum. Mean effective concentrations to inhibit mycelial growth by 50% (EC50 values), as determined by the spiral gradient method, were 0.90 μg/ml for Alternaria alternata, 0.76 μg/ml for Botrytis cinerea, 3.20 μg/ml for Geotrichum citri-aurantii, 0.17 μg/ml for Monilinia fructicola, 1.54 μg/ml for P. digitatum, 1.14 μg/ml for P. expansum, and 0.48 μg/ml for Rhizopus stolonifer. Distributions of EC50 values for each pathogen were unimodal and mostly normal with no outliers detected. Natamycin was also inhibitory to spore germination with values for five of the species similar to those for mycelial growth. Microscopically, natamycin generally arrested spores at the pregermination swelling stage. Mass platings of a conidial mixture of 10 isolates of P. digitatum were inoculated on agar media with 2.5-log radial concentration gradients of natamycin or fludioxonil, and a conidial mixture of 10 isolates of G. citri-aurantii were plated on media amended with natamycin or propiconazole. No resistant isolates were observed for both species to natamycin or for G. citri-aurantii to propiconazole, whereas a resistance frequency of 4.5 × 10−6 to 3.1 × 10−6 was calculated for P. digitatum to fludioxonil. The wide spectrum of activity against different fungal pathogens and a low resistance potential support the registration of natamycin as a postharvest treatment and its integration into an integrated pest management program with other practices including sanitation and rotation of other fungicides with different modes of action.