UC Riverside

In California, the main postharvest decay pathogens of citrus are Penicillium digitatum causing green mold and Geotrichum citri-aurantii causing sour rot. Both pathogens cause significant crop losses if fruit are not treated with fungicides. I sought to determine the efficacy of natamycin against P. digitatum and G. citri-aurantii in vitro and in fruit efficacy experiments. Natamycin is derived as a fermentation product from Streptomyces species. It is incompatible with oxidative sanitizers, thus, organic acids were tested as alternatives.

The results of this dissertation have been obtained from experiments on the 1) in vitro sensitivity of seven postharvest decay fungi; 2) efficacy of natamycin with different application methods in laboratory, experimental packingline and commercial packingline settings; and 3) in vitro sanitation and compatibility of natamycin with organic acids.

In vitro sensitivity studies established ranges of EC50 values for P. digitatum and G. citri-aurantii and other important decay fungi of stone and pome fruits. Distributions of EC50 values were unimodal, and no resistance was identified. Natamycin was effective in inhibiting mycelial growth of all fungi tested at a range of 0.17 to 3.20 µg/ml.

In laboratory and packingline studies, natamycin reduced the incidence of green mold and sour rot of inoculated citrus fruits. Natamycin was compatible in mixtures and in sequential applications with a storage fruit coating, but not in mixtures with a packing fruit coating. Mixtures of natamycin and propiconazole in aqueous solutions or in storage fruit coatings were highly effective against both pathogens.

Citric and lactic acids were tested against Escherichia coli and Pseudomonas syringae in vitro, with or without the surfactant sodium dodecylbenzenesulfonate (SDBS). P. syringae was highly susceptible to acidic conditions. E. coli was effectively sanitized by heated organic acid solutions with SDBS. In fruit studies, natamycin was compatible with citric acid and SDBS against green mold. Lower pH values and higher temperatures improved the efficacy of citric acid and SDBS as a sanitizer.

In summary, this research determined baseline sensitivities of decay fungi to natamycin, optimized utilization of natamycin in a disease management program, and demonstrated effective sanitation of natamycin solutions using organic acids and surfactants.