Resistance in Postharvest Pathogens of Citrus in the United States

Abstract

Among citrus postharvest pathogens, fungicide resistance in the United States to date has only been reported for species of Penicillium. Except for the inorganic salts, widespread resistance has developed to all of the older fungicides such as the still registered phenylphenols o-phenylphenol and sodium o-phenylphenate, the methyl benzimidazole carbamate, thiabendazole, and the demethylation inhibitor imazalil. The almost simultaneous introduction of several new compounds in the early 2000s that include the anilinopyrimidine pyrimethanil, the phenylpyrrole fludioxonil, and the quinone outside inhibitor azoxystrobin offered a unique opportunity in keeping the development of resistance to a minimum. Fungicide modes of action could be mixed and rotated from the first introduction before resistance had occurred. Resistance to pyrimethanil, however, has developed in some packinghouse populations of P. digitatum because end users did not follow these guidelines and used the fungicide exclusively. For azoxystrobin, resistance in P. digitatum has only been described for laboratory mutants and for fludioxonil only in mass platings of conidia on selective media in the laboratory or in packinghouse air samplings. Thus, practical resistance to azoxystrobin and fludioxonil has not occurred. Natural resistance frequencies and molecular mechanisms for thiabendazole, imazalil, pyrimethanil, fludioxonil, and azoxystrobin have been studied, and resistant pathogen isolates have been evaluated for their fitness. Anti-resistance strategies focus on sanitation of fruit, equipment, and storage rooms; limitation of pathogen sporulation and spore dispersal; use of fungicide mixtures, pre-mixtures, and rotations; as well as the early detection of resistance.