Organic Acids as Sanitizers for Natamycin and other Fungicides in Postharvest Recirculating Application Systems for Citrus.

Abstract

Natamycin is a new postharvest biofungicide for citrus and some other fruit crops in the United States that can be effectively used in recycling drench or flooder treatments. These types of applications necessitate sanitation of the fungicide solution to ensure that it remains free from contamination by bacteria that potentially are human pathogens. In in vitro experiments, heated (48°C) citric acid (1,100 μg/ml or 2,200 μg/ml) amended with sodium dodecylbenzensulfonate (SDBS; 55 μg/ml or 110 μg/ml, respectively) significantly reduced viability of E. coli in natamycin solutions by >5 log10 as compared to the control. In laboratory studies with Penicillium digitatum-inoculated lemon fruit, 1,000 μg/ml natamycin mixed with lactic or citric acids (1,000 μg/ml each) and with or without SDBS (55 μg/ml) was effective in significantly reducing green mold. Natamycin mixed with lactic acid at 2000 μg/ml or higher, however, caused fruit injury resulting in browning and pitting of the rind. Natamycin was incompatible with peroxyacetic acid resulting in reduced efficacy of the fungicide against green mold, whereas sodium hypochlorite lost its toxicity to a non-pathogenic Escherichia coli strain although the performance of natamycin was not affected. In heated (average 49°C) drench treatments on an experimental packingline, natamycin (1,000 μg/ml), fludioxonil (300 μg/ml), or azoxystrobin (300 μg/ml) mixed with citric acid (1,000 μg/ml)-SDBS (55 μg/ml) were effective against green1 mold without fruit injury. At a pH between 3.6 and 3.8, citric acid-SDBS significantly reduced the viability of E. coli by approximately 4 log10 in mixtures with fludioxonil or azoxystrobin, but not with natamycin. Natamycin (1,000 μg/ml) mixed with 2000 μg/ml citric acid and SDBS (4 μg/ml), however, significantly reduced E. coli counts by >4 log10 within 4 min when the pH was maintained between 3.0 and 3.3, and efficacy of the fungicide was retained. The use of citric acid with a surfactant can be a viable alternative sanitation method for natamycin in citrus packinghouses utilizing heated recirculating fungicide systems.