Postharvest use of natamycin to control Alternaria rot on blueberry fruit caused by Alternaria alternata and A. arborescens

Highlights

• Natamycin inhibited Alternaria alternata and A. arborescens in vitro.

• Natamycin reduced incidence and severity of Alternaria rot in blueberry fruit.

• Natamycin is an effective tool for control of Alternaria rot in blueberry.

Abstract

Alternaria rot caused by Alternaria alternata and A. arborescens is a major postharvest disease affecting blueberries in California. The effectiveness of natamycin, a newly registered biofungicide in the United States, was evaluated for its potential as a postharvest treatment to control Alternaria rot. The baseline sensitivities to natamycin in A. alternata and A. arborescens were established. The effective concentrations of natamycin for 50 % reduction in growth relative to control (EC₅₀) of 34 A. alternata isolates ranged from 0.640 to 1.032 mg L⁻¹ with a mean of 0.778 ± 0.097 mg L⁻¹ for conidial germination, and from 2.660 to 4.330 mg L⁻¹ with a mean of 3.390 ± 0.435 mg L⁻¹ for mycelial growth. EC₅₀ values of natamycin for 34 A. arborescens isolates ranged from 0.540 to 1.335 mg L⁻¹ with a mean of 0.950 ± 0.162 mg L⁻¹ for conidial germination, and from 0.610 to 3.430 mg L⁻¹ with a mean of 1.920 ± 0.637 mg L⁻¹ for mycelial growth. For A. alternata isolates, the minimum inhibitory concentrations (MICs) for conidial germination were 5 mg L⁻¹ for 27 isolates and 10 mg L⁻¹ for 7 isolates, and the MICs for mycelial growth were 25 mg L⁻¹ for all 34 isolates. For A. arborescens isolates, the MICs for conidial germination were 10 mg L⁻¹ for 5 isolates and more than 10 mg L⁻¹ for 29 isolates, and the MICs for mycelial growth were 10 mg L⁻¹ for 6 isolates and 25 mg L⁻¹ for 28 isolates. Control tests on blueberry fruit inoculated with four isolates of each Alternaria alternata and A. arborescens showed that even a quarter concentration of the label rate of natamycin significantly reduced disease incidence and severity of Alternaria rot, regardless of application method or incubation condition. Our results suggest that natamycin can be an effective tool for control of postharvest Alternaria rot in blueberry.

Introduction

Recent development of low-chill southern highbush blueberry cultivars made it possible for blueberry production in the Central Valley of California (Jimenez et al., 2005). Since then, blueberry production has steadily been increasing and California has now become one of the major blueberry production regions in the United States, with a production value exceeding $137 million in 2017 (USDA-National Agricultural Statistics Service, 2017).

Blueberries produced in California are mainly for the fresh market (USDA- National Agricultural Statistics Service, 2014), and thus prolonging storage life is essential to extend marketing opportunities and withstand long distance transits to overseas markets. However, storage rots caused by various fungal pathogens are a limiting factor to the storage life of blueberry (Beaudry et al., 1992). Previously, a survey was conducted to determine major postharvest diseases of blueberries in California, and the primary postharvest diseases were found to be Alternaria rot caused by Alternaria spp. and gray mold caused by Botrytis spp. (Xiao and Saito, 2017; Saito et al., 2016). The isolates of Alternaria spp. collected from the survey were then further identified at the species level, by morphological and phylogenetic analyses based on multiple DNA sequences. Of the 283 Alternaria spp. isolates examined, five Alternaria species were identified with A. alternata and A. arborescens being the most prevalent species, accounting for 61.5 % and 32.9 % of the isolates, respectively (Zhu and Xiao, 2015).

Several site-specific fungicides with different modes of action are available to control Alternaria rot on various crops in California, including anilinopyrimidines (AP), quinone outside inhibitors (QoIs), phenylpyrroles (PP), and succinate dehydrogenase inhibitors (SDHIs) (Avenot et al., 2008, 2014; Avenot and Michailides, 2015; Karaoglanidis et al., 2011). However, development of fungicide resistance renders management of Alternaria diseases difficult.

Natamycin, also known as pimaricin, is a natural polyene macrolide antibiotic produced by Streptomyces spp. (i.e., Streptomyces natalensis, S. chattanoogensis, S. gilvosporeus, and S. lydicus) (Aparicio et al., 2016). Its action is correlated with the alteration of the normal functioning of ergosterol in the fungal membrane, not forming any membrane disruption complexes, and resistance to natamycin has never been reported in filamentous fungi (Aparicio et al., 2016). Because of its broad spectrum of activity against fungi, no effect on bacteria, protozoa or viruses, and low toxicity to mammalian cells, it has been widely used as a preservative in certain foods and beverages, such as cheese, meat, sausages, yoghurt, fruit juice, beer, and wine (Delves-Broughton et al., 2005). More recently, natamycin has been classified as biofungicide by the U. S. Environmental Protection Agency (U.S. EPA, 2012).

BioSpectra, whose active ingredient is natamycin (10.34 %), has been registered for postharvest use on citrus, pome and stone fruit, and other fresh fruit such as avocado, kiwi, mango and pomegranates (U.S. EPA, 2016). Recent studies showed that natamycin was highly effective against Botrytis cinerea on mandarin fruit and Colletotrichum acutatum on strawberry (Haack et al., 2018; Saito et al., 2020). Currently no postharvest fungicides/biofungicide are commercially available for postharvest use on blueberries for control of postharvest diseases. There is no information available regarding effectiveness of natamycin in controlling Alternaria rot on blueberries.

The aim of this study was to evaluate the potential use of natamycin as a postharvest treatment to control Alternaria rot of blueberry. The specific objectives were (i) to establish baseline sensitivity of A. alternata and A. arborescens isolates to natamycin and (ii) to evaluate efficacy of natamycin in controlling Alternaria rot on blueberry fruit.