A Coating Based on Bioactive Compounds from Streptomyces spp. and Chitosan Oligomers to Control Botrytis cinerea Preserves the Quality and Improves the Shelf Life of Table Grapes

Abstract

Botrytis cinerea is the most harmful postharvest disease of table grapes. Among the strategies that can be envisaged for its control, the use of coatings based on natural products is particularly promising. The study presented herein focuses on the assessment of the antagonistic capacity of two Streptomyces species and their culture filtrates against B. cinerea. Firstly, the secondary metabolites were characterized by gas chromatography-mass spectrometry, with N1-(4-hydroxybutyl)-N3-methylguanidine acetate and 2R,3S-9-[1,3,4-trihydroxy-2-butoxymethyl]guanine acetate as the main compounds produced by S. lavendofoliae DSM 40217; and cyclo(leucyloprolyl) and cyclo(phenylalanylprolyl) as the most abundant chemical species for S. rochei DSM 41729. Subsequently, the capacity of S. lavendofoliae DSM 40217 and S. rochei DSM 41729 to inhibit the growth of the pathogen was tested in dual culture plate assays, finding 85-90% inhibition. In agar dilution tests, their culture filtrates resulted in effective concentration values (EC₉₀) in the 246-3013 μg·mL^-1 range. Upon the formation of conjugate complexes with chitosan oligomers (COS) to improve solubility and bioavailability, a synergistic behavior was observed, resulting in lower EC₉₀ values, ranging from 201 to 953 μg·mL^-1. Ex situ tests carried out on 'Timpson' and 'Red Globe' table grapes using the conjugate complexes as coatings were found to maintain the turgor of the grapes and delay the appearance of the pathogen by 10-15 days at concentrations in the 750-1000 µg·mL^-1 range. Hence, the conjugate complexes of COS and the selected Streptomyces spp. culture filtrates may be put forward as promising protection treatments for the sustainable control of gray mold.

Keywords: Botrytis cinerea; Streptomyces spp.; antifungal; conjugate complexes; postharvest diseases; synergism; table grapes.

Figure 1

Compounds identified by GC–MS in S. lavendofoliae DSM 40217 extract.

Figure 2

Compounds identified by GC–MS in S. rochei DSM 41729 extract.

Figure 3

Radial growth values of B. cinerea in the presence of the different treatments under study at different concentrations (in μg·mL⁻¹). Concentrations labeled with the same uppercase letters are not significantly different at p < 0.05 by Tukey’s test. All values are presented as the average of three repetitions. Error bars represent the standard deviation across three replicates.

Figure 4

General appearance of cv. ‘Timpson’ and cv. ‘Red Globe’ clusters on days 1, 5, 10, and 15. From left to right: negative control; positive control (clusters inoculated with 1 × 10⁴ CFU·mL⁻¹ of B. cinerea); clusters treated with COS–S. lavendofoliae DSM 40217 metabolites conjugate complex (COS–ML) at 1000 μg·mL⁻¹ and inoculated with 1 × 10⁴ CFU·mL⁻¹ of B. cinerea; clusters treated with COS–S. rochei DSM 41729 metabolites conjugate complex (COS–MR) at 750 μg·mL⁻¹ and inoculated with 1 × 10⁴ CFU·mL⁻¹ of B. cinerea. Only one replicate per treatment per day is shown.