Isolation and in vivo and in vitro antifungal activity of phenylacetic acid and sodium phenylacetate from Streptomyces humidus

Abstract

The antifungal substances SH-1 and SH-2 were isolated from Streptomyces humidus strain S5-55 cultures by various purification procedures and identified as phenylacetic acid and sodium phenylacetate, respectively, based on the nuclear magnetic resonance, electron ionization mass spectral, and inductively coupled plasma mass spectral data. SH-1 and SH-2 completely inhibited the growth of Pythium ultimum, Phytophthora capsici, Rhizoctonia solani, Saccharomyces cerevisiae, and Pseudomonas syringae pv. syringae at concentrations from 10 to 50 microg/ml. The two compounds were as effective as the commercial fungicide metalaxyl in inhibiting spore germination and hyphal growth of P. capsici. However, the in vivo control efficacies of the two antifungal compounds against P. capsici infection on pepper plants were similar to those of H(3)PO(3) and fosetyl-AI but less than that of metalaxyl.

FIG. 1

EI mass spectrum (A) of the antifungal substances SH-1 (phenylacetic acid), and EI (B) and ICP (C) mass spectra of SH-2 (sodium phenylacetate).

FIG. 2

(A) Correlations of partial structures of the antifungal substance SH-1 from HMBC spectra and (B) structures of the antifungal substances SH-1 (R = H) and SH-2 (R = Na) isolated from S. humidus strain S5-55.

FIG. 3

In vivo control efficacy of SH-1, SH-2, H₃PO₃, fosetyl-AI, and metalaxyl against P. capsici infection on pepper plants at the first-branch stage. (A) Foliar spray treatment on pepper plants just before stem wound inoculation. (B) Soil drench treatment 1 day before soil drench inoculation. The disease severity rating is based on a scale of 0 to 5 scale, with a score of 0 for no visible symptoms and a score of 5 for a dead plant. Means at each concentration followed by the same letter are not significantly different (P = 0.05) according to the least significant difference test.

FIG. 4

In vitro and in vivo efficacy of the authentic phenylacetic acid, H₃PO₃, fosetyl-AI, and metalaxyl against mycelial growth of P. capsici (A) and the disease development in pepper plants (B). Mycelial growth was measured on potato dextrose agar containing different concentrations of the chemicals when the control plates (9 cm in diameter) were completely covered by the fungus. Each chemical was sprayed on the foliage of plants 1 day before inoculation. Disease severity was rated 7 days after inoculation on pepper plants at the first-branch stage. Means at each concentration followed by the same letter are not significantly different (P = 0.05) according to the least significant difference test.