Inhibitory effect of biochar-enriched biocontrol agents on Phytophthora capsici

Biochar is known to effectively control the Phytophthora blight of pepper. To further understand the mechanisms of biochar-mediated disease control, we screened the biochar-enriched beneficial microorganisms and evaluated their antagonistic activities against Phytophthora capsici. A pot experiment was conducted to determine the effects of straw biochar on the control of Phytophthora blight. Both qPCR and dilution-plate methods were used to identify the biochar-enriched biocontrol microorganisms in rhizosphere soils. Potential antagonistic strains, which were biochar-enriched, were screened using the selective isolation method and rhizosphere colonization assay, after which their antagonistic activity against P. capsici in soil was determined. The results showed that biochar amendment could significantly reduce the abundance of P. capsici and severity of the Phytophthora blight of pepper by 95.1% and 91.1%, respectively. In addition, biochar amendment significantly increased the abundances of Trichoderma, Penicillium, Aspergillus, Bacillus, Pseudomonas, and Sphingomonas by 2.22, 4.09, 3.89, 2.45, 1.45, and 1.30 times, respectively. Twenty-two potential biocontrol strains that can be enriched by biochar were screened from biochar-amended rhizosphere soils using the selective isolation method. Comparing the colonization between the biochar-amended and control rhizosphere soils, two strains of Trichoderma, three strains of Penicillium, two strains of Aspergillus, three strains of Bacillus, three strains of Pseudomonas, three strains of Streptomyces, and two strains of Sphingomonas were confirmed to be enriched by biochar. Trichoderma (TR1 and TR3), Penicillium (PE1), Aspergillus (AS1 and AS2), Bacillus (BA1, BA2, and BA3), Pseudomonas (PS1 and PS3), and Streptomyces (ST1, ST4, and ST5) strains could significantly reduce the abundance of P. capsici in the soil. Among these antagonistic stains, Trichoderma (TR1 and TR3), Aspergillus (AS1 and AS2), Bacillus (BA1 and BA2), Pseudomonas (PS1 and PS3), and Streptomyces (ST1) strains in synergism with biochar facilitated a significant increase in the inhibition of P. capsici. Thus, Trichoderma, Aspergillus, Bacillus, Pseudomonas, and Streptomyces enriched by biochar might play an import role in the suppression of Phytophthora blight of pepper under biochar amendment.