Bacillus velezensis HN-2 crude extract inhibits Erysiphe quercicola infection of rubber tree leaves

Abstract

The rubber tree (Hevea brasiliensis) is an important tree that produces rubber, which is a strategic industrial raw material. Erysiphe quercicola is a fungal pathogen that causes powdery mildew infection of rubber trees and is difficult to control. Large-scale use of inorganic pesticides to control the pathogen, Bacillus sp. is an environment friendly biocontrol agent that is effective against E. quercicola, but little is known about how it controls powdery mildew. In this study, we performed a crude extraction of the active ingredient in the fermentation broth of Bacillus velezensis HN-2 by using n-butanol and treated E. quercicola spores with the crude extract, and the 50% effective concentration was determined to be 22.5 μg/mL. Optical microscopic analysis revealed that the crude extract caused morphological changes in E. quercicola spores. We subsequently treated rubber leaves with this crude extract and examined changes in the activity of antioxidant enzymes in the leaves. The activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase were elevated in rubber leaves treated with the crude extract. Quantitative Real Time PCR (RT-qPCR) assays indicated that the crude extract upregulated the expression of defence-related genes, whereas HbLFG1 (a negative feedback-regulated gene associated with E. quercicola infestation) expression was suppressed, indicating suppression of E. quercicola infection. Our findings show that lipopeptides of the biocontrol strain B. velezensis HN-2 have effective biocontrol potential and that LFG1 expression can be detected to determine the effectiveness of an agent in controlling E. quercicola. On the basis of existing experiments, we will further develop and utilize strain HN-2.