Plant Growth Promotion and Biocontrol of Leaf Blight Caused by Nigrospora sphaerica on Passion Fruit by Endophytic Bacillus subtilis Strain GUCC4

Abstract

Passion fruit (<i>Passiflora edulis</i> Sims) is widely cultivated in tropic and sub-tropic regions for the production of fruit, flowers, cosmetics, and for pharmacological applications. Its high economic, nutritional, and medical values elicit the market demand, and the growing areas are rapidly increasing. Leaf blight caused by <i>Nigrospora sphaerica</i> is a new and emerging disease of passion fruit in Guizhou, in southwest China, where the unique karst mountainous landscape and climate conditions are considered potential areas of expansion for passion fruit production. <i>Bacillus</i> species are the most common biocontrol and plant-growth-promotion bacteria (PGPB) resources in agricultural systems. However, little is known about the endophytic existence of <i>Bacillu</i>s spp. in the passion fruit phyllosphere as well as their potential as biocontrol agents and PGPB. In this study, 44 endophytic strains were isolated from 15 healthy passion fruit leaves, obtained from Guangxi province, China. Through purification and molecular identification, 42 of the isolates were ascribed to <i>Bacillus</i> species. Their inhibitory activity against <i>N. sphaerica</i> was tested in vitro. Eleven endophytic <i>Bacillus</i> spp. strains inhibited the pathogen by >65%. All of them produced biocontrol- and plant-growth-promotion-related metabolites, including indole-3-acetic acid (IAA), protease, cellulase, phosphatase, and solubilized phosphate. Furthermore, the plant growth promotion traits of the above 11 endophytic <i>Bacillus</i> strains were tested on passion fruit seedlings. One isolate, coded <i>B. subtilis</i> GUCC4, significantly increased passion fruit stem diameter, plant height, leaf length, leaf surface, fresh weight, and dry weight. In addition, <i>B. subtilis</i> GUCC4 reduced the proline content, which indicated its potential to positively regulate passion fruit biochemical properties and resulted in plant growth promotion effects. Finally, the biocontrol efficiencies of <i>B. subtilis</i> GUCC4 against <i>N. sphaerica</i> were determined in vivo under greenhouse conditions. Similarly to the fungicide mancozeb and to a commercial <i>B. subtilis</i>-based biofungicide, <i>B. subtilis</i> GUCC4 significantly reduced disease severity. These results suggest that <i>B. subtilis</i> GUCC4 has great potential as a biological control agent and as PGPB on passion fruit.