Suppression of pine pitch canker disease and promotion of seedling growth in Japanese red pine using a newly isolated strain of Bacillus subtilis

Abstract

Fusarium circinatum, the causal agent of pine pitch canker, can cause large-scale tree mortality in pine forests. Asymptomatic seedlings infected with F. circinatum can spread the disease at planting sites, highlighting the need for nursery-level control and using vigorous seedlings. Accordingly, this study isolated the plant growth-promoting bacterium (PGPB) Bacillus subtilis SJ 910 and evaluated its efficacy in controlling F. circinatum and enhancing pine-seedling growth. B. subtilis SJ 910 produced lytic enzymes (chitinase, protease, and β-1,3-glucanase) associated with the degradation of major fungal cell wall components. Crude enzyme extracts inhibited F. circinatum spore germination by 72.20%, 79.06%, and 87.36% and mycelial growth by 7.08%, 15.93%, and 21.24% at concentrations of 25, 50, and 100 μL/mL, respectively, and induced abnormal morphologies in spores and hyphae. Application of B. subtilis SJ 910 significantly suppressed pitch canker, reducing disease incidence to less than half and markedly lowered disease severity compared with the control and pink broth (PB) media treatments, while also increasing seedling survival. Moreover, B. subtilis SJ 910 produced indole-3-acetic acid and solubilized nitrogen and phosphorus, contributing to enhanced seedling growth. B. subtilis SJ 910 markedly enhanced seedling biomass, substantially increasing both shoot and root dry weights compared with control and PB media treatments. These findings suggest that B. subtilis SJ 910 is a promising biocontrol agent against F. circinatum and a growth promoter for pine seedlings, offering a sustainable alternative to chemical treatments and contributing to the production of vigorous seedlings for successful afforestation.