Effect of Microbial Inoculants Endowed with Multifarious Plant Growth-Promoting Traits on Grape Growth and Fruit Quality under Organic Fertilization Scenarios

Plant growth-promoting bacteria (PGPB) have growth-promoting and disease-resisting effects and can be used as new types of plant growth promoters. This study was conducted to determine the plant growth-promoting traits of five strains and follow a 2-year field trial to evaluate their effects on grape growth and fruit quality. The five PGPB were combined with two organic fertilizers (cow dung fertilizer and distillers’ grain fertilizer) for application on grape fields; the control group only received the corresponding organic fertilizer. The five strains showed different growth promoting abilities, as indicated by their differing production of indole acetic acid (IAA) and siderophores and ability to dissolve phosphorus and potassium, fix nitrogen, and resist saline and alkali. During the field trial, vine growth and fruit quality were significantly better in the distillers’ grain fertilizer (high nutrient content) alone treatment than in the cow dung fertilizer (low nutrient content) alone treatment. However, after the two fertilizers were inoculated with the five different PGPB, only the five treatments with cow dung fertilizer inoculated with PGPB showed significant improvement. The five treatments of cow dung fertilizer inoculated with PGPB exhibited varied impacts on plant growth and fruit quality. And the promotion effects persisted significantly after two consecutive years. Among the PGPB, Bacillus velezensis 18, B. velezensis 20, and Rahnella aquatilis 5 emerged as consistently effective performers over the two-year period, demonstrating stable and commendable outcomes. These strains are recommended for prolonged application in grape cultivation to optimize growth and yield. This study provides a theoretical reference and an experimental basis for organic fertilizer inoculated with PGPB to improve grape production.