Comparative Effects of Plant Growth–Promoting Rhizobacteria and their Formulations with Compost and Mineral Fertilizer on Onion Productivity and Disease Incidence in Burkina Faso

Abstract

Soil infertility is a major constraint to onion production in Burkina Faso. This study aimed to evaluate the combined and separate effects of Plant Growth–Promoting Rhizobacteria (PGPR), mineral fertilizer (NPK), and organic (compost) fertilizer on onion growth and disease incidence. Five bacterial isolates—VDK5AN, VDK12AE, M14AN, TI13AN, and M16AN—were characterized for plant growth-promoting properties and antifungal activity against Fusarium sp. and Aspergillus sp. Their effect on onion seed germination and early growth was also evaluated. In addition, soil samples were collected in field and used in pot experiments involving several fertilization modes, including mineral fertilizer (NPK and compost applied at two rates (2.5% and 5%), and their combinations with selected bacterial isolates. Germination dynamics, plant biomass, root length, and disease incidence were monitored throughout the experiment. To assess the individual and combined effects of variety, treatment, and evaluation period on onion performance, an ANOVA test was used for data analysis. Germination tests showed a highly significant varietal effect (p < 0.01), with the Violet de Galmi variety reaching up to 25% germination compared to only 2–14% for Prema-178. In the pot experiments, treatments significantly influenced seedling height and root length (p < 0.05), neck height, and leaf number (p < 0.05). Compost-based inoculations—particularly Compost (5%)–TI13AN, Compost (5%)–M16AN, and Compost (2.5%)–M14AN—significantly increased root length (P < 0.001) and biomass (P < 0.05), yielding the longest roots and highest biomass. Conversely, NPK–M14AN yielded the lowest biomass. Disease incidence reached 100% under NPK+M16AN but declined to 0–16.67% under Compost (2.5%)–M16AN. Overall, compost-based PGPR formulations, particularly with TI13AN (Bacillus), M14AN (Lysinibacillus), and M16AN (Terrilactibacillus), significantly enhanced the germination, growth, and disease resistance of onion seedlings, offering a sustainable alternative to mineral fertilizer dependency in onion production in West Africa.