The Effect of Some Bacteria and Arbuscular Mycorrhizal Fungi on Rhizoctonia Root rot and Growth Parameters of Common Bean Plants

The seeds of common bean plants (Phaseolus vulgaris) are among the most important food sources worldwide. The plant pathogenic fungus Rhizoctonia solani causes seed rot and seedling damping-off of bean plants and is one of the most important soil-borne pathogens in most cultivation areas of this plant. R. solani plays an important role in reducing the yield and quality of this crop. Biological control is a promising, effective, and sustainable solution for managing soil-borne fungal diseases. The simultaneous use of biological control agents, especially agents with different mechanisms of action, might increases the chance of success in disease management. Indeed, two living agents can show different types of interactions based on their inherent characteristics and ecological conditions. In the current research, the potential of co-application of mycorrhizal fungi and bacterial strains from different taxonomic groups was evaluated on increasing the growth of beans and suppression of bean damping-off.

In this research, the effect of 14 different bacterial strains on the mycelial growth of R. solani was investigated by dual culture and by the volatile compound production tests in the laboratory. The list of bacterial strains included: Bacillus megaterium B15, Bacillus thuringiensis B48Pet, Bacillus subtilis BS (B.s), B. subtilis AS, Lysinibacillus boronitolerans RUPB71 (Lysin), Pseudomonas putida RUP1, Arthrobacter citreus B27Pet, Alcaligenes faecalis 1624, Bacillus pumilus INR7, Bacillus thuringiensis 32B (B.thur), Azotobacter vinelandii RUA1 (Azoto), Delftia tsuruhatensis PIIR (Delft), Stenotrophomonas maltophilia S37 and Bacillus velezensis JPS19 (B.vel). In the dual culture test, B.s, B.vel, Alca, Azoto, Lysin, and Delft strains significantly reduced the mycelial growth of the pathogen. In volatile compounds production test, three strains, Alca, Azoto, and B.thur, significantly reduced the mycelial growth of the pathogen. Four bacterial strains, including B.s, B.vel, Alca, and Azoto, were selected for greenhouse experiment. The selected bacterial strains as separate and combined application with two species of arbuscular mycorrhizal fungi (AMF), including Funneliformis mosseae (F.moss) and Glomus claroideum (G.cla) were investigated for inhibiting the disease caused by R. solani, and for their effect on the growth parameters of bean plants in the presence of the pathogen. The greenhouse experiment was conducted as a completely randomized design with 16 treatments and five replications.

Many of the greenhouse treatments that were applied in this study, significantly reduced the severity of the disease compared to the diseased control. They significantly increased the fresh and the dry weight of the shoots, root fresh weight and root dry weight. Four treatments (F.moss, B.s, Alca and Azoto) significantly enhanced the root length and four treatments (Alca, B.s, B.vel+G.cla and Alca+G.cla) increased the shoot length. In terms of the effect on the disease, the B.s strain was the best treatment, which reduced the disease severity by 36.5% and showed the greatest effect in promoting the vegetative characteristics of the plant. Although the bacterial and mycorrhizal strains showed compatibility in many combined treatments, G.cla+B.vel was the only double treatment that showed an increasing effect on the fresh weight and shoot length compared to the separate applications. The other double treatments did not have an additive or synergistic effect in reducing the disease and improving plant growth. Even in some double treatments, in terms of reducing disease and improving plant growth, a weaker effect was seen than their single treatments. Among different treatments, Alca+G.cla and Alca+F.moss showed the highest and B.s+G.cla showed the lowest mycorrhizal colonization.

The effect of bacteria on mycorrhizal colonization was increasing, decreasing, or null depending on the combination of bacterial and mycorrhizal strains used. In many cases, there was no direct relationship between the amount of mycorrhizal colonization and disease control. Overall, before the commercial use of the combination of biological agents, it is necessary to check their interactions in greenhouse and field experiments and ensure their compatibility and lack of antagonistic effect on each other. If the biological agents present in a consortium are compatible, its biocontrol effect will be more stable, efficient and reliable, and it will be possible to use it in a wider range of variable soil ecological conditions. Moreover, the interaction of individuals within a given consortium is strain-specific, and the results of an experiment on specific strains cannot be generalized to other strains of the same species.