Razi University

Sugar beet (Beta vulgaris L.) is one of the main sources of sugar production worldwide, and soil-borne fungal diseases—particularly damping-off and root rot caused by Rhizoctonia solani—are among the most important factors reducing the yield of this crop. Given the limitations of chemical control alone and the potential of plant growth-promoting rhizobacteria (PGPR) in biological control, the present study aimed to evaluate the efficacy of several bacterial strains and chemical compounds, applied individually and in combination, against R. solani on sugar beet under laboratory and greenhouse conditions.In the first stage, the antagonistic potential of 14 bacterial strains against the pathogenic isolate (belonging to anastomosis group AG-4HGI) was evaluated using two methods: dual culture and antifungal volatile compound assays on PDA medium. Two-way analysis of variance showed a highly significant "strain × assay method" interaction (P<0.0001); only 7 of the 14 strains showed significant inhibition in at least one of the two methods. Bacillus subtilis BS (70.48%) and Bacillus velezensis JPS19 (61.90%) showed the highest inhibition in dual culture but performed very poorly in the volatile compounds assay; this crossover pattern indicated differing antagonistic mechanisms among strains. Based on these results, Azotobacter vinelandii and Bacillus subtilis BS were selected for further study.In the next stage, the inhibitory effect of eight commercial fungicides on the pathogen's mycelial growth was examined, and the effective concentration values (EC25 to EC90) of each fungicide were calculated using probit analysis. The compatibility of these fungicides at their effective concentration levels with the two selected bacteria was then assessed. The results showed that fungicide type, more than concentration level, was the main determinant of damage to the bacteria (P<0.0001 for both factors); Copper (for Azotobacter) and Captan (for Bacillus) caused the greatest damage, while Celest caused the least damage to both bacteria. Based on this, Celest and Rovral (due to their lowest EC90 values and acceptable compatibility with the bacteria) as well as Fosetyl (due to its resistance-inducing role) were selected for the greenhouse experiment.In the greenhouse experiment, the effect of 25 treatments (healthy and infected controls, the two bacteria alone, three fungicides at different concentrations, and bacterium × fungicide combinations) was evaluated on 14 traits related to emergence, disease, and seedling growth. The treatment effect was highly significant for all traits (P<0.0001; R² between 0.47 and 0.78). Individual application of either probiotic bacterium, without fungicide, showed no significant advantage over the infected control. In contrast, the combination of bacteria with fungicides—particularly Rovral at 50% and 75% of its EC, together with either bacterium—consistently ranked among the best treatments (after the healthy control), producing the lowest disease severity index and disease incidence along with improved growth traits (fresh and dry weight, root volume, green area).Overall, the findings of this study showed that, under the tested conditions, the fungicide (particularly Rovral) plays the central role in controlling R. solani and improving sugar beet growth indices, while individual application of the tested probiotic bacteria, without an accompanying fungicide, had no appreciable efficacy under greenhouse conditions. Combined application of bacteria with reduced fungicide concentrations may serve as a suitable strategy for reducing reliance on fungicides in the integrated management of this disease, although validation of these findings under field conditions and selection of bacteria from a larger pool of strains is recommended.