Background White mold disease, caused by Sclerotinia sclerotiorum the devastating pathogen, attacks green beans (Phaseolus vulgaris L.) and several crops worldwide. The present investigation was carried out to introduce some antagonistic microorganisms as novel antifungal substances to be an alternative and secure method to effectively control the disease. Results Three Streptomyces species, i.e., S. griseus (MT210913 “DG5”), S. rochei (MN700192 “DG4”), and S. sampsonii (MN700191 “DG1”) were isolated, biologically, molecularly characterized, and evaluated in vitro and in vivo. Molecularly, polymerase chain reaction (PCR) amplification and nucleotide sequencing were used to characterize the pathogen and bio-agents. PCR amplification of the pathogen and Streptomyces species (bioagents) exhibited amplicons of around 535 bp and 1300 bp, respectively. The nucleotide sequence analysis of the three Streptomyces spp. indicated that S. rochei was closely related to S. griseus, and both had a distance relationship with S. sampsonii. The evaluation of bioagents was carried out against S. sclerotiorum. Reduction percentages in the mycelial growth of the pathogen ranged between 60.17 and 52.30%, indicating that S. rochie gave the highest inhibition percent. Incorporations of Streptomyces spp. culture filtrate components into culture media proved that S. sampsonii was more efficient as a bioagent in reducing mycelial growth pathogen by 84.50%. The effectiveness of the bioagent volatile compounds inhibited the pathogen growth at a rate of 54.50-72.54%, respectively, revealing that S. rochei was the highest inhibitor followed by S. griseus. The parasitic activity of Streptomyces spp. upon S. sclerotiorum showed deformation, contraction, and collapse when observed by light and scanning electron microscopy. Molecular characterization of the 3 Streptomyces spp. revealed that S. griseus was closely related to S. sampsonii (96%), secondly ranked by S. rochei (93.1%). Viability and germination of pathogen sclerotia were reduced when they dipped into the Streptomyces spore suspensions for 10, 20, and 30 days. Application of the 3 Streptomyces spp. in the field proved a great potential to control the disease. Conclusions The results suggested that the 3 Streptomyces strains and their secondary metabolites can be potential biocontrol agents and biofertilizers for controlling S. sclerotiorum, the causative agent of bean white mold disease.

Background Soil-borne plant pathogenic fungi with a wide host range of crops cause a significant limitation on the global production of agronomic crops. Applications of synthetic pesticides are an important tool for managing plant diseases, but have deleterious influences on the environment as well as its incompatibility with organic agriculture. Recently, Streptomyces spp. became one of the best bio-control agents as a promising environmentally eco-friendly method for effective management of plant diseases. Results In a previous research, three species of Streptomyces spp., i.e., S. griseus (MT210913 “DG5”), S. rochei (MN700192 “DG4”) and S. sampsonii (MN700191 “DG1” strains) were identified, as exhibiting potent antifungal activities against plant pathogenic fungus, Sclerotinia sclerotiorum in vitro and greenhouse. GC–Mass analysis revealed the presence of 44, 47 and 54 substances of S. sampsonii DG1, S. griseus DG5 and S. rochei DG4, respectively. GC–MS revealed substances, with bio-control activity, were categorized as volatile organic compounds (VOCs), fatty acids and plant growth regulators, etc. GC–MS analysis exhibited the presence of 7, 13 and 20 volatile compounds produced by S. sampsonii, S. rochei and S. griseus, respectively. These substances exhibited potent antifungal activity against various plant pathogenic fungi, i.e., Botrytis cinerea, Macrophomina phaseolina, Rhizoctonia solani and S. sclerotiorum in vitro, by dual-culture assay. The three strains inhibited all the pathogenic fungi in dual-culture assay in the range of 30–73.67%. Also, the produced substances were applied in vivo (in the field) and supported their potential biocontrol agent against S. sclerotiorum as well as possessed significant biological properties for plant health and growth. Applying Streptomyces spp. culture broth in the field enhanced physiological responses of phenols, sugar, chlorophyll, protein contents and parameters as well as the yield of bean plants. Conclusion In field experiments, foliar application of Streptomyces spp. and their metabolites proved to be a great potential, as promising biocontrol agents, for controlling S. sclerotiorum and enhanced plant growth and yield. S. rochei and S. griseus proved to be strong antifungal, plant growth promoters and environmentally eco-friendly fungicides.