Viral plant diseases represent a serious problem in agricultural production, causing large shortages in the production of food crops. Eco-friendly approaches are used in controlling viral plant infections, such as biocontrol agents. In the current study, Streptomyces cellulosae isolate Actino 48 is tested as a biocontrol agent for the management of tobacco mosaic virus (TMV) and inducing tomato plant systemic resistance under greenhouse conditions. Foliar application of a cell pellet suspension of Actino 48 (2 × 10⁷ cfu. mL⁻¹) is performed at 48 h before inoculation with TMV. Peroxidase activity, chitinase activity, protein content, and the total phenolic compounds are measured in tomato leaves at 21 dpi. On the other hand, the TMV accumulation level and the transcriptional changes of five tomato defense-related genes (PAL, PR-1, CHS, PR-3, and PR-2) are studied. Treatment with Actino 48 before TMV inoculation (48 h) induced tomato plants to increase their levels of peroxidase and chitinase enzymes. Furthermore, a significant increase in the concentration of total phenolic compounds was observed in Actino 48 and TMV-treated tomato plants compared to TMV-treated tomato plants alone. Treatment with Actino 48 reduced the TMV accumulation level (53.8%) compared to treatment with the virus alone. Actino 48 induced plant growth, where the fresh and dry weights of tomato plants increased. Additionally, significant increases of the PAL, PR-1, CHS, and PR-3 transcripts were observed. On the other hand, a higher induction of PR-2 was only observed in TMV-treated tomato plants. In conclusion, S. cellulosae isolate Actino 48 can be used as a biocontrol agent for the reduction of symptoms and severity of TMV.

Culture broth, cell pellet suspension and cell free supernatant of 14 Bacillus subtilis isolates obtained from different Egyptian locations were checked for their ability to repress egg hatching and juvenile (J2) activity of root-knot nematode, Meloidogyne incognita under laboratory environment. Treatments using culture broth of B. subtilis isolates B10 and B8, at a concentration of 50%, recorded lowest percentages of hatched eggs reaching 44.7 and 46.3%, respectively. Culture broth of B. subtilis isolate B10 at the same concentration showed a higher percentage of juvenile mortality reached 99.7%. Batch fermentation was completed, using B. subtilis isolate B10 (Accession No. EF583055), which gave the lowest percentage of hatched eggs and the highest percentage of juvenile mortality of M. incognita, for maximizing biomass production and suppression effects of culture broth. Batch fermentation no. 2, which started in a bioreactor with optical density of 0.5, was the best process that achieved a higher cell biomass and percentage of juvenile mortality of 4.52 g/l and 74.3%, respectively, using culture broth of 5%. Under greenhouse conditions, culture broth, cell pellet suspension, and cell-free supernatant of B. subtilis isolate B10 were used to test their potential for reducing number of galls and egg masses in the roots of tomato plants. The treatment of using culture broth at a concentration 10 ml/pot, 2 × 10⁹ cfu/ml in a soil infested with M. incognita, was highly significant in decreasing number of galls and egg masses reaching the average of 9.3 and 6.7, with reduction percentage of 81.1 and 89.5%, respectively, compared with the control treatment of M. incognita only. In addition, B. subtilis isolate B10 was formulated and applied as bionematicide to test its efficiency in reducing number of galls and egg masses. Treatment with bioformulation at a concentration of 0.1 g/pot was more significant than the other concentrations in reducing number of galls and egg masses, reaching the average of 12 and 7 with a reduction percentage of 69.7 and 71.2%, correspondingly.

Of ten actinobacterial isolates, Streptomyces cellulosae Actino 48 exhibited the strongest suppression of Sclerotium rolfsii mycelium growth and the highest chitinase enzyme production (49.2 U L⁻¹ min⁻¹). The interaction between Actino 48 and S. rolfsii was studied by scanning electron microscope (SEM), which revealed many abnormalities, malformations, and injuries of the hypha, with large loss of S. rolfsii mycelia density and mass. Three talc-based formulations with culture broth, cell-free supernatant, and cell pellet suspension of chitinase-producing Actino 48 were characterized using SEM, Fourier transform infrared spectroscopy (FTIR), and a particle size analyzer. All formulations were evaluated as biocontrol agents for reducing damping-off, root rot, and pods rot diseases of peanut caused by S. rolfsii under greenhouse and open-field conditions. The talc-based culture broth formulation was the most effective soil treatment, which decreased the percentage of peanut diseases under greenhouse and open-field conditions during two successive seasons. The culture broth formulation showed the highest increase in the dry weight of peanut shoots, root systems, and yielded pods. The transcriptional levels of three defense-related genes (PR-1, PR-3, and POD) were elevated in the culture broth formulation treatment compared with other formulations. Subsequently, the bio-friendly talc-based culture broth formulation of chitinase-producing Actino 48 could potentially be used as a biocontrol agent for controlling peanut soil-borne diseases caused by S. rolfsii.

Twenty fluorescent Pseudomonas isolates were tested for their ability to produce siderophores on chrome azurol S (CAS) agar plates and their antagonistic activity against six plant pathogenic fungal isolates was assessed. Scaling-up production of siderophores from the promising isolates, P. aeruginosa F2 and P. fluorescens JY3 was performed using batch and exponential fed-batch fermentation. Finally, culture broth of the investigated bacterial isolates was used for the preparation of two economical bioformulations for controlling Fusarium oxysporum and Rhizoctonia solani. The results showed that both isolates yielded high siderophore production and they were more effective in inhibiting the mycelial growth of the tested fungi compared to the other bacterial isolates. Exponential fed-batch fermentation gave higher siderophore concentrations (estimated in 10 µL), which reached 67.05% at 46 h and 45.59% at 48 h for isolates F2 and JY3, respectively, than batch fermentation. Formulated P. aeruginosa F2 and P. fluorescens JY3 decreased the damping-off percentage caused by F. oxysporum with the same percentage (80%), while, the reduction in damping-off percentage caused by R. solani reached 87.49% and 62.5% for F2 and JY3, respectively. Furthermore, both formulations increased the fresh and dry weight of shoots and roots of wheat plants. In conclusion, bio-friendly formulations of siderophore-producing fluorescent Pseudomonas isolates can be used as biocontrol agents for controlling some plant fungal diseases.

Microbial secondary metabolites are biologically active natural compounds that exhibit a broad range of antimicrobial properties. Investigating rhizobacterial resources could lead to the development of new antimicrobial compounds to enhance biocontrol agents. In this study, six Bacillus isolates (KSAS15-KSAS20) were isolated from the potato rhizosphere and screened in vitro for their ability to inhibit the growth of the phytopathogenic fungus, Sclerotium bataticola. The antifungal efficacy of the bacterial strains was assessed in vitro using a dual culture antagonism assay. The results demonstrated that strain KSAS17 had the most significant antagonistic activity, achieving a growth inhibition rate of 31.4%. The sequencing and analysis of the 16S rRNA gene identified the bacterial isolates as Bacillus licheniformis (KSAS15, KSAS16, and KSAR19), Bacillus cereus (KSAS17 and KSAR18), and Lysinibacillus fusiformis (KSAR20). Gas chromatography-mass spectrometry analysis of ethyl-acetate extract of the bacterial culture filtrate revealed that the compound most likely responsible for the effects in strain KSAS17 is pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl). Furthermore, significant bioactive aliphatic and aromatic compounds were identified, including oleic acid, hexadecanoic acid, 1-tetradecanol, heptadecanoic acid, 5-fluoro-2,2-dimethylchroman-4-one, pentadecanoic acid, 3′,8,8′-trimethoxy-3-piperidyl-2,2′-binaphthalene-1,1′,4,4′-tetrone, and phthalic acid derivatives. These findings indicate that the secondary metabolites of the rhizobacteria B. cereus strain KSAS17 exhibit antifungal properties and may be effectively employed as a biocontrol agent for managing fungal diseases.