Introduction
The root-knot nematode has proliferated significantly because of its ease of transmission through infected seedlings and contaminated soil. Species of the genus Meloidogyne are highly destructive due to their rapid reproduction rate and broad host range, making their control particularly challenging. Given the risks and limitations associated with chemical control, alternative management strategies have attracted increasing attention. Nowadays, the use of chemical pesticides for controlling plant diseases is recommended to be combined with biocontrol agents. Bioactive compounds derived from Streptomyces exhibit a wide spectrum of chemical diversity and have been shown to cause mortality of second-stage juveniles (J2), reduce egg hatching, decrease nematode populations in soil, and mitigate pathogenicity indices of root-knot nematodes. Moreover, the previous research also revealed that the metabolites produced by Streptomyces species are effective in controlling root-knot nematodes and improving plant growth indices.
 
Materials and Methods
Streptomyces isolates were obtained from the soils of healthy tomato (Meloidogyne javanica) and eggplant plants in Khorasan Razavi province. The biocontrol potential of these isolates against root-knot nematodes was first assessed under in vitro conditions. The hatching rate of eggs was assessed at 24, 48, 72, and 120 hours. Additionally, larval mortality was evaluated at three-time intervals including 24, 48, and 72 hours. Their growth-promoting features consisting of auxin, ACC deaminase production, phosphate solubilization) and antagonistic properties (siderophores and chitinase) were also evaluated in the laboratory assays. The most effective biocontrol isolates were subsequently tested in greenhouse experiments to determine their impact on reducing pathogenicity indices of Meloidogyne javanica and improving tomato growth parameters. All tests were conducted twice with a minimum of three replicates. For statistical analysis, one-way analysis of variance (ANOVA) was employed. Means were compared using Duncan's multiple range test, and values at the five percent level were considered statistically significant. Molecular identification of the selected biocontrol isolates was performed based on 16S rDNA sequencing, and phylogenetic analysis was carried out to assess their relatedness to reference strains. The phylogenetic tree was constructed with a bootstrap value of 1000 replicates using the neighbor-joining method.
 
Results and Discussion
In vitro bioassays revealed that among the 18 Streptomyces isolates tested, strains BAD, ANA, and S116 caused the greatest reduction in egg hatching and second-stage juvenile mortality compared with the nematode-infected control. The BAD isolate, obtained from the soil of eggplant in Bojan–Neyshabur, exhibited significantly higher levels of phosphate solubilization (1.43 mm), indole-3-acetic acid production (36.4 µg mL^-1), and ACC deaminase activity (1.38 µmol mL^-1) than the uninoculated control and other isolates. In laboratory tests, after 72 hours, isolates S116 and BAD inhibited egg hatching by 37.1% and 38.5%, respectively, compared to the nematode control. Larval mortality rates for these two isolates reached 100% and 65.67%, respectively. Based on laboratory results, including their effects on nematode suppression, plant growth-promoting traits, and chitinase activity, five isolates (BAD, ANA, GHR, S104, and S116) were selected for greenhouse evaluation. Under greenhouse conditions, In the treatment of nematode + isolate BAD, the number of galls and egg masses on the roots decreased by 19.6% and 35.4%, respectively, compared to the control. The number of larvae in the pot soil of this treatment was estimated to be half that of the control pot. Individually and in the presence of the nematode, isolate BAD positively influenced the growth parameters of tomato plants compared to both healthy and infected controls. In the nematode + BAD isolate treatment, the fresh weight of the shoot and root increased by 21.4% and 27.5%, respectively, compared to the nematode-infected control. Furthermore, the shoot and root length in the individual BAD isolate treatment showed the highest values of 46.50 cm and 16.30 cm, respectively, compared to the healthy control. Although this difference was not statistically significant at the five percent level compared to the ANA isolate, an increase in shoot length (43%) and root length (17.9%) was observed in plants infected with nematodes in the presence of the BAD isolate compared to the infected control. Phylogenetic analysis based on 16S rDNA sequencing revealed that isolate BAD clustered together with S. carpaticus and S. harbinensis within the same group.
 
Conclusion
Based on the nematicidal activity of isolate BAD under laboratory conditions, its ability to produce plant growth-promoting metabolites, and its effectiveness in enhancing tomato growth parameters while reducing nematode pathogenicity indices (egg masses, galls, and J2 populations) in greenhouse experiments, isolate BAD appears to be the most effective biocontrol candidate. It represents a promising option for further field evaluations and potential use in integrated management of root-knot nematodes.