**3. Discussion**

The obtained results proved that the studied *Actinomycetes* isolates were able to promote the growth of tomato plants by improving the eco-physiological characteristics and also are promising for the biocontrol of *S. sclerotiorum* on tomato seedlings. In particular, the biological activity and growth-promoting effect of the studied *Actinomycetes* strains may be due to their ability to produce some bioactive metabolites, such as growth hormones, which enhance the tomato seedlings' growth [26–29]. The application of microbial plant stimulants is considered an important strategy for sustainable agriculture systems for enhancing plant growth and increasing production, especially under abiotic stress [30].

Sousa and Olivares [31] concluded that plant-growth-promoting Streptomyces (PGPS) was able to biostimulate plant growth by direct and indirect pathways such as phytohormones production, phosphate solubilization, and alleviation of various abiotic stresses. In particular, endophytic *Actinobacteria* can biostimulate the secretion of plant growth hormones such as indole acetic acid (IAA), as reported by Manulis et al. [32] and Dochhil et al. [33].

The treatments with *Streptomyces* sp. and *Arthrobacter humicola* showed high reduction in disease symptoms on tomato seedlings against the tested pathogenic fungi. Furthermore, the bacterization treatments induced a significant disease protection of tomato seedlings compared with non-bacterized plants against fungal infection with *S. sclerotiorum*. These results are in agreement with those of several researchers who reported that many soilborne *Actinomycetes* are able to reduce the growth of some pathogenic fungi such as *Colletotrichum gloeosporioides*, *C. capsici*, and *Fusarium solani* f. sp. *pisi* [34–37].

The production of hydrolytic enzymes can also play an important role in controlling phytopathogenic fungi [38,39]. The cell wall lytic enzymes glucanase and protease can contribute to the degradation of fungal cell wall (skeletal) components through embedment in its protein matrix [40,41]. In addition, Ordentlich et al. [40] reported that chitinase and other lytic enzymes produced by *Serratia marcescens* were able to control the pathogenic fungus *Sclerotium rolfsii*, causing the release of *β*-glucanase, which can increase the chitinolytic activity in hyphal degradation [42].

Chaudhary et al. [43] studied the antagonistic activity of some *Actinomycetes* strains isolated from different niche habitats of Sheopur (India) and observed that some strains were highly active against *Bacillus cereus*, *Enterococcus faecalis*, *Shigella dysenteriae, Streptococcus pyogenes*, *Staphylococcus saprophyticus*, *S. epidermidis*, methicillin-resistant *Staphylococcus*, and *S. xylosus*. The same authors also reported that all studied isolates were able to inhibit the extracellular growth of tested microorganisms, whereas they were not able to inhibit intracellular growth of mycelium. The latter phenomena may be due to the production of some bioactive secondary metabolites that may not reach to the intracellular cells of the tested bacteria and hence were not able to denature their cell walls [43].

In a recent study conducted by Odumosu et al. [42], it was reported that some species of *Streptomyces* sp. showed promising antibacterial activity against some food and human pathogens such as *S. aureus*, *E. coli*, *Klebsiella pneumonia*, and *Salmonella typhi*. The same authors also chemically analyzed the secondary metabolites produced by the studied species using GC-MS and verified their antibiotic properties may be used in novel antimicrobials. The antifungal activity of *Streptomyces* strains may also be due to their ability to produce some bioactive secondary metabolites such as isoikarugamycin, a novel polycyclic tetramic acid macrolactam produced by *Streptomyces zhaozhouensis* active against *C. albicans*, as reported by Lacret et al. [44].
