The suppressive effect could be more likely related to a combination of biotic and abiotic parameters in soils [13]. Chemical and physical attributes of soil, including pH, organic matter, and clay content, can impact soil microbial activity [14]. The degree of suppressiveness is related to physical soil conditions, fertility level, biodiversity, water supply, and populations of soil organisms and soil management [15]. Soil texture and structure affect holding capacity, nutrient status, gas exchange, and root growth related to the percentage of sand, silt, and clay particles in the soil [13]. A significant negative correlation between the sand content of soil and its suppressiveness to Fusarium wilts of flax and Armillaria root disease on lodgepole pine was found [16]. Poor soil aeration was associated with cavity spot (Pythium spp.) disease in carrot. The pea root rot complex (Fusarium spp.) is also affected by soil compaction [17].

Soil organisms are concentrated in the top 10 cm of soil; the minimum tillage use maintains the biota near the surface instead of diluting them through a greater depth [14]. Rotations that include a break crop significantly reduce root disease in cereals, for example [13]. Stubble retention has an essential effect on the level of organic material (carbon). Carbon-based material represents an energy source for soil biota, helps in fast multiplication of the soil population, and reduces moisture evaporation that also benefits some antagonist organisms [17]. The general suppression in soils is not related to a specific group of microorganisms; many factors and populations cooperate to inhibit the pathogen or the disease's development. The disease suppression might be related to soil microorganisms specific functions; many authors have found consistent correlations among specific functional parameters and disease suppression. One of the best examples is the production of antibiotic compounds [13].

Bacteria from the plant environment can produce different molecules with antagonist activity against fungal and oomycetes plant pathogens. Many of these beneficial microorganisms can be selected with a relatively high effectivity and multiplied on artificial media [18] to study its compounds. However, in vitro cultures' main disadvantage is that metabolites' production depends on medium nutrient concentration and composition, generally higher in culture media than in natural habitats, conditions that could not correctly predict the real antagonism effect [19,20]. Nonetheless, there has been an increasing interest in discovering the metabolites of biocontrol agents for plant disease control. According to Colavolpe et al. (b) [21], the use of active compounds can overcome the challenge associated with the use of living microorganisms for the biocontrol purpose.

Bacteria from the Bacillus genus inhabit many different habitats [22] and offer many compounds displaying a broad range of antagonist functions [23]. This enormous versatility increases the industrial and environmental interest, especially in B. subtilis strains [24]. The volatile compounds include benzene compounds, pentadecane, tetradecane, and some ketones, which are of particular interest due to their long-distance action [25,26]. Volatile fatty acids and their derivatives are the most important produced by microbes, and in the case of B. subtilis, represent up to 87% of known antimicrobial produced by the bacteria [27].

According to Méndez-Bravo et al. [26], a bacteria isolate (A8a) closely related to B. acidiceler can inhibit the oomycete Phytophthora cinnamomi growth in vitro by 76% through the production of volatile compounds identified as 6,10-dimethyl-5,9-undecadien-2-one, 2,3,5-trimethylpyrazine, and 3-amino-1,3-oxazolidin-2- one. The total ion chromatogram of isolate A8a displayed components identified to belong to the chemical categories of ketones, aldehydes, alkyls, sulfoxides, pyrazines, and alcohols [25]. The strain G341 of B. velezensis produced antifungal volatiles and inhibited the mycelial growth of S. sclerotiorum, R. solani, and B. cinerea. Volatile profiles indicated that strain G341 produces three volatile compounds: dimethylsulfoxide, 1-butanol, and 3-hydroxy2-butanone (acetoin) [23].

Some of the more diverse plant-associated bacteria are β-proteobacteria belonging to the genus Burkholderia [28]. The volatile profile of B. tropica strain MTo431 was studied by Tenorio-Salgado et al. [29], and different compounds were identified, including sulfur metabolites, dimethyldisulfide, toluene, and terpenoid compounds, such as α-pinene, limonene, and ocimene. Several of these volatile compounds have been previously detected in cultures of many strains of Pseudomonas spp and Burkholderia cepacia [29]. Many soil-derived Pseudomonas strains are potentially useful biocontrol agents of oomycetes. These include P. putida, P. fluorescens, Pseudomonas sp. SH-C52 and P. chlororaphis [30]. Dimethyl disulfide and other sulfur-containing compounds emitted by Pseudomonas species were recently shown to stop the growth of Phytophthora infestans. Other volatile compounds, such as 1-Undecene [31], can also reduce sporangia formation and zoospores' release in P. infestans. The volatile compound 2-phenyl ethanol inhibited the mycelial growth of Penicillium digitatum, P. italicum, and B. cinerea. Additionally, the 2,3,5-trimethylpyrazine compound has been identified as an abundant component in several volatile profiles from fungistatic soils against Rhizoctonia and Fusarium phytopathogens [26].

Important bioactive molecules from the genus Bacillus are non-ribosomally synthesized peptides [9], metabolites with very complex biosynthesis mechanisms catalyzed by non-ribosomal peptide synthetases, large enzyme modular structure complexes, each module being in charge of incorporating particular amino acid [22]. B. subtilis and B. amyloliquefaciens can produce different lipopeptides in which antifungal activity varies according to the targeted phytopathogen [9]. The representative lipopeptides can be classified into three families based on their amino acid configuration: surfactin, iturins (mycosubtilin, iturin A, and bacillomycin), and fengycin. Both iturins and fengycin have antifungal activities [32]; meanwhile, the surfactin family has antiviral activity. Some antifungal activity presents synergistic actions when applied in combination with iturin A or fengycin [9]. B. amyloliquefaciens synthesizes iturin and fengycin and can inhibit the growth of Alternaria panax, Botrytis cinerea, Colletotrichum orbiculare, Penicillium digitatum, Fusarium oxysporum, F. solani, Verticillium dahlia, Phytophthora parasitica [21], Ralstonia solanacearum, and Rhizoctonia solani [20]. Most important molecules from this group, circular lipopeptides from surfactin, iturin, and fengycin families (Asari et al., 2017), affect the target cells of the pathogens on the membrane level [22].

Another antibiotic lipopeptide, called viscosinamide, is produced by Pseudomonas fluorescens. It has been shown to increase membrane permeability in plant cells facilitating pectolytic attack and has been further reported to act as a biosurfactant. Pythium ultimum and R. solani resulted in inhibited by specific Pseudomonas in vitro antagonism [20]. Antibiotics isolated from strains of P. cepacia include cepacin A, cepacin B, altericidins, 2-[2-heptenyl]-3- methyl-4-quinolinol, pyrrolnitrin [3-chloro-4- (2'-nitro-3'- chlorophenyl)-pyrrole], and 2-[2- nonenyl)-3-methyl-4- quinolinol [33].

The analysis of fungal pathogens co-cultivated with antagonistic bacteria producing the mentioned compounds shows changes in hyphal morphology, including hyphal swelling, distortion, large amounts of balloon-shaped cells, and cytoplasm protoplasm aggregation. Degradation of fungal cell walls, cell breakage, leakage of intracellular substances, alterations in hyphal morphology, lysis of fungal hyphae, vacuolization, granulation in mycelium structures, and ruptured of mycelia were also detected changes [29]. These effects on pathogen cells result from direct defense due to the production of antibiotic compounds by bacteria. This potent inhibition leads to thinking that specific bacteria antagonistic compounds are key for supporting future biotechnological research in controlling plant disease.