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Assessment of the fungicidal effect of silicon on mycelial growth of Fusarium spp. and Cochliobolus sativus Abstract Soluble silicon (Si) helps field crops, whether directly or indirectly, counteract fungal diseases, but in vitro inhibition of Fusarium head blight (FHB) species and Cochliobolus sativus causing spot blotch (SB) and common root rot (CRR) has not been reported. The effect of Si on the mycelial growth of 16 FHB isolates of four species (F. culmorum, F. verticillioides, F. solani and F. equiseti) and 54 C. 

sativus cultures (32 SB isolates + 22 CRR isolates) was investigated in four experiments. The growth of 70 isolates was not significantly inhibited (P>0.05) on Si amended potato dextrose agar (PDA) plates in the first experiment. However, when the pH of Si amended PDA plates was raised to 8.79, 9.57, 10.19 and 10.59, there were differences (P<0.05) in fungal growth of 16 FHB and 54 C. sativus isolates compared to Si amended PDA plates (pH was equivalent to 6.56) in the second experiment. NaOH amended PDA medium was tested for 6 fungal isolates of the five analyzed species in the third experiment and it was found to have similar results to NaOH amended Si-PDA. 

More importantly, differences in growth rates were observed for the 6 isolates in the fourth experiment when discs were transferred from NaOH amended Si-PDA to PDA without NaOH and soluble silicon, indicating that NaOH amended Si-PDA was fungicidal rather than fungistatic. Our results provide conclusive evidence that inhibition of fungal growth is primarily due to the alkaline pH effect of NaOH amended to Si PDA plates. No fungicidal effects of Si were observed, suggesting that reduction of FHB, SB and CRR diseases can not be achieved through direct silicon application. 

Thus, our data highlight that other physical, biochemical, and/or molecular defense mechanisms could be behind the possible decreases in severity of FHB, SB and CRR diseases in cereal plants treated with low concentrations of Si. To our best knowledge, this is the first report investigating the fungicidal effect of Si on mycelial growth of causative agents of FHB, SB and CRR diseases in vitro. Key words: cereal phytopathogenic species, colony diameter, fungal inhibition, media pH, mycelial growth. 

Introduction Globally, small-grain cereals can be heavily damaged by pathogenic deuteromycetes Fusarium fungi causing Fusarium head blight (FHB) and ascomycetous Cochliobolus sativus species responsible for spot blotch (SB) and common root rot (CRR), account for most of the major devastating diseases of cereal cultivation (Nutter et al., 1985; Parry et al., 1995; Mathre et al., 2003). FHB results from the development of a complex of at least 17 species of Fusarium genus, mainly by F. graminearum and F. culmorum. In addition, other causal agents are less frequently encountered species such as F. poae, F. cerealis and F. equiseti, and, to a lesser extent, F. solani, F. oxysporum and F. verticillioides (Bottalico and Perrone, 2002). 

Outbreaks of soil-borne FHB species occurring in seasons with frequent rainfall and high humidity can reduce yield and contaminate wheat and barley grains with dangerous mycotoxins (McMullen et al., 2012). C. sativus, a common foliar pathogen of barley, is an economically important agent because it can cause marked reduction in yield and quality of the crop (Mathre, 1997). Also, C. sativus acting as a soil-borne fungus is responsible for large economic losses in dry-land barley farming (Fernandez et al., 2009). 

Effective management of FHB, SB and CRR cannot be achieved through the use of single control strategy because each has its own limitations. Employing different control strategies including cultural, biological, chemical and host resistance is a powerful tool for FHB, SB and CRR management (Gyawali, 2010; Aada, 2013; Dahl <https://www.sciencedirect.com/science/article/pii/S0308521X17309666> and Wilson <https://www.sciencedirect.com/science/article/pii/S0308521X17309666>, 2018; Sakr, 2020a). 

In fact, the application of bioactive silicon (Si) has shown potential to enhance crop production, alleviation of metal toxicity and nutrient imbalance, provision of structural rigidity and resistance to fungal diseases (Ma and Yamaji, 2006; Sakr, 2016). To date, the underlying mechanisms that govern fungal disease protection by silicon treatments are not yet fully understood (Debona et al., 2017; Sakr, 2018). However, silicon can assist plants in alleviating disease severity via prohibiting pathogen penetration (1) via structural strengthening, (2) by preventing pathogen infection and establishment through stimulating systemic acquired resistance, (3) throughout antifungal component production, as well as (4) through enhancing plant resistance by stimulating multiple signaling pathways and defense-related gene expression (Debona et al., 2017; Sakr, 2018). Concerning FHB disease, Yobo et al., 

(2019) found that granulated and foliar potassium silicate had a potential capacity to reduce FHB as compared to the pathogen-inoculated control treatment in wheat plants. However, no study has been undertaken to characterize the effect of soluble silicon on reducing the incidence of SB and CRR diseases on barley. The fungicidal effect's hypothesis suggests that silicon might operate as the first protecting barrier in plants treated with silicon, and may prevent pathogen colonization and consequential infection by suppressing fungal growth on the plant surface. 

Generally, there is a significant body of literature highlighting that reduction of fungal disease severity via foliar silicon treatments is the result of a direct effect on the pathogen rater than one alleviated by the plant (Reviewed in Sakr, 2016; Debona et al., 2017). Till now, some in vitro studies have been investigated this hypothesis in several phytopathogens; however, the contradictory results showed the presence or absence of antifungal activity following silicon treatment (Bi et al., 2006; Bekker et al., 2009; Li et al., 2009; Liu et al., 2010; Shen et al., 2010; Fayadh and Aledani, 2011; Khan et al., 2013; Ge et al., 2017). 

To our best knowledge, there are no reports about the fungicidal effect of silicon on mycelial growth of causative agents of FHB, SB and CRR diseases. With a view to better characterizing the role of silicon in FHB, SB and CRR control, experiments were performed to 1) determine the direct effect of soluble silicon on the growth rate of 70 fungal isolates of five cereal phytopathogenic species (F. culmorum, F. verticillioides, F. solani, F. equiseti and C. sativus) in vitro and (2) to explore the possible antifungal activity of soluble silicon. Materials and Methods Fungal isolates Sixteen fungal isolates belonging to four FHB species, i.e., F. 

culmorum, F. solani, F. verticillioides (synonym F. moniliforme) and F. equiseti, were obtained from heads displaying observable disease symptoms collected during the 2015 growing season in several localities of Ghab Plain, one of the principal Syrian wheat production areas. Fifty five C. sativus isolates were obtained in 2015 from both naturally infected barley leaves exhibiting spot blotch (SB) symptoms (32 isolates) and barley sub-crown internodes showing common root rot (CRR) symptoms (22 isolates) in different regions of Syria. 

Isolates were grown separately in 9 cm diam. Petri dishes containing potato-dextrose agar (PDA, HiMedia, HiMedia Laboratories) for 10 days at 22 ± 1 ?C in an incubator (JSPC, JS Research Inc.) in the dark to allow mycelial growth. Recently, the 16 FHB fungal isolates were molecularly analyzed by random amplified polymorphic DNA (Sakr, unpublished data). The 70 monosporic isolates were maintained in sterile distilled water at 4°C and frozen at -16°C until needed (Sakr 2019, 2020b). Culture media preparation Si amended PDA To determine the effect of soluble silicon on the 70 fungal isolates, Si-PDA plates containing 0.00, 1.67, 3.33, 5.00, or 6.67 mM soluble silicon (SiO2, Kieselsaure, Carl Roth GmbH + Co. 

KG) were prepared by adding sterile SiO2 powder to warm (50 to 60°C) PDA and then pouring the mixture into Petri dishes. The mean pH value of Si-PDA plates containing all tested above SiO2 concentrations was equivalent to 6.56. Sodium hydroxide amended Si-PDA Amendment with soluble silicon did not increase the pH of the PDA medium. To determine whether the increase of pH affect the growth of the fungal isolates, the pH of the soluble silicon amended medium containing 1.67, 3.33, 5.00, or 6.67 mM soluble silicon was adjusted to 8.79, 9.57, 10.19 and 10.59 with 4 M NaOH (Avonchem, SK 116PJ) (the pH of 4 M NaOH was equivalent to 14) before sterilization. The pH of the NaOH amended Si-PDA was equivalent to the pH (8.79, 9.57, 10.19 and 10.59, respectively) of the PDA medium amended with four potassium silicate concentrations (1.67, 3.33, 5.00 and 6.67 mM (K2SiO3), respectively) as described by Shen et al., (2010). 

NaOH amended Si-PDA were prepared by adding sterile SiO2 powder and sterile sodium hydroxide powder to PDA and then pouring the mixture into Petri dishes. The mean pH value of PDA control plates was equivalent to 6.56. Sodium hydroxide amended PDA medium To determine whether NaOH amended PDA affected the growth of the fungal six isolates (F1 (F. culmorum), F7 (F. solani), F15 (F. verticillioides), F43 (F. equiseti), C.S. 14, a SB isolate and C.S. 41, a CRR isolate (C. sativus)) of the five tested species, PDA plates containing 1.67, 3.33, 5.00, or 6.67 mM NaOH were prepared by adding sterile sodium hydroxide solution to PDA and then pouring the mixture into Petri dishes. 

The pH of the NaOH amended PDA was equivalent to the pH of NaOH amended Si-PDA (8.79, 9.57, 10.19 and 10.59, respectively). The mean pH value of PDA control plates was equivalent to 6.56. For the three experiments, the pH of each solution was measured with a pH meter (Hanna Ins., HI 8521). The pH of the amended PDA medium could not be measured directly because it was solid; however, the measurements of the soluble silicon, NaOH amended Si-PDA and NaOH-PDA amended potato dextrose broth (PDB) were conducted before the addition of agar. 

In vitro assays Fungicidal activity was measured as the growth rate method adapted by Li et al., (2009). A 7 mm disk cut was cut from the margin of actively growing 10 day old culture plate of each fungal isolate and placed at the centre of each treatment or control plate. The plates were then randomly placed in an incubator at 22 ± 1 ?C under continuous darkness. Colony diameter was obtained by measuring two perpendicular axes at 24 h intervals until 72 h. 

Each treatment was replicated five times and each experiment was repeated twice. Determining whether NaOH amended Si-PDA is fungicidal or fungistatic To determine whether NaOH amended Si-PDA is fungicidal or fungistatic, growth rates were observed for six isolates (F1, F7, F15, F43, C.S. 14 and C.S. 41) of the five tested species when discs were transferred from 72 h-old-NaOH amended Si-PDA to PDA without NaOH and soluble silicon. The plates with disks were then randomly placed in an incubator at 22 ± 1 ?C in the dark until 72 h. 

Each treatment was replicated five times and each experiment was repeated twice. Statistical analyses All statistical analyses were performed with DSAASTAT add-in version 2011. Data were analyzed by one-way analysis of variance (ANOVA). For each isolate, treatment means within an incubation time were compared using the Fisher's LSD test. Differences at P<0.05 were considered to be significant. Results Effect of soluble silicon on colony growth in vitro The mean growth of 70 fungal isolates of five species (F. culmorum, F. verticillioides, F. solani, F. equiseti and C. 

sativus) causing FHB, SB and CRR diseases was not inhibited on silicon amended plates (Table 1 and Figure 1). Colony diameters of all species were not significantly (P>0.05) smaller than the control at all soluble silicon concentrations. These results imply that amendment of soluble silicon at all tested concentrations did not raise the pH of medium (equivalent to 6.56). Thus, the pH value of 6.56 was a crucial factor in no inhibiting the colony growth of all tested isolates. Table 1: Mean colony diameters of 5 isolates of F. culmorum, 6 isolates of F. solani, 4 isolates of F. verticillioides, one isolate of F. equiseti, 32 isolates for Cochliobolus sativus causing spot blotch and 22 isolates for C. 

sativus causing common root rot grown on PDA medium amended with soluble silicon. The pH of the culture medium was not adjusted Colony diameter (mm) Incubation time (h) Fungal species S4 S3 S2 S1y Control 7.7±0.4 a 7.6±0.3 a 7.9±0.5 a 7.3±0.8 a 7.5±0.9az 24 F. culmorum 21.0±4.5a 21.9±4.5a 21.5±4.0a 21.0±4.6a 20.3±3.7a 48 39.6±10.4a 40.4±11.0a 39.6±10.0a 39.1±10.4a 39.5±10.4a 72 9.6±1.2a 9.5±1.3a 9.7±1.3a 9.4±1.4a 9.8±1.7a 24 F. solani 33.6±8.4a 33.8±8.3a 33.5±8.2a 33.7±7.3a 33.0±7.6a 48 56.7±8.7a 56.2±8.9a 55.9±8.8a 56.7±8.8a 56.3±9.0a 72 9.9±0.9a 9.7±1.1a 9.9±1.0a 9.7±1.2a 9.3±1.2a 24 F. verticillioides 29.4±7.4a 29.6±7.0a 29.5±7.6a 29.1±6.6a 28.9±6.8a 48 54.4±13.0a 53.9±13.0a 53.2±12.5a 53.8±13.6a 53.5±13.0a 72 7.0±0.7a 6.4±1.1a 6.8±0.8a 6.8±0.8a 7.0±1.2a 24 F. equiesti 29.4±1.3a 29.8±0.8a 30.2±1.1a 30.6±1.1a 30.4±0.9a 48 48.4±0.9a 48.6±0.5a 49.2±1.1a 48.4±0.5a 48.4±1.1a 72 12.0±4.6a 12.1±4.8a 12.1±4.7a 12.1±4.6a 12.0±4.7a 24 Cochliobolus sativus causing spot blotch 32.3±9.8a 32.3±9.9a 32.3±9.9a 32.6±9.9a 32.2±9.6a 48 50.1±11.7a 50.2±11.9a 50.3±11.9a 50.2±11.7a 50.2±11.9a 72 10.6±4.4a 10.5±4.4a 10.6±4.1a 10.5±4.3a 10.4±4.3a 24 C. sativus causing common rot root 27.6±6.7a 27.9±6.8a 27.5±6.7a 27.8±6.5a 27.5±6.7a 48 48.0±11.5a 48.2±11.7a 48.1±11.5a 48.4±11.8a 48.2±11.7a 72 y S1, S2, S3 and S4 represent PDA plates amended with 1.67, 3.33, 5.00 and 6.67 mM soluble silicon, respectively. z Data are the means ± standard deviation (SD). Means ± SD within a row followed by the same letter are not significantly (P<0.05) different based on Fisher's LSD test. Figure 1: Growth of fungal colonies for C.S. 50 (common root rot) on PDA medium amended with 0.00, 1.67, 3.33, 5.00 and 6.67 mM soluble silicon at 72 hours after infection. 

Effect of NaOH amended Si-PDA on colony growth in vitro NaOH amended Si-PDA markedly inhibited (P<0.05) mean mycelial growth for 70 fungal isolates of F. culmorum, F. verticillioides, F. solani, F. equiseti and C. sativus, with greater inhibitory effects at higher concentrations (Table 2). Colony diameters of all species were significantly smaller than the control (pH was equivalent to 6.56) measured on medium at pH values of 9.57, 10.19 and 10.59. It was possible that it was the higher pH of the amended medium which inhibited colony growth of all fungal isolates caused FHB, SB and CRR diseases. Table 2: Mean colony diameters of 5 isolates of F. culmorum, 6 isolates of F. solani, 4 isolates of F. 

verticillioides, one isolate of F. equiseti, 32 isolates for Cochliobolus sativus causing spot blotch and 22 isolates for C. sativus causing common root rot grown on PDA medium amended with soluble silicon. The pH of the soluble silicon amended medium was adjusted with NaOH that it was equivalent to the pH (8.79, 9.57, 10.19 and 10.59, respectively) of the PDA medium amended with four potassium silicate concentrations (1.67, 3.33, 5.00 and 6.67 mM (K2SiO3), respectively) as described by Shen et al., (2010) Colony diameter (mm) Incubation time (h) Fungal species S4 S3 S2 S1y Control 2.7±1.0d 3.8±0.7cd 4.5±0.7bc 5.2±1.1b 7.5±1.2az 24 F. culmorum 13.3±2.8c 14.2±2.2bc 16.4±2.1bc 18.0±3.5ab 20.4±3.8a 48 20.6±6.1b 23.3±6.9b 26.5±8.2b 31.9±11.5ab 39.7±10.4a 72 4.8±1.2c 5.3±1.6c 6.2±0.8bc 7.3±1.5b 9.4±1.0a 24 F. solani 20.5±7.2b 23.0±7.2b 25.4±7.4ab 27.7±7.8ab 33.7±7.4a 48 30.7±6.6d 33.1±6.7cd 40.9±7.9bc 47.0±7.7b 56.3±7.9a 72 5.6±0.7c 5.9±0.3c 6.7±0.6bc 7.8±1.1b 9.4±1.7a 24 F. verticillioides 16.6±5.1c 17.9±3.6bc 21.2±7.6abc 26.1±5.5ab 29.5±6.7a 48 33.0±9.6b 33.8±11.9b 37.1±10.5ab 47.5±14.4ab 54.1±13.0a 72 3.8±0.4c 4.6±0.9bc 5.2±1.3b 4.8±0.4bc 7.8±1.1a 24 F. equiesti 15.5±0.7d 18.6±0.5c 20.1±0.7b 20.4±0.8b 31.8±1.1a 48 28.5±0.5e 30.4±0.3d 32.8±0.7c 44.0±0.5b 49.2±2.2a 72 5.6±2.4d 6.8±3.0cd 8.3±3.4c 10.1±4.1b 12.2±4.6a 24 Cochliobolus sativus causing spot blotch 17.0±6.2c 20.1±6.7c 24.2±7.7b 28.0±8.5b 32.4±9.6a 48 26.9±8.4e 32.4±8.5d 38.4±9.6c 44.3±10.5b 50.4±11.9a 72 4.5±1.5d 5.9±2.8cd 7.3±2.7bc 8.7±3.5ab 10.5±4.3a 24 C. sativus causing common rot root 12.7±4.7d 16.8±5.7c 21.1±5.9b 23.9±6.8ab 27.4±6.7a 48 23.6±7.4d 30.3±8.9c 37.1±9.2b 41.8±9.7b 48.1±11.3a 72 y S1, S2, S3 and S4 represent PDA plates amended with 1.67, 3.33, 5.00 and 6.67 mM soluble silicon, respectively. The pH of the soluble silicon amended medium was adjusted with NaOH that it was equivalent to the pH (8.79, 9.57, 10.19 and 10.59, respectively). 

The mean pH value of PDA control plates was equivalent to 6.56. z Data are the means ± standard deviation (SD). Means ± SD within a row followed by the same letter are not significantly (P<0.05) different based on Fisher's LSD test. Effect of NaOH amended PDA on colony growth in vitro NaOH amended PDA significantly suppressed (P<0.05) mycelial growth for 6 fungal isolates of the five tested species, with greater inhibitory effects at higher concentrations (Table 3). Colony diameters of all isolates were significantly smaller than the control (pH was equivalent to 6.56) measured on medium at pH values of 9.57, 10.19 and 10.59. 

Table 3: Colony diameters of 4 isolates of four Fusarium head blight species and 2 isolates of Cochliobolus sativus causing spot blotch and common root rot grown on PDA medium amended with sodium hydroxide Colony diameter (mm) Incubation time (h) Isolates (identification) S4 S3 S2 S1y Control 2.2±0.4c 2.8±1.1bc 4.8±1.9b 4.0±1.9b 8.2±2.5az 24 F1 (F. culmorum) 13.0±1.6d 14.8±2.0cd 16.6±1.7bc 18.8±3.6b 23.6±1.5a 48 31.6±3.5d 31.8±2.1d 39.4±2.6c 49.5±3.8b 54.2±2.9a 72 3.0±0.7c 3.0±0.4c 4.2±0.8b 5.4±1.5b 8.0± 0.7a 24 F7 (F. solani) 16.4±1.1c 17.6±2.3c 17.2±1.5c 21.6±2.2b 24.6±2.1a 48 20.0±1.6e 25.6±1.8d 31.8±4.2c 38.4±3.3b 46.0±1.6a 72 5.8±1.1c 6.2±1.1c 6.8±1.1b 7.8±0.8a 8.8±1.3a 24 F15 (F. verticillioides) 10.7±1.0d 13.6±1.8c 14.2±1.6c 21.3±1.3b 24.2±1.3a 48 22.3±1.5d 22.3±1.2d 27.7±1.1c 30.5±0.7b 41.4±1.5a 72 3.6±0.5c 5.0±1.7bc 5.0±1.0b 5.2±0.4bc 8.2±1.5a 24 F43 (F. equiesti) 15.7±1.0d 19.0±1.2c 20.5±1.1b 20.6±1.0b 32.2±1.5a 48 28.6±0.6e 30.0±0.6d 33.0±0.6c 43.6±0.6b 48.8±1.6a 72 3.6±1.1c 4.6±0.5bc 6.4±2.5bc 6.6±3.0b 9.6±1.9a 24 C.S. 14 (spot blotch) 18.0±3.2c 19.2±1.1c 20.0±1.9c 22.2±1.8b 29.0±2.4a 48 33.2±2.3c 33.8±2.4c 41.6±1.6b 47.9±1.2a 49.4±3.8a 72 4.2±0.9c 7.8±1.1b 8.0±0.7b 9.2±1.5a 10.8±1.6a 24 C.S. 41 (common root rot ) 15.8±2.2d 18.6±1.8c 25.4±3.6b 27.0±3.0ab 29.6±1.7a 48 30.6±1.9e 35.2±1.1d 41.8±2.7c 48.2±0.8b 49.8±1.3a 72 y S1, S2, S3 and S4 represent PDA plates amended with 1.67, 3.33, 5.00 and 6.67 mM NaOH, respectively. 

The pH of the NaOH amended PDA was equivalent to the pH of NaOH amended Si-PDA (8.79, 9.57, 10.19 and 10.59, respectively). The mean pH value of PDA control plates was equivalent to 6.56. z Data are the mean of five replicates ± standard deviation (SD). Means ± SD within a row followed by the same letter are not significantly (P<0.05) different based on Fisher's LSD test. Determining whether NaOH amended Si-PDA is fungicidal or fungistatic Significant differences in growth rates (P<0.05) were observed for the six isolates of the five tested species when they were transferred from 72 h-old-NaOH amended Si-PDA to PDA without NaOH and soluble silicon (Table 4). 

Table 4: Comparisons among reductions in colony diameters measured after 72 h of 4 isolates of four Fusarium head blight species and 2 isolates of Cochliobolus sativus causing spot blotch and common root rot grown on PDA medium when they were transferred from 72 h-old-NaOH amended Si-PDA Isolates (identification) Colony diameter (mm) S1x PDAy S2 PDA S3 PDA S4 PDA F1 (F. culmorum) 49.3±3.9az 39.0±0.7b 39.3±2.7a 31.4±0.9b 31.8±2.1a 24.0±1.0b 30.5±3.7a 22.8±1.8b F7 (F. solani) 38.4±3.3a 32.3±0.9b 31.6±1.3a 23.8±1.3b 26.0±1.4a 20.4±0.5b 20.0±1.5a 14.1±2.7b F15 (F. verticillioides) 30.5±0.7a 25.6±1.7b 26.9±1.3a 20.8±1.3b 21.5±1.0a 15.4±0.9b 22.3±1.2a 15.0±1.0b F43 (F. equiesti) 43.6±0.6a 34.6±1.1b 33.0±0.6a 22.4±0.9b 30.3±0.3a 22.0±1.0b 27.8±1.6a 16.4±1.4b C.S. 14 (spot blotch) 48.7±1.9a 40.2±0.4b 41.0±1.5a 34.8±1.1b 33.4±2.1a 26.8±1.9b 32.9±2.7a 23.4±1.1b C.S. 41 (common root rot ) 48.6±0.5a 37.2±1.5b 41.6±2.3a 35.2±1.9b 35.0±1.2a 28.0±1.0b 29.4±0.9a 21.8±1.1b x Colony diameter of fungal isolates measured after 72 h on PDA plates amended with 1.67 (S1), 3.33 (S2), 5.00 (S3) and 6.67 (S4) mM soluble silicon, respectively. 

The pH of the soluble silicon amended medium was adjusted with NaOH that it was equivalent to the pH (8.79, 9.57, 10.19 and 10.59, respectively). y Colony diameter of fungal isolates measured after 72 h when they were transferred from 72 h-old-NaOH amended Si-PDA on PDA plates without NaOH and soluble silicon. z Data are the mean of five replicates ± standard deviation (SD). Means ± SD within a row followed by the same letter are not significantly (P<0.05) different based on Fisher's LSD test. Discussion Important fungal diseases of wheat and barley, i.e., 

FHB, powdery mildew, spot blotch, blast, leaf blast, have been efficiently controlled following soluble silicon supplying (Reviewed in Sakr (2016) and Debona et al., (2017)), Yobo et al., 2019) because these small grain cereals are classified as high silicon absorbers and accumulators (Ma and Yamaji, 2006). An in vitro screening procedure which provides rapid and in situ repeatable results is an important step in screening efficient control measures of FHB, SB and CRR species. However, such in vitro procedures are not applicable yet on the growth of F. 

culmorum, F. verticillioides, F. solani, F. equiseti and C. sativus. Thus, the findings in this study are the primary data in detecting the potential antifungal effect of soluble silicon in a large set of fungal cultures recovered from several Syrian cereal growing area and exhibiting several morphological, pathogenic and genetic characteristics (Sakr, 2020a; unpublished data). 

In the current research, we do not utilize potassium silicate or sodium silicate as the source of silicate commonly used in studies investigating the effect of silicon on the growth of fungi in vitro (Bi et al., 2006; Bekker et al., 2009; Li et al., 2009; Liu et al., 2010; Shen et al., 2010; Fayadh and Aledani, 2011; Khan et al., 2013; Ge et al., 2017). However, the soluble silicon concentrations and the values of pH in NaOH amended Si-PDA applied here are similar to those of potassium silicate reported by Shen et al., 

(2010) The compound in our experiment can be comparable to the sodium or potassium silicate media in terms of containing soluble silicon and sodium and its similar adjusted alkalinity. Therefore, we can compare our results generated in media containing NaOH amended Si-PDA with those obtained with sodium silicate amended PDA. NaOH amended Si-PDA had an inhibitory effect on the growth of all tested FHB, SB and CRR isolates of species F. culmorum, F. verticillioides, F. solani, F. equiseti and C. sativus. These results agree with those reported by Bi et al., 

(2006) for Alternaria alternata, F. semitectum and Trichothecium roseum on sodium silicate amended medium, Bekker et al., (2009) for eleven phytopathogenic fungi (F. solani, F. oxysporum, Colletotrichum coccodes, Mucor pusillus, Drechslera spp, Phytophthora cinnamomi, Pythium F-group, Alternaria solani, Verticillium theobromae, Curvularia lunata, Stemphylium herbarum and Sclerotinia sclerotiorum) on medium amended with potassium silicate, Li et al., (2009) for F. sulphureum on sodium silicate amended medium, Liu et al., (2009) for Penicillium digitatum on sodium silicate amended medium, Shen et al., (2010) for F. oxysporum f. sp. fragariae, F. 

oxysporum, Pestalotiopsis clavispora and Rhizoctonia solani on PDA medium amended with potassium silicate, Fayadh and Aledani (2011) for R. solani on sodium silicate amended PDA medium, Khan et al., (2013) for Macrophomina phaseolina on sodium silicate amended medium, and Ge et al., (2017) for Trichothecium roseum on medium amended with sodium silicate. These published findings and our data indicate the sensitivity of several species within deuteromycetes Fusarium genus to in vitro potassium silicate, sodium silicate or NaOH amended Si-PDA application. 

Also, this report highlighted, for the first time, the inhibition of colony growth of ascomycetous C. sativus species causing SB and CRR diseases. Furthermore, it is worthwhile to note that potassium silicate, sodium silicate or NaOH amended Si-PDA inhibit mycelial growth not only for soil-borne fungi but also for air-borne pathogens. More importantly, differences in growth rates were observed for some isolates of the five tested species when discs were transferred from NaOH amended Si-PDA to PDA without NaOH and soluble silicon. 

This indicated that NaOH amended Si-PDA was fungicidal rather than fungistatic against F. culmorum, F. verticillioides, F. solani, F. equiseti and C. sativus. Although the suppressive effect that NaOH amended Si-PDA had on fungal growth is clear (the pH values were equivalent to 8.79, 9.57, 10.19 and 10.59, respectively), the growth of 70 fungal isolates of five species was not inhibited when the PDA culture medium was amended only with soluble silicon concentrations similar to sodium silicate concentrations (the mean pH value was equivalent to 6.56). 

Thus, results from this study indicated that the inhibition of fungal growth is primarily due to the alkaline pH effect of NaOH to amended Si PDA plates. During the current investigation, NaOH amended PDA medium was tested for some fungal isolates of the five analyzed species and it was found to have similar results to NaOH amended Si-PDA. Our data agree with Shen et al., (2010) position; they argue that silicon does not seem to directly affect pathogens, being no fungicidal. The inhibition of colony growth of four pathogens on PDA medium amended with potassium silicate was due to a pH effect (Shen et al., 2010). 

In fact, the most optimal mycelial growth for most fungi was obtained at pH 5 to 6.5; however, alkaline conditions can inhibit fungal growth (Ingold 1973). In contrast to our findings and those obtained by Shen et al., (2010), Bekker et al., (2009) and Fayadh and Aledani (2011) noted that the suppressive effect of potassium silicate and sodium silicate, respectively, on fungal growth in vitro was primarily due to an antifungal activity effect. The differences in these data may be attributable to that the potassium silicate and sodium silicate concentrations used by Bekker et al., 

(2009) and Fayadh and Aledani (2011) were 50 to 60 times and 30 to 74 times, respectively, higher than that in our experiment and the research conduced by Shen et al., (2010). It should be indicated that the concentrations tested by Bekker et al., (2009) and Fayadh and Aledani (2011) are inapplicable for field application due to the high pH of the resulting potassium silicate/sodium silicate solutions could cause phytotoxicity. In fact, potassium silicate/sodium silicate concentrations commonly do not exceed 1.67 mM in field studies investigating the effeteness of silicon for decreasing plant disease incidence (Laane, 2018; Zhou et al., 2018). 

It is worthwhile to note that the range of NaOH amended Si-PDA concentrations tested in our study is advisable for field treatment. However, there is still a need to further analyses need to be done to assess the effect of higher silicate concentrations on plant development. Based on the information generated in this study, we conclude that although sodium silicate acts as a promising effective controlling measure against fungal diseases, our results revealed that the NaOH amended Si-PDA at low concentrations (1.67 to 6.67 mM) did exhibit antifungal activity against the causative agents of FHB, SB and CRR diseases because of the alkaline pH effect of NaOH amended to Si PDA plates. 

No fungicidal effects of soluble silicon were observed, suggesting that reduction of FHB, SB and CRR disease severity can not be achieved through direct silicon application. Thus, our data highlight that other physical, biochemical, and/or molecular defense mechanisms could be behind the possible decreases in FHB, SB and CRR disease severity in cereal plants treated with low concentrations of silicon. In our experimental station, studies aimed to investigate the role of silicon to reduce FHB (F. culmorum, F. verticillioides, F. solani and F. 

equiseti) incidence and severity on wheat and barley plants are in progress under controlled and field conditions, and encouraged results were obtained. Further research should be conducted to better understand the bioactive role of silicon on biological functions involving cell turgor pressure of hyphae and spores, mycelium symmetry and asymmetry, hyphal swelling, curling, cupped shape, reproductive growth including asexual and sexual spore germination, and the cellular architecture alterations in FHB, SB and CRR fungi. 

In future, the use of silicon along with fertilizers and other biological control agents to reduce the severity of these important fungal diseases should be investigated. Acknowledgements The author would like to thank the Atomic Energy Commission of Syria for providing assistance for this research. Also, the author gratefully thanks Dr. I. Mubarak for helping in stats and Eng. F. Al-Ain for helping in measuring the pH values of media. References Aada, A. M. M. A. 2013. Identification of pathogens and control of spot blotch disease of barley (Hordeum vulgare) by combining plant resistance and biological control. 

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