Foliar Application of Bio-Stimulants Enhancing the Production and the Toxicity of Origanum majorana Essential Oils Against Four Rice Seed-Borne Fungi

In the present study, the enhancement of the production of Origanum majorana essential oils (EOs) was studied by treating plants with ascorbic acid (AA) and tryptophan (Trp) at concentrations of 100, 200 and 300 mg/L and Moringa oleifera leaf extract (MLE) at 2.5%, 5% and 10% as foliar applications during the seasons 2018–2019. The toxicities of the EOs were assayed against four seed-borne fungi (Bipolaris orzyae, Curvularia lunata, Fusarium verticilliodies and F. graminearum) isolated from rice grains (Oryzae sativa). Vegetative growth parameters and EO production were enhanced by the application of AA, Trp and MLE in both seasons. Analysis of the EOs by Gas chromatography–mass spectrometry (GC-MS) showed that the main chemical constituents were terpineol (cis-β-(1-terpinenol)), terpinen-4-ol, 4-thujanol (sabinene hydrate), α-terpineol, cymene and sabinene. The highest fungal mycelial growth inhibition (FMGI) percentages against F. verticilliodies were 94.57% and 92.63% as MLE at 5% and 10%, respectively, was applied to plants and 85.60% and 82.19% against F. graminearum as Trp was applied to plants at 300 and 200 mg/L, respectively. EOs from the treated plant with MLE (10%) observed the highest FMGI (84.46%) against B. oryzae, and EOs from plants treated with AA as foliar application at 300 and 200 mg/L showed the highest FMGI values of 81.11% and 81.85%, respectively, against the growth of C. lunata. Application of EOs extracted from plants treated with Trp, AA and MLE at 300 mg/L, 300 mg/L and 10%, respectively, or untreated plants to rice seeds inhibited or decreased the fungal infection percentage from 82.5% (naturally infected grains) to 1.75%, 10.5%, 17.5% and 18.5%, respectively. In conclusion, the extracted EOs affected by the foliar application of O. majorana plants with Trp, AA, and MLE could be useful as a biofungicide against rice seed-borne fungi.