Essential oils (EOs) are a new class of crop protectant. Naturally occurring biologically active compound from plants are generally regarded as safe (Jobling 2000). EOs are a complex mix of secondary metabolites (terpenes, phenolic compounds, alcohol) and possess biological activities such as antibacterial, antifungal, antiviral, insecticidal and antioxidant property (Burt 2004; Bassole and Juliani 2012; Falleh 2020). Thymus and oregano EOs contain phenolic components, thymol and carvacrol exhibiting high antimicrobial activity (Lambert et al., 2001; Sokovic et al., 2009; Bassole and Juliani 2012). The phenolic compound carvacrol, thymol and isoeugenol show antifungal activity. Origanum acutidens EO containing carvacrol and thymol completely inhibited mycelial growth of 17 phytopathogenic fungi and their antifungal effect was higher than the commercial fungicide benomyl (Kordali et al., 2008).
The major compound of the EOs from Baccharis dracunculifolia was spathulenol and trans-nerolidol both are oxygenated sesquiterpene. The EOs showed bacteriostatic and bactericidal activities as well as fungistatic and fungicidal activities (Cazella et al., 2019). The carbonylation of some monoterpene increase bacteriostatic and fungistatic activities (Naigre et al., 1996). The study confirms the possibility of using Pinus patula in controlling plant diseases. Pinus patula EOs showed antibacterial and antifungal activity (Claude et al., 2014). The mono- and sesquiterpenes are the major component of EOs isolated from Hypericum hyssopifolium and Hypericum heterophyllum exhibiting antifungal activity (Cakir et al., 2003).
EOs of rosemary and lavender were inhibitory at high concentration. Spore germination and germ tube growth elongation was inhibited by EO tested. EOs cause morphological degeneration of fugal hyphae such as cytoplasmic coagulation, vacuolations, hyphal shrivelling and protoplast leakage and loss of conidiation (Soylu et al., 2010). EO from Foeniculum vulgare was found to be more effective than the EO of Syzygium aromoticum against grey mold disease of apple (Peighami-Ashnaei et al., 2009). Rice false smut caused by Villosiclava virens is a devastating disease. Antifungal test showed cinnamon bark oil and cinnamon oil had stronger fumigation and contact effects on V. virens. The hyphae surface morphological changes by cinnamon bark oil, cinnamon oil and trans-cinnamaldehyde included shrivelling, vacuolation and exfoliation (Zheng et al., 2019).
Essential oil decrease the activity of cell wall degrading enzymes produced by the fungal phytopathogens causing disease (Khaledi et al., 2014). EOs of Mentha piperita, Bunium persicum and Thymus vulgaris had fungicidal and fungistatic activity against Rhizoctonia solani and Macrophomina phaseolina (Khaledi et al., 2014). The antifungal activity of phenol is based on the interaction with mitochondrial membrane (Gallucci et al., 2014). The hydrophobicity of EOs enables them to partition, in the lipids of cell membrane and mitochondria, making them permeable leading to leakage of cell contents (Burt 2004). There are several factor that influence the chemical composition of oils such as (Regnault-Roger and Philogene 2008):
- Genetics: The amount of allelochemicals varies between species. For example 1,8-cineol is the main compound of Eucalyptus globulus EO, whereas, linalol is abundant in Coriandrum sativum EOs and within the same species. The example is thyme (Thymus vulgaris L.) which exhibits many chemotype according to main compound (thymol, carvacrol, terpineol etc.).
- Physiology: The secondary metabolism of the plant is not identical at all steps of its development. The proportion of menthone and menthol in Mentha piperata plant vary with photoperiod and vegetative cycle.
- Pedological and climatic parameters involve acidity of the ground, heat, hygroscopy etc. which directly influence the secondary metabolism of the plant. Example the amount of EOs in Citrus species is higher with warm temperature.
- Analytical: The nature of the compounds of EOs is linked with processes of extraction.
The compounds of EOs such as carvacrol, carvone, cinnamaldehyde, citral, p-cymene, eugenol, lomonene, menthol and thymol pose no health risk (Cazella et al., 2019). Foliar application of bio-stimulant on Origanum majorana plant enhanced the toxicity of O. majorana EOs against rice seed-borne fungi (Mohamed et al., 2020), suggesting use of EOs as a biofungicide against seed-borne fungi, especially before sowing.
Plant EOs can control plant pathogens. EOs show a wide spectrum of activity and the EO-based pesticides is available as green pesticides (El-Mohamedy 2017). EOs from red thyme, summer savory, cinnamon and clove were phytotoxic and caused electrolyte leakage resulting in cell death (Tworkoski 2009). EOs extracted from plant, may be useful as natural product herbicides or as biopesticide. EOs exhibiting potential as a natural herbicide, the optimum dose of application must be identified to control weed and simultaneously to see that there is no negative impact on soil microorganisms (Jouini et al., 2020). Fungal disease alter physiology of plant, disrupting their normal functioning. Therefore EOs are used to control plant pathogen (Arraiza et al., 2017).
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