Essential oils (EO), also known as volatiles, ethereal oils, or essences, are biochemical chemicals produced by several different species, most probably plants (Vasconcelos et al., 2019). These oils are commonly obtained using classical approaches, including distillation, pressurizing, or solvent-based extraction (Ali, Chua, & Chow, 2019). Because of their nature and characteristics, EOs are extensively used in scents, foodstuffs, skincare, and pharmacology (Szutt, Dołhańczuk-Śródka, & Sporek, 2019). The biochemical makeup of various plant oils varies depending on wherever they grow and particularly on the climatic conditions (Arraiza, 2017). Nevertheless, the essential oil composition may change with the methods employed for drying plant materials, extraction, and distilling process (Merah et al., 2020). These oils are valued for their pleasant fragrance and bioactive components and characteristics (Liao et al., 2017). Essential oils are typically extracted via conventional hydro-distillation (Elyemni et al., 2019), which decreases oil quality and exhibits minimal extraction efficiency when certain volatile compounds can be lost during the extraction and the deprivation of unsaturated compositions temperatures fluctuations and nature of solvents used (Chen et al., 2020). Microwave-assisted extraction, a unique and inventive methodology, has received considerable research and is suitable for small materials and simple and effective control overheated mechanisms (Vinatoru, Mason, & Calinescu, 2017). The primary advantages of microwave-assisted extracting are affordable, time-saving, eco-friendly, and solvent-free extraction (Y. Wang et al., 2018).
Taro Colocasia esculenta L. is a well-known tuber crop that is extensively grown in many regions of the world, especially China, Africa, and European countries, consumed as a staple food (Ahmed et al., 2020). Taro corms and leaves are rich in carbohydrates, proteins, minerals, and vitamins (Sharma, Jan, Kaur, & Riar, 2020), and essential elements, including calcium, phosphorus, iron, vitamin C, thiamine, riboflavin, and niacin (Otieno, 2020). Furthermore, taro has medicinal potentials against several pathogenic diseases and physiological disorders in humans. Taro is vulnerable to over 23 various diseases that negatively impact plant development and production. Taro leaf blight is the most significant damaging of these diseases, resulting in reductions in good crop production (M. E. Bi, Teke, Christopher, Annih, & Charles, 2020). This disease is very common and has resulted in significant production losses for the crop in different cultivation areas of China (Z. Wang et al., 2021). Phytophthora colocasiae predominantly infects foliar parts of plants, but in severe cases can also appear on petioles and damage the corms of the hosts. The disease develops around 2 to 4 days following infection, during warmer humid climates, and is particularly noticeable on the top portion of plant leaves.
The initial symptoms appear as small, light, or dark brown specks, mainly at the lamina and margins of leaves where water accrues. The patches expand rapidly, fetching spherical, zonate, and purple-brown colors. A watery substance oozes out underneath the spots and appears as a dry gray and solid bubbles like plant exudate. (Jeeva, Veena, Makeshkumar, & Arutselvan, 2020). In favorable conditions, the disease becomes severe, the lesions become dark brown with yellow edges, and an ash-colored ring of sporangia at the borders of the lesions becomes apparent. Lesions turn papery and drop off under dry and hot environments, causing shot-holes appearance in leaves. However, grown taro leaves become destroyed in few days, dangling like flags and possibly falling on the ground whenever petioles get damaged. The disease causes 50-60% losses, leading to significant corms deterioration immediately or after harvesting (Takor, Monono, Ntane, Ngale, & Fontem, 2020).
Phytophthora colocasiae is an oomycete fungus and has a complicated lifecycle depending on growth habits, including hemibiotrophic, biotrophic, and necrotrophic (Alexandra, Jamora, Smale, & Ghanem, 2020). The survival temperature for pathogen ranges between 15-35 ºC. The optimum temperature from 25-30 ℃ when the night temperature is around 20 ℃, and the moisture content exceeds 90%, encourages zoospore to germinate rapidly, triggering disease outbreaks. P. colocasiae is a heterothallic species that produce asexual sporangia, sporangiophores, zoospores, and sexually oospores that become vested in plant debris and corms inoculum for upcoming cropping season. The pathogen overwinters by forming chlamydospores through conversion of thick-walled zoospores, hibernating for persistent viability in the plant debris and corms (Brooks, 2015). Such process enables P. colocasieae, as a dangerous pathogen, while continual farming harbors pathogen in field biomass continuously persist in the soil.
The disease can be cured effectively with systemic fungicides, but on the other hand, it creates an immediate risk to human and environmental health as well as long-term consequences on biodiversity and the ecosystem. People are increasingly opting for chemical-free products, particularly those produced in organic agriculture. Thus, researchers focus on bio-pesticides and phytochemicals as eco-friendly, non-hazardous, long-lasting, and effective alternatives to manage plant diseases.
Essential oils or combinations of different phytochemicals exhibit synergistic potential with additional properties against many plant pathogens. This advocates a cost-effective and natural alternative to both agro-food production and consumers, at the same while to the obstacles in innovations to inhibit proliferation of plant pathogens. Sage (salvia sclarea) and tea tree (Melaleuca alternifolia) essential oils contain high antibacterial activities. They are being used as a natural cure and preventive for many plant diseases (Chidi, Bouhoudan, & Khaddor, 2020; Ebani et al., 2018). As far as our knowledge is concerned, there is no prior information for using essential oil from sage and tea tree against taro leaf blight. The present study was designed to explore and develop an easy, compact and effective system for extracting essential oils for convenient and consistent control of plant diseases for sustainable food production.