Agriculture in the 21st century faces multiple challenges: emerging resistant pests, drought, climate change, etc. In addition, United Nations had been reported that the world’s population is expected to increase by 2 billion in the next 30 years, from presently 7.7 billion to 9.7 billion in 2050. To feed this huge growing population, more activities have been directed toward the sustainable agriculture practices. Due to increased demand for foods to feed the ever-growing population led to development and adoption of synthetic chemicals as a quick and effective strategy of managing crop pests and diseases [1].

Some scientists claimed that the status of food security and food safety are alarming in Bangladesh due to crop losses by pests, emerging newly and resistant crop pathogens, and use of excessive synthetic pesticides [2] [3] [4].

Increasing rate of emerging fungal diseases in crop plants is a serious threat to food and nutritional security of increasing population in the world [5] [6]. One of the striking examples is the new emergence and re-emergence of blast disease which is caused by distinct pathotypes of a filamentous fungus Magnaporthe oryzae [7] [8]. Presently, wheat blast disease has emerged as a threat for global wheat production [9].

Bangladesh is an agro-based country. Livelihood of 80% population of Bangladesh is directly or indirectly dependent on agriculture. In last couple of decades, Bangladesh has progressed remarkably in food production and developing high yield seeds of vegetables and crops which leading Bangladesh to be self-sufficient of food. On the contrary, Bangladesh has not yet developed environmental friendly chemicals to control diseases of vegetables and crops caused by fungi. As a result, farmers are forced to heavily rely on heavy metal containing synthetic fungicides to control diseases of vegetables and crops. These heavy metals (such as copper, cadmium, tin, etc) containing fungicides are imposing enormous threat to human health (cancer, neurological defects, asthma, allergies, birth defects, etc) and soil fertility. Excessive use of non-biodegradable pesticides may lead to the destruction of biodiversity. Many birds, aquatic and soil organisms and animals are under threat of harmful, synthetic and persistent (non-biodegradable) pesticides for their survival which challenges food security [10].

Wheat is the second staple food in Bangladesh after rice. Lately, wheat blast, a new devastating fungal disease caused by Magnaporthe oryzae Tritichum (MoT) has been observed in eight southwestern districts, viz., Meherpur, Chuadanga and so on. In February 2016, Bangladesh was reported as the first Asian country havingan outbreak of worrisome wheat blast disease caused by a South American lineage of a hemibiotrophic filamentous fungus MoTpathotype [1] [11] [12].

Wheat blast disease is wide spread in Bangladesh at this moment. The epidemic spread to an estimated 15,000 hectares , about 16% of the cultivated wheat area in Bangladesh, with yield losses reaching up to 100% which threatening food security. The existing synthetic fungicides are inactive against notorious wheat blast fungus to be physiologically different from true fungi. Moreover, the currently available synthetic fungicides are persistent chemicals, whose toxicity represents enormous threats to the ecosystem, human health, and the environment as well. Therefore, it has become inevitable to search for safer and environmentally friendly fungicides from natural sources including medicinal plants.

Most of the agricultural research conducted in the 20^th century focused on increasing crop productivity but not developing environmental friendly natural pesticides for sustainable food production. The synthetic pesticides were immediately accepted after development due to their effectiveness and efficacy in managing serious crop diseases [13]. Consequently, the use of natural products of plant origin slowly faded until recently when use of synthetic pesticides started threatening human health and environmental safety [14]. Currently, organic foods produced by using plant-based pesticides are gaining popularity due to detection of hazardous residues of synthetic pesticides in foods and increased awareness among consumer on food safety [15] [16].

Natural products which are safe for the environment and have low toxicity to living organisms are gaining interest as important sources for the development of fungicides, and these may serve as effective substitutes for synthetic fungicides [17] [18]. Therefore, to increase food security and food safety and sustainable food production and to control wheat blast disease, we need urgently natural, environment friendly pesticides. The main focus of this study was to evaluate in vitro efficacy of 12 medicinal plant extracts (methanol extracts) against wheat blast caused by notorious fungal pathogen, Magnaportheoryzae Triticum (MoT) pathotype.

In vitro antifungal activity of 12 medicinal plants was evaluated against notorious wheat blast causing pathogen, MoT. The zones of inhibition (mm) exhibited by plant extracts are listed in Table 2. Among the studied plant extracts, the remarkable inhibitory activity {(29.6 ± 01.5) mm, (25.1 ± 01.0) mm and (20.0 ± 02.0) mm zone of inhibition} showed (5 mg/disk) obtained from Artemisia indica (Nagdona), Persicariaorientalis (Biskatali) and Clerodendrumindicum (Bamonhati), respectively. Other extracts (1 and 8) showed zone of inhibition ranged between 8-12 mm (Table 2 and Figure 1). It is noted that extracts 2, 3, 4, 5, 7, 10 and 12 did not show any activity against tested pathogen. The same extracts were tested for their activity against another pathogen Penicilliumsp. (common endophytic fungus of cereal plants like wheat and soybean). Unfortunately, all extracts did not show any activity against tested pathogen (Figure 2) at applied concentrations (5 mg/disc), whereas the standards showed activity (zone of inhibition 20.0 ± 01.0 mm) against the said pathogen.

Presently, wheat production in Bangladesh is under threat due to outbreak of devastating blast disease caused by MoT [21]. Although some synthetic fungicides are being used for controlling wheat blast but heavily and frequent use of these fungicides imposing enormous threat to human and plant health as well as beneficial microorganisms, and ultimately leading to develop resistance by the pathogens.

Application of natural pesticides is one of the sustainable approaches to manage wheat blast especially originated from plant sources. Plant extracts have been well-known for their medicinal and antimicrobial properties since the history of

“-” indicate not active in tested concentration. Mean value ± standard deviation of 3 replications were used in Table.

mankind [22]. They offer a greater scope for sustainable agriculture than synthetic chemicals as they are relatively safe, easily biodegradable and eco-friendly. Natural pesticides from plants are cheap, readily available and cost-effective for poor farmers in developing countries where synthetic fungicides are scarce and expensive [23].

Fungi are the main casual agents of plant diseases which cause a considerable loss of crop yield across the world. Some scientists had been reported that in anin vitro experiment, garlic clove extract (1:10 dilution) completely inhibited mycelia growth (up to 93.33%) of MoTand minimized disease incidence and severity with promotion of yield parameters [24]. The plant extracts obtained from processed coffee (C. arabica) exhibited 81.12% and 89.40% inhibitory effect at concentrations of 10% and 25% (w/v), respectively against Pyriculariagrisea-the causal agent of rice blast disease with no phytotoxic to rice seedlings in applied concentration [25].

The methanol extract obtained from stems of a tree, Catalpa ovata, exhibited potent in vivo antifungal activity against the most severe rice blast disease causal agent, Magnaporthegrisea [26]. In an in vivo experiment with methanol extracts of invasive plants, Amorphafruticosa and Phytolaccaamericana, showed potent antifungal activity at 3000 ppm against severe crop fungal diseases viz., rice blast, tomato gray mold, pepper anthracnose and tomato late blight [27]. Methanol extract of Lawsoniainermis leaves exhibited potent antifungal activity against major plant pathogenic fungi [28]. Pythiumdebaryanum is a fungal species causes diseases many wild and cultivated plants against which methanol extracts of Lawsoniainermis, Mimosa pudica and Phyllanthusniruri revealed marked activity (25 mm, 20 mm and 20 mm inhibition zone, respectively) at a concentration of 40 mg/disk [29].

In the present study, when zone of inhibition against MoT is compared in same concentration (5 mg/disk) among the most active extract obtained from Artemisia indica (Nagdona), Persicariaorientalis (Bishkatali) and Clerodendrumindicum (Bamonhati), respectively and standard (iprodione), it has been shown that iprodione produced less zone of inhibition (15 ± 01.0 mm) than extract of these three plants {(29.6 ± 01.5) mm, (25.1 ± 01.0) mm and (20.0 ± 02.0) mm zone of inhibition, respectively)}. Note that in case of iprodione, it is a pure compound whereas extract of these plants may contain many compounds out of which 2 - 3 compounds of each may be active. Accordingly, the pure form of these 2 - 3 compounds of plant extract may be more active than standard, iprodione. It may conclude that these plants extract will be a hopeful source for developing natural fungicides against wheat blast pathogen like MoT.

There are several approaches in developing plant-derived natural pesticides in a cost-effective way: 1) Traditional extraction of bioactive secondary metabolites by solvents (e.g., water, methanol etc) under conditions from the field-grown plants that produce the highest levels of the compounds, determination of the dose of the bioactive extracts against target pests through lab and field level in vivo activity test; 2) Bioassay guided isolation and characterization of the plant-derived active secondary metabolites and subsequently their production through synthesis from inexpensive precursors or through fermentation by gene transfer to microorganisms and fixing dose through lab and field level experiments; 3) Enhancing biosynthesis of the target plant-derived bioactive compounds through inexpensively synthesized metabolic precursors; 4) Using plant growth regulators, elicitors, and metabolic blockers with a view to increase production of plant-derived target pesticides and so on. Therefore, active compounds of Artemisia indica (Nagdona), Persicariaorientalis (Bishkatali) and Clerodendrumindicum (Bamonhati) against wheat blast may be subjected to developing eco-friendly and cost-effective natural fungicides for sustainable agricultural practices following any one of the above appropriate approach.

The current findings suggest that extract obtained from leaves and stems of Artemisia indica (Nagdona), Persicariaorientalis (Bishkatali) and Clerodendrumindicum (Bamonhati) possess significant antifungal properties {(29.6 ± 01.5) mm, (25.1 ± 01.0) mm and (20.0 ± 02.0) mm zone of inhibition, respectively)} against MoT(Magnaporthe Oryzae Triticum) pathotype. The methanol extracts obtained from these medicinal plants are affordable, safe and eco-friendly. However, the further research is required to identify and characterize the active antifungal compounds in the extract and their role in wheat blast disease control in order to develop natural and eco-friendly fungicides.

I express my sincere gratitude to the authority of Bangladesh Open University for funding this research. I would like to thank Dr. Md. Israil Hossain, Director General, Bangladesh Wheat and Maize Research Institute (BWMRI), Nashipur, Dinajpur for providing MoT(Magnaporthe Oryzae Triticum) pathotype. I also extend my deep gratitude to Bangladesh Rice Research Institute (BRRI), Gazipur, Bangladesh for providing Penicilliumsp. pathotype.

The author declares no conflicts of interest regarding the publication of this paper.

Shamim, A.H.M. (2021) Medicinal Plants, A promising Source of Natural Fungicides against Magnaporthe oryzae Triticum, Causal Agent of Wheat Blast. American Journal of Plant Sciences, 12, 748-758. https://doi.org/10.4236/ajps.2021.125051