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    Plant Extracts for Postharvest Protection

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    Submitted by: Kwanele Nxumalo


    Various medicinal plant parts have different phytochemicals and antioxidants that can be used in crop protection and preservation. Extracts from plants such as Ruta chalepensis, Eucalyptus globulus, etc., have proven to be effective in controlling postharvest pathogens of horticultural crops and increased their shelf life when used as a substitute for synthetic chemicals. Furthermore, extracts from neem and other medicinal plants contain a predominant and insecticidal active ingredient. The application of medicinal plant extracts could be a useful alternative to synthetic chemicals in the postharvest protection and preservation of horticultural crops.

    1. Introduction

    Medicinal plants have been the basis of the treatment of various diseases in African traditional medicine and other forms of treatment from diverse cultures of the world [1][2][3]. Despite the well-documented ethnobotanical literature, very little scientific information is available on the efficacy and phytochemistry of indigenous medicinal plants and plant extracts in postharvest protection and preservation of horticultural crops [4][5]. In contrast with countries such as China and India, the use of medicinal plants and plant extracts in Africa is greatly underdeveloped for crop protection and preservation [2][3]. Countries such as China, India, Japan, Brazil, Mexico, South Africa, Kenya, Morocco, Tunisia and Egypt are major international role players in the production and export of fresh produce globally [6]. However, these horticultural crops are highly perishable and do experience various physiological and biochemical changes which lead to the development of undesirable physiological disorders and quality degradation, leading to major economic losses [7][8][9]. Fungal infections are the major causes of postharvest losses of fresh fruits and vegetables either in transit or storage [10]. They cause significant economic losses in the commercialization phase and are rendered unfit for human consumption [7][8][9][10].
    About one-third of the food produced in the world per year for human consumption is lost or wasted [11]. In Africa, postharvest losses of fruit and vegetables could be as high as 70%, while the global quantitative food losses and wastes during the year are around 40–50% for fruits and vegetables only [12]. Every year, consumers in developed countries lose almost as much food (over 220 million tons) as the total net food production in sub-Saharan Africa (around 230 million tons) [13]. Not only are losses a waste of food, but they also represent a similar waste of human effort, farm inputs, livelihoods, investments and scarce resources such as water [13][14]. Some of the major causes of these postharvest losses are physical damage, poor handling, transportation and storage, poor packaging, postharvest pathogens (Rhizoctonia solani, Alternaria alternata, Colletotrichum gloeosporioid, Penicillium digitatum and Botrytis cineria) and senescence [11][12]. The horticulture industry relies on the use of capital-intensive technologies during the postharvest phase of production and fungicides are also applied to reduce the losses due to postharvest diseases or decay [15][16][17]. However, there is a growing global concern about the use of fungicides. The use of synthetic chemicals is becoming increasingly restricted locally and internationally due to health concerns and consumers’ requests for safe and sustainable natural alternatives. As a result, the commercial success of the horticulture industry is threatened [10][11][18][19].
    Crop protection and preservation are central entities in global food sustainability and security [20][21]. Several methods of preservation have successfully prohibited food waste caused by insect infestation, environmental conditions and microbial attacks [20][21]. However, studies have revealed vast health issues relating to applications of synthetic pesticides and preservatives in crop protection and preservation [22]. This has prompted the exploration of safe and cost-effective constituents without harmful or detrimental effects to the health of consumers and the environment at large [23]. Natural preservatives and pesticides have been formulated and applied in food, pharmaceutical and agrochemical industries [23][24][25][26].
    A wide range of phytochemicals such as alkaloids, cyanogenic glycosides, phenylpropanoids, polyketides, anthocyanins, carbohydrates, amino acids, lipids, nucleic acids, terpenoids, flavonoids, phenols, saponins and tannins found in most medicinal plants are essential materials in the production of several pesticides and fungicides that can be helpful in crop protection and preservation of horticultural crops [2][27][28][29][30]. For example, the antimicrobial and antioxidant properties of the medicinal plant extracts have been attributed but not limited to phytochemicals such as citral, aspilactonol B and 8-methyl-6-prenylquercetin found in Cymbopogon citratus [31], fukugetin and fukugiside found in Geophagus brasiliensis [31] and carnosic acid, carnosol and rosmanol found in Lepidium meyenii [32]. Therefore, there is a need to undertake different phytochemical analysis (active ingredients, nutritional and mineral content), biological activities (e.g., antimicrobial, anti-inflammatory and antioxidant) and safety evaluation (cytotoxicity and genotoxicity) of medicinal plants as a substitute for synthetic pesticides and fungicides to be used in protection and preservation of horticultural crops [4][33][34].
    Regular monitoring of the pest population dynamics in agroecosystems can reveal the economic losses and importance of a particular pest which can be mitigated by medicinal plant extracts [35][36]. Despite the relatively low rates of expansion of botanically based pesticides, regulatory changes in many parts of the world are driving a renaissance for the development of new natural pest control products that are safer for human health and the environment [37][38]. Therefore, botanical pesticides can help provide new ideas for the development of new pest management products [39][40]. Hundreds of indigenous and exotic species with insecticidal properties have been reported around the world through various farmer surveys and subsequent research, many of which have been confirmed to be active against a wide range of arthropod pests [39][40]. On-farm use of insecticidal plants, particularly among resource-poor smallholder farmers, is widespread and familiar to many African and Asian farmers [38][39][40].
    By 2015, more than 400,000 plant species had been identified, a majority of which are flowering plants (369,000), and each year nearly 2000 others are discovered [41][42]. These plants produce a needed wide range of primary and secondary metabolites that have antibacterial and antifungal properties [43][44][45]. Several medicinal plant extracts such as neem (Azadirachta indica) leaf extract [46], turmeric (Curcuma longa) leaves [47] and lemongrass extracts (Cymbopogon citratus) [48] have been successfully applied as fungicides. The activity of neem leaf extract can be attributed to the presence of compounds such as dibutyl phthalate, phytol, nonanoic acid, tritriacontane and 1,2-benzenedicarboxylic acid in the crude extracts [46]. Studies have shown that some plants produced secondary metabolites, such as essential oils and volatile compounds that can have a biocidal action against postharvest pathogens [49][50]. Commercial products of natural fungicides such as rosemary oil, neem oil, Aloe vera gel (AVG), tea tree oil and jojoba oil, among many others, are now used in crop protection and preservation as fungicides [51][52][53], while commercial natural insecticides include nicotine and pyrethrum, amongst others [54][55]. Medicinal plant extracts could be a useful alternative to synthetic fungicides in the control of rot fungi when handling fruits and vegetables after harvest.

    2. The Renaissance of Medicinal Plants as Antimicrobial Agents in Postharvest Preservation

    The possibility to control many postharvest pathogens using medicinal plants has been investigated on a wide range of horticultural crops [56][57]. In modern agriculture, the application of synthetic fungicides remains the most effective and common method to control postharvest rot of horticultural crops [57][58]. However, increasing requests by consumers for fresh produce that is free of fungicide residues has contributed to the interest of researchers in the development of alternative methods for controlling postharvest decay of fresh produce [59][60]. Increasing health hazards such as the development of cancer, infertility and effects in the offspring of pregnant women caused by the application of postharvest fungicides have led to their restriction in some commodities or total ban in organic agriculture [61][62]. In the last 10 years, the Pesticide Action Network International has banned the use of many highly hazardous pesticides for use in agriculture (Table 1).
    Table 1. Pesticide Action Network (PAN) International selected list of highly hazardous pesticides (2021) for use in agriculture in the last 10 years [62].
    Chemical Application Classification of Pesticides Year of Ban
    1,2-dibromoethane It is used as a soil fumigant to control nematodes and other soil pests in crops such as vegetables, ornamentals, pineapples and tobacco Classified as a probable carcinogen by the US EPA 2010
    Ethylene dibromide It is used as a fumigant to protect against insects, pests and nematodes in citrus, vegetable and grain crops Classified as a probable carcinogen by the US EPA 2010
    Hydrogen cyanide It is used in the treatment of citrus and other fruits for the control of scale insect and thrips, in quarantine treatments of bananas, pineapple and other commodities for the control of aphids, mealybugs and other exposed insects. It is also used in a vacuum treatment for bulbs, rhizomes, tubers, asparagus roots and strawberry plants to control certain mites and nematodes Classified as “fatal if inhaled” (H330) according to the EU GHS. 2010
    Lindane An insecticide used to control a broad spectrum of insects in fruits and vegetables Classified in several categories, and in 2018, IARC classified it as “Carcinogenic to humans” 2010
    Metaflumizone It is used to control the diamondback moth on Brassica leafy vegetables Is very persistent in the water-sediment environment and the bio-concentration factor is over 5000. It is classified as P = Persistent and B = Bio-accumulative 2010
    Noviflumuron Prevents the successful molting and development of subterranean termites and eventually eliminates the colony that can cause damage to fruit tree plantations Classified as a probable carcinogen by US EPA Annual Cancer Report and classified as WHO Class 1a 2010
    Vinclozolin It is used to control blights, rots and molds in vineyards and on raspberries, lettuce, kiwi, snap beans and onions. It is also used to protect crops against Botrytis cinerea and Sclerotinia sclerotiorum Classified as a reproductive toxicant and endocrine disruptor 2010
    Cyproconazole It is used to control powdery mildew in cucurbits, rust on cereals and apple scab Classified as presumed human reproductive toxicant according to EU GHS 2011
    Spirodiclofen It is used as an acaricide and insecticide on citrus, grapes, pome fruit, stone fruit and tree nut crops Classified as a probable carcinogen by the US EPA and is now also classified as “Carc 1B” by the EU GHS 2011
    Ethiofencarb It is used as an insecticide in controlling aphids on hard and soft fruits and some vegetables Classified as WHO Class 1b 2012
    Methomyl It is used as a broad-spectrum insecticide that inhibits cholinesterase activity. It is used in vegetables, fruit crops, cereals and orchard crops for the control of a wide range of insect species Classified as WHO Class 1b 2015
    Diquat It is used for pre-emergence weed control on the potato and also to defoliate seed or root crops for pre-harvest desiccation Classified as a probable carcinogen by the US EPA and is now also classified as “Carc 1B” by the EU GHS 2016
    Flumioxazin It is used as a herbicide for pre- and post-emergence control of susceptible weeds on fruit orchards, vegetables and other field crops Classified as a reproductive toxicant 2016
    Flupyradifurone It is used to prevent sucking insects such as aphids, leafhopper, whitefly and Lygus sp. on citrus, pome and stone fruits, tree nuts, grapes, coffee, cocoa and leafy vegetables Highly toxic to honey bees (oral LD50) and aquatic life 2016
    Malathion It is used to control aphids, red spider mites, mealybugs, thrips, scales and whiteflies on ornamentals, fruits and vegetables Classified as a probable carcinogen by the US EPA and is now also classified as “Carc 1B” by the EU GHS 2016
    Maneb It is ethylene (bis) dithiocarbamate fungicide used in the control of early and late blights on potatoes and tomatoes and many other diseases of fruits, vegetables, field crops and ornamentals Classified as an endocrine disruptor 2016
    Pymetrozine It is used to control aphids, brown planthopper and whiteflies in field vegetables, ornamentals, deciduous fruit and citrus Classified as a probable carcinogen by the US EPA and is now also classified as “Carc 1B” by the EU GHS 2016
    Quizalofop-p-tefuryl Used as a selective post-emergence control of annual and perennial grass weeds in potatoes, soya beans, sugar beet, peanuts, oilseed rape, sunflowers, vegetables, cotton and flax. Classified as an endocrine disruptor (EDC) 2016
    Thiram It is used to control stem gall of coriander, damping-off on allium crops and neck-rot of onion Classified as toxic to aquatic zooplanktons 2016
    Zineb It is used as a broad-spectrum fungicide to control the scab in apples and pears, leaf curl in peaches and anthracnose and early blight in tomatoes Classified as an endocrine disruptor 2016
    Ziram It is used as a broad-spectrum-use fungicide to control scab in apples and pears, leaf curl in peaches and anthracnose and early blight in tomatoes, controlling leaf blight and scab in almonds, shot-hole in apricots, brown rot and leaf spot in cherries, scab and anthracnose in pecans and leaf spot, rust and powdery mildew in ornamentals Toxic to aquatic zooplanktons 2016
    Propiconazole In bananas, it is used to control Mycosphaerella musicola and Mycosphaerella fijiensis var. difformis; in coffee, it is used against Hemileia vastatrix; in stone fruits, it is used against Monilinia spp., Podosphaera spp., Sphaerotheca spp. and Tranzschelia spp.; soft rot on stone fruits Classified as presumed human reproductive toxicant according to EU GHS 2018
    Propineb It is used to control apple scab, leaf and fruit spots on pomegranate, control chili die-back and buckeye rot on tomato Classified as a probable carcinogen by the US EPA Annual Cancer Report 2018
    The most commonly used fungicides in postharvest preservation of horticultural crops are azoxystrobin, fludioxonil, imazalil, pyrimethanil and thiabendazole, which are synthetic compounds with different modes of action that can be applied either in waxes or water [63][64]. However, the overuse of fungicides and pesticides in agriculture is now a public concern because of the harmful potential these substances have in the environment, and the food chain represents a risk for human health [61][62]. Moreover, the overuse of these synthetic fungicides has resulted in the emergence of resistant strains of pathogens and this has become a major global problem because the frequency of mutant phenotypes in the populations is high [65][66]. Some of these fungicides are suspended because of their high toxicity, and there is increased pressure on the food value chain to either remove these agents or to adopt natural alternatives for the maintenance or extension of a product’s shelf life [67]. Such obstacles provide new opportunities for those seeking natural alternatives for new preservatives to be applied on horticultural crops. The control of postharvest diseases in fruits and vegetables using synthetic chemicals is associated with several hazardous effects (Table 2).
    Table 2. Common control of postharvest diseases in fruits and vegetables using synthetic chemicals and their hazardous effect on human health.
    Disease Crop Affected Symptoms Control Reference Hazardous Effect According to PAN [62]
    Anthracnose Apples Black spots appear on skin of the affected fruits which gradually become sunken and coalesce. Before storage, treat with hot water (50–55 °C) for 15 min or dip in benomyl solution (500 ppm) or thiobendazole (1000 ppm) for 5 min. [68] Can affect the reproductive system in males
    Stem end rot Avocado The affected area enlarges to form a circular, black patch around the base of the pedicel. The pulp becomes brown and softer during storage. Prune and destroy infected twigs and spray carbendazim or thiophanate methyl (0.1%) or chlorathalonil (0.2%) on a fortnightly interval during the rainy season. [69] Can cause infertility and destroy the testicles
    Soft-rot Potato Young spots start from the stem end of the fruit as light brown watery rot. As the fruit ripens, area of the rotting increases, and the skin becomes wrinkled. A peculiar musty odour is later emitted. Careful handling of potatoes without causing any wounds and dipping the potatoes in aureofungin-sol at 500 ppm for 20 min to control infection in storage. [70] Highly carcinogenic
    Bitter-rot Apple Faint, light brown discolouration beneath the skin develops. The discolouration expands in a cone shape. The circular, rough lesions become depressed. Pink masses of spores are found arranged in defined rings. Treatment with mancozeb to check the disease in storage. [71] Has detrimental effects on the nervous system and should be used with caution.
    Alternaria rot Stone fruits Alternaria rot is characterized by circular, dry, firm, shallow lesions covered with dark, olive green to black surface mycelial growth. The infected tissue is brown, such as that caused by brown rot. Postharvest sprays with imazalil, azoxystrobin, fludioxonil or mixtures of these may provide control. [72] Can cause developmental effects in the offspring of pregnant women
    Botrytis rot Brinjal The fruits show water-soaked and softened tissue. The water-soaked spots are irregular in shape and are approximately 25 mm in diameter. The fungus that develops on the surface of the fruit shows a dark grey growth. A pre-harvest spray of pyraclostrobin or fludioxonil will give some control. [73] Can cause eye injury and skin irritation
    Rhizopus stolonifer Banana The infection starts as a circular tan area around an island of fruit. The skin will slip off from the flesh if you put slight pressure on it. Next, the fluffy white growth of the fungus becomes visible near the centre and rapidly colonizes the whole area. Use postharvest fungicides such as benomyl, fenbuconazole and fludioxonil. [74] Longer exposure can result in severe liver damage
    Penicillium italicum Citrus Early symptoms include a soft water-soaked area on the peel, followed by the development of a circular colony of white mould. Bluish asexual spores (conidia) form at the centre of the colony, surrounded by a broad band of white mycelium. The fruit rapidly spoils and collapses, with sporulation sometimes occurring internally. Add sodium bicarbonate to either imazalil, thiabendazole, pyrimethanil or fludioxonil for improved performance. [75] Exposure to these chemicals can have negative effects on the respiratory system and they are known to be a carcinogen
    Penicillium digitatum Sacc. Citrus Symptoms include a soft water-soaked area on the peel, followed by the development of a circular colony of white mould, up to 4 cm in diameter. Green asexual spores (conidia) form at the centre of the colony, surrounded by a broad band of white mycelium. Add sodium bicarbonate to either imazalil, thiabendazole, pyrimethanil, or fludioxonil for improved performance. [75] Exposure to these chemicals can have negative effects on the respiratory system and they are known to be a carcinogen
    Brown-rot Stone and pome fruits The infection of the fruit usually occurs as the fruit approaches full ripeness. A rapidly spreading firm brown rot develops, and the fungus produces masses of fawn-coloured spores often in concentric zones. Infected fruit shrivels to a ‘mummy’. Brown rotted fruit in cold storage appear black and there may be no signs of sporulation Spray with fungicides such as Merivon, Luna Sensation and Fontelis. [76] Highly toxic to beneficial insects such as bees
    Sour-rot Citrus Lesions often occur near the stem-end scar, are water-soaked and may have a white scummy growth in the cracks. The odour of these lesions is distinctive and is similar to that produced by lactic acid bacteria The use of guazatine is effective in controlling this disease, while SOPP results in some protection. [75] Can cause skin cancer
    The international community has taken several important initiatives to protect the environment and people’s health from chemicals. These include the Montreal Protocol on the protection of the ozone layer, the Basel Convention on the Transboundary Movement of Hazardous Wastes, the Rotterdam Convention on the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides in International Trade, the Stockholm Convention on Persistent Organic Pollutants and the Strategic Approach to International Chemicals Management [62]. Initiatives are being led by governments and non-governmental organisations (NGOs) for improving pesticide and crop protection policies towards safer, socially just, environmentally sustainable and economically viable pest management systems [61][62].
    Medicinal plants have been used for thousands of years to treat human health disorders and prevent diseases. These medicinal plants have bioactive compounds that can be considered as good alternatives to synthetic antimicrobial and antioxidant preservatives in horticultural crops [77][78][79][80]. Results from several publications in the last two decades show that compounds derived from plants and their antimicrobial and antioxidant capacity tested both in vitro and in vivo gave positive results and are a viable alternative to the use of synthetic chemicals [2][15][81][82].

    3. Medicinal Plant Extracts against Pathogens in Horticultural Crops

    Although there is an array of indigenous floras in tropical, semi-arid and humid regions currently used worldwide for human medical or treatments, only a few of them have been studied for their use in protecting horticultural crops against pathogen infection [83]. Indigenous knowledge has already identified medicinal plant extracts as traditional means to control plant diseases [77][80]. The application of these medicinal plant extracts in controlling postharvest pathogens of horticultural crops has become an important field of study [78]. The family of higher plants and shrubs, particularly, tropical flora, has been shown to provide a potential source of naturally produced inhibitory chemicals [77]. The natural product of medicinal plant extracts such as volatile chemicals, essential oils and phenolic compounds has been applied successfully to control postharvest diseases of stored fruits and vegetables [84][85][86]. Documented medicinal plant extracts for use in indigenous knowledge (IK) or used in the modern day as alternatives for synthetic chemicals for crop protection and preservation are summarised in Table 3 and Table 4.
    Table 3. List of commonly used medicinal plant extracts used as pesticides.
    Plant Name Country of Origin Plant Part Used Focus of the Study Treatment Application Key Findings Reference
    Azadiracta indica India Aqueous leaf extracts To control Pieris brassicae on cabbages Aqueous concentration (10, 5, 2.5 and 1.0%) were sprayed on cabbage foliage. At 5%, had an anti-feedant of 82.5%. The anti-feedant effect of the different concentrations decreased with a decrease in concentration. [87]
    Azadiracta indica India Kernel aqueous extract Control red slug caterpillar on tea plants Tea leaves were sprayed with different neem kernel aqueous extract (NKAE) concentrations at 2, 4, 6 and 8%. The anti-feedant activity was in ascending order with an increase in concentrations. The leaf area consumed was highest at 1158.6 + 254.79 cm2 at 2% concentration in 5th instar, and it was lowest at 8% concentration in the 1st instar larva as 92.2 cm2. The anti-feedant of 87.5% over control was attained in 3rd instar larva at 8% concentration, while it was lowest as 22.74% in the 2nd instar larva at 2% concentration. [37]
    Bobgunnia madagascariensis Senegal Dried pods Aqueous extracts dried pods in controlling ladybird beetle on Brassica napus Aqueous extracts applied separately at 5, 10, 15, 20 and 25% w/v under laboratory conditions. The mortality of H. variegata was recorded 24, 48 and 72 h post-exposure. B. madagascariensis (25% w/v) caused the highest mortality (90%). [88]
    Lippia javanica Botswana The essential oil of leaves To control F. gramenearum in sweet corn The bioassays were carried out at concentrations of 0.87, 0.65, 0.43, 0.22, 0.11, 0.054 and 0.027 mg mL−1 (essential oil mL−1). The maximum antifungal activity was recorded from the concentration of 0.87 g mL-, and the least activity was recorded for the least concentration of 0.027 g mL-. After the 3rd day, the inhibition zone for 0.87 g/mL was larger (25.00 mm) while 0.027 g/mL had the smallest inhibition zone (3.33 mm). After 14 days, 0.87 g/mL had an inhibition zone of 7 mm while 0.027 g/mL had 0 mm. [89]
    Lippia javanica Botswana Leaf powder Control mustard rape aphids and tomato spider mites on tomatoes Plant extracts from leaf powder at 12.5% w/v were mixed with 0.1% v/v soap. The treatments were applied 24 h post mixing the plant materials with water at a rate of 1 L on an area of 5 m2 using a knapsack sprayer fitted with a hollow cone spray nozzle. Plant extracts from leaf powder at 12.5% w/v using 0.1% v/v soap can be used against rape aphids and tomato spider mites. L. javanica at 12.5% reduced aphids by 83% and 75.9% tomato mites. [90]
    Melia azedarach India Aqueous leaf extracts Control Pieris brassicae on cabbages Aqueous concentration (10, 5, 2.5 and 1.0%) was sprayed on cabbage foliage. At 5%, had an anti-feedant of 88.3%. The anti-feedant effect of the different concentrations decreased with a decrease in concentration. [87]
    Melia azedarach India Leaves plant powder To control cucumber pests Crushed fruits of M. azedarach, were tested at the rates of 30 and 60 g kg−1, All the concentrations were effective in controlling 90% of pests than the control [91]
    Solanum incanum Madagascar Fruits were used as a paste To control cabbage aphids Bitter apple extract (BA) was mixed with distilled water to obtain BA fruit concentrations (30, 60 and 90 mL L−1) The treatments were mixed with 3, 6 and 12 g of sugar, respectively. The concentration of 90 mL L−1 had the highest mortality of cabbage aphids, and the cabbages had a good appearance.  
    Solanum incanum Madagascar Aqueous crude fruit sap extract To control green peach aphids (Myzus persicae) on kale Kale was routinely sprayed with 10, 25, 50 and 75% S. incanum extract. The crude extract was effective in controlling the green peach aphids. The order of the insecticidal activity of the four different concentrations was 75 > 50 > 25 > 10%. [92]
    Solanum incanum Madagascar Leaf powder To control mustard rape aphids and tomato spider mites on tomatoes Plant extracts from leaf powder at 12.5% w/v were mixed with 0.1% v/v soap. The treatments were applied 24 h post mixing the plant materials with water at a rate of one liter on an area of 5 m2 using a knapsack sprayer fitted with a hollow cone spray nozzle. Plant extracts from leaf powder at 12.5% w/v using 0.1% v/v soap can be used against rape aphids and tomato spider mites. Solanum delagoense (25%) reduced aphids and mites by 86.5% and 75%, respectively. [90]
    Solanum incanum Madagascar An aqueous crude fruit extract To control ladybird beetle on Brassica napus Concentration extracts at 5, 10, 15, 20 and 25% were applied by spraying Brassica napus plants under greenhouse conditions. Concentration extracts at 25% caused the highest mortality of 80% on collected dead ladybird beetles. [88]
    Tephrosia vogelii Zimbabwe Leaf extracts Control green bean aphids Leaf extracts were made at three different concentrations (0.5%, 2% and 5% w/v). T. vogeii at 5% w/v reduced aphid infestation by 60%, while control reduced pest infestation by 27%. This resulted in 5% T. vogeii having yield of 1100 kg/ha compared to 190 kg/ha of the control. [93]
    Table 4. List of selected medicinal plant extracts used for antimicrobial (antifungal).
    Plant Name Country of Origin Plant Part Used Focus of the Study Treatment Application Key Findings Reference
    Acorus calamus China Root Control banana fruit crown rot Evaluation of plant extracts at various concentrations (1%, 5%, 10%, 25% and 50%) against C. musae was carried out by the ‘poisoned food technique’. The banana fruits were then dipped in plant extracts for 5 min and allowed to air dry for 6 h. Banana hands dipped in chemical benomyl (0.1%) served as positive control while distilled water was used as a negative control. One group was incubated at room temperature (28 ± 2 °C), and another group was held in low-temperature storage (14 °C and 90% RH) conditions. Extracts of A. calamus (50%) significantly reduced crown-rot disease by up to 75% at room temperature (12 d of incubation) and up to 85% at cold storage (35 d of incubation) conditions. [94]
    Allium cepa × Allium sativum Croatia Leaves Control banana fruit crown rot Evaluation of plant extracts at various concentrations (1%, 5%, 10%, 25% and 50% concentration) against C. musae was carried out by the ‘poisoned food technique’. The banana fruits were then dipped in plant extracts (at 25% concentration) for 5 min and allowed to air dry for 6 h. Banana hands dipped in chemical benomyl (0.1%) served as positive control while distilled water was used as a negative control. One group was incubated at room temperature (28 ± 2 °C), and another group was held in low-temperature storage (14 °C and 90% RH) conditions. The dipping of banana fruits in zimmu leaf extract at 25% concentration exhibited 100% inhibition of crown-rot disease in cold storage (14 °C) up to 35 d and increased the shelf life to 64 d. However, at room storage (28 ± 2 °C), the same treatment exhibited 86% inhibition of crown-rot disease up to 12 d. [94]
    Aloe vera Oman Leaves Use of Aloe vera gel solution in controlling nectarine Rhizopus stolonifer, Botrytis cinerea and Penicillium digitatum The fruits were treated by dipping with the corresponded Aloe vera gel solution for 10 min and allowed to dry at room temperature. After 24 h, the fruits were inoculated with R. stolonifer, B. cinerea or P. digitatum by depositing 20 μL the fungi stock (50 spores) inside the artificial injury made (2 × 2 × 2 mm of length, width and depth) on the nectarine cultivars and then stored in room temperature for 6 d. Aloe vera (alone or with the addition of thymol) was effective in reducing fruit decay in the two nectarine cultivars by 50 and 70% depending on nectarine cultivar and fungus species. [95]
    Aloe vera Oman Leaves Aloe vera gel edible coating in delaying rachis browning on grapes The treatments were Aloe vera gel diluted 1:3 with distilled water, and distilled water served as control. The grapes were immersed in 5 min in respective treatments, air-dried before storage at 1 °C and 95% RH in permanent darkness for 35 d. Ten samples for both treated and control clusters were taken after 7, 14, 21, 28 and 35 d; half of them were immediately analyzed (cold storage), and the remainder were transferred to a chamber under controlled conditions at 20 °C and 90% RH and analyzed after 4 d to simulate market operations. Results indicate severe symptoms of dehydration and browning in control rachises (plus SL scores > 3) and low effects for those clusters treated with A. vera gel (plus SL scores < 3) after 28 d of cold storage. After 35 days of storage, grapes treated with Aloe vera got plus SL score < 4, while the control got plus SL score > 5. [96]
    Datura stramonium Mexico/Colombia Leaves Controlling soft-rot on mango fruits Datura stramonium extracts were tested at 10%, 25% and 50% dilutions. Control had higher mean soft-rot severity of 93.4%, while the Datura stramonium extracts at 25% reduced the severity of soft-rot by 41%. [97]
    Galenia africana South Africa Dried leaves Effect of Galenia africana extracts alone and in combination with kresoxim-methyl for controlling B. cinerea on apples The apple cultivar, Granny Smith, was wounded (5 mm in diameter and 3 mm in depth) three times halfway between the calyx and the stem end. A 20 μL drop of each plant extract and kresoxim-methyl was placed in the wounds and allowed to air-dry for 2 h before application of a 20 μL conidial suspension (1 × 104 spores mL−1); the 20 μL drops had final plant extract doses of 0.0, 1.95, 3.91, 7.81, 15.63, 31.25 and 62.5 mg·mL−1, with or without kresoxim-methyl at 0.0 and 0.005 mg mL−1. Kresoxim-methyl (2.5 mg·mL−1) in combination with G. africana extract at doses of 125.0, 250.0 and 500.0 mg·mL−1 showed high inhibition levels (73, 83.8 and 90.8%, respectively) compared to the kresoxim-methyl (72.5%). Inhibition of decay progression by 67.1% for the plant extract only (62.5 mg·mL−1) was achieved compared to 37% of the control. [98]
    Moringa olifera India Leaf extracts Effect of gum arabic (GA) coatings and moringa (M) leaf extract in controlling Colletotrichum gloeosporioides on ‘Maluma’ avocado fruit Fruits were dipped into the treatments: GA 10%, GA 15%, GA 10% + M, GA 15% + M and CMC 1% + M, and the fruits were then stored at 5.5 °C (95% relative humidity (RH)) for 21 d and moved to ambient conditions at 21 ± 1 °C (60% RH) for 7 d to simulate a retail condition. The study demonstrated that GA 15% + M (62.37 N) and CMC 1% + M (59.93 N) retained fruit firmness and lowered weight loss by 3.66% and 6.19%, respectively, and both suppressed mycelial growth of C. gloeosporioides on ‘Maluma’ avocado fruit by 33%. [99]
    Phyllanthus niruri India Leaves Phyllanthus niruri as an edible coating to control postharvest anthracnose in dragon fruits The fruits were inoculated by dipping for 2 min in a spore suspension of C. gloeosporioides (105 spores mL-) with 0.1% (v/v) Tween 80 and air-dried at ambient (25 ± 2 °C). The fruits were then dipped for 2 min in 5.0 g L−1, 10.0 g L−1 and 15.0 g L−1 for Phyllanthus niruri crude extract and left to dry again at room temperature. Fruits dipped in spore suspension (105 spores’ mL-) with 0.1% (v/v) Tween 80 for 2 min served as control. All inoculated treated and untreated fruits were then packed in commercial packaging cartons and stored at 11 ± 2 °C, 80% RH for 28 d. Phyllanthus niruri extracts at 5.0 g L−1 or 10.0 g L−1 significantly controlled anthracnose by 80 and 90%, respectively, after 28 d of cold storage at 11 ± 2 °C and 80% RH. [100]
    Plumbago zeylanica Australia Leaves Control banana fruit crown rot Evaluation of plant extracts at various concentrations (1%, 5%, 10%, 25% and 50% concentration) against C. musae was carried out by the ‘poisoned food technique’. The banana fruits were then dipped in plant extracts (at 25% concentration) for 5 min and allowed to air dry for 6 h. Banana hands dipped in chemical benomyl (0.1%) served as positive control while distilled water was used as a negative control. One group was incubated at room temperature (28 ± 2 °C), and another group was held in low-temperature storage (14 °C and 90% RH) conditions. Extracts of P. zeylanica (25%) recorded a significant reduction of crown-rot disease up to 75% at room temperature (12 d of incubation) and up to 85% at cold storage (35 d of incubation) conditions. [94]
    Ruta chalepensis Egypt Leaves Controlling soft-rot on mango fruits Ruta chalepensis extracts were tested at 10%, 25% and 50%. Higher mean soft-rot severity was recorded on the untreated control 4.67 (93.4% fruit area affected); while the greatest reduction in the severity of soft-rot 1.33 (26%) was recorded in the extract of Ruta chalepensis at 50% concentration. [97]
    Thymus vulgaris L. Italy Leaves The effect of edible coatings alone or in combination with thyme oil on anthracnose incidence and severity in inoculated avocado fruits Evaluation of plant extracts at various concentrations was carried out by the ‘poisoned food technique’. The inoculated fruits were dipped in commercial treatment (prochloraz 0.05% for 5 min dip), chitosan (CH), aloe (AL), thyme oil (TO), CH+TO (3:1) and AL+TO (3:1), allowed to air dry at room temperature and stored for 5 d. Coating with CH +TO and AL+TO combination was the most effective, and both combination treatments significantly reduced the percentage disease incidence by 80% and 75%, respectively. [101]
    Zataria multiflora Iran An essential oil from leaves Preventing browning of button mushrooms (Agaricus bisporus) The treatments were control (water), TG (Tragacanth gum coating, 0.6%), TGZEO1 (0.6% TG + 1.0% 122 sorbitol + 100 ppm ZEO), TGZEO5 (0.6% TG + 1.0% sorbitol + 500 ppm ZEO), TGZEO10 (0.6% TG + 1.0% sorbitol + 1000 ppm ZEO) and SM (1000 ppm sodium metabisulphite). Mushrooms were dipped into their respective solutions for 5 min, and browning of button mushrooms was evaluated upon 16 d of storage at 4 °C. Control and SM-treated samples had higher open cap mushrooms (82.2% and 80.0%, respectively). Over the same period, the percentage of open cap mushrooms coated with TGZEOs, TSs and TG were in the range 66.7–75.6%. After 16 days, the control had higher PPO and POD activity (75 and 25 units/mg protein, respectively) resulting in higher browning rate, while TG-coated mushrooms had lower browning rate in the range of 25–70 units/mg protein PPO and 15–20 units/mg protein POD. [102]
    Zehnerria scabra Angola Tubers Control banana fruit crown rot Evaluation of plant extracts at various concentrations (1%, 5%, 10%, 25% and 50% concentration) against C. musae by dipping the fruits in plant extracts (at 25% concentration) for 5 min and allowed to air dry for 6 h. Banana hands dipped in chemical benomyl (0.1%) served as positive control while distilled water was used as a negative control. Extracts of Z. scabra (25%) and recorded significant reduction of crown-rot disease up to 75% at room temperature (12 d of incubation) and up to 85% at cold storage (35 d of incubation) conditions. [94]

    3.1. Medicinal Plant Extracts against Microbes in Horticultural Crops

    Medicinal plants produce secondary metabolites with antimicrobial properties. Thus, their screening can provide an alternative for producing chemical fungicides that are relatively non-toxic and cost-effective [37][103]. Most of these compounds are terpenes with fungicide properties and can be used as phenolic compounds or essential oils to inhibit microorganisms [3][104]. Medicinal plant extracts can be directly used, or substances responsible for the antimicrobial properties can be isolated [105][106]. Although several studies on the antimicrobial effects of plant extracts have been performed, many medicinal plants used in different rural communities have never been evaluated for their antimicrobial effects [105][107].
    Alemu et al. [97] investigated extracts from four plants (Ruta chalepensis (fringed rue), Eucalyptus globulus (eucalyptus), Vernonia amygdalina (bitter leaf) and Datura stramonium (jimsonweed)) at 10%, 25% and 50% dilutions in controlling soft-rot on mango fruits while in storage for 16 d at 25 °C (65 ± 5% RH). Higher mean soft-rot severity was recorded on the untreated control 4.67 (i.e., nearly 93.4% fruit area affected), while the most significant reduction in the severity of soft-rot 1.33 (26%) was recorded in the extract of fringed rue at 50% concentration. Extracts from jimsonweed at 25% and 50%, eucalyptus at 50% and bitter leaf at 25% dilution also reduced the soft-rot severity within a range of 2.07–2.40. In an in vivo study reported by Navarro et al. [95] on two nectarine cultivars (‘Flavela’ and ‘Flanoba’) dipped in Aloe vera gel alone or with the addition of thymol (99.5%) followed by inoculation with R. stolonifer, B. cinerea or P. digitatum, findings show that the application of aloe treatments (alone or with the addition of thymol) led to significantly lower fungus infection volume than in non-treated nectarines after incubation at 25 °C (85% RH) for 6 d. The results show that extracts of the different plant species are substantially varied in their antifungal potentials. According to Ogbebor and Adekunle [108], these differences are expected because plants vary in their chemical constituents, habitats and stages at which they were collected.
    Bordoh et al. [100] applied extracts from Zingiber officinale (ginger), Curcuma longa (turmeric) and Phyllanthus niruri (gulf leaf-flower) as an edible coating to control postharvest anthracnose in dragon fruits after inoculating them with C. gloeosporioides. After storage in commercial packaging cartons at 11 ± 2 °C and 80% RH for 28 d, results show that gulf leaf-flower extracts at 10 g L−1 significantly reduced the disease followed by turmeric extracts at 10 g L−1 compared to control. On the other hand, dipping avocado fruits in chitosan + thyme oil (3:1) and aloe gel + thyme oil (3:1) significantly reduced the disease severity of C. gloeosporioides 80% and 75%, respectively, after storage at 20 °C and 70 ± 5% RH for 5 d [101]. The results also show that preventative dip treatment with chitosan or Aloe vera gel incorporated with thyme oil or stand-alone treatments showed lower incidence of anthracnose severity. According to Singh et al. [109], the antifungal activity of plant extracts against C. gloeosporioides could be due to the presence of bioactive compounds such as gingerols (in ginger), curcumin (turmeric), loin and aloe-emodin (aloe) and alkaloids in dukung anak.
    In trying to reduce the severity of anthracnose on naturally infected berries, Cruze et al. [110] immersed the berries in extracts of neem (Azadirachta indica), orange (Citrus sinensis) extracts, essential oil emulsions of garlic (Allium sativum), diesel tree (Copaifera langsdorfii), cinnamon (Cinnamomum zeylanicum) and clove (Eugenia caryophyllata) extracts before storage at 24 ± 2 °C and RH of 85 ± 5% for 11 d. Results show that neem and citric extract at 4% was the most efficient treatment because the disease incidence was 19.44% and the disease severity was 9.34%, while the control showed disease severity of 75.13%. Less severity and, consequently, more disease control were also achieved by immersing the berries into the emulsion of essential oil of garlic, followed by treatments with diesel tree, clove and cinnamon (Table 3). According to Bautista-Baños et al. [111], compounds such as nimbin and quercetine present in the neem have fungicide activities, and thus they were more effective than the other medicinal extracts.
    Sangeetha et al. [94] dipped banana fruits (cv. Robusta) in plant extracts of sweet flag (Acorus calamus), haritaki (Terminalia chebula), dawidjies (Zehneria scabra), doctorbush (Plumbago zeylanica), shallot (Allium cepa × Allium sativum), mamijava (Enicostemma littorale), orange climber (Toddalia asiatica) and arni (Clerodendron phlomoides) at 25% concentration to control crown-rot of banana. After storage for 12 d at room temperature (28 ± 2 °C and 80% RH) and 35 d at low-temperature storage (14 °C and 90% RH), results show that dipping of banana fruits in aqueous leaf extract of shallot significantly reduced the crown-rot disease by 86% compared to control and other treatments (Table 4). Gosh et al. [112] reported that the antimicrobial compounds are abundantly present in medicinal plants, and these might be involved in the defence of plants against microbial pathogens in addition to their direct antimicrobial activity against crown-rot disease in bananas.

    3.2. Medicinal Plant Extracts against Pests in Horticultural Crops

    About 50% of total crops are lost annually because of insect and pest attacks, which adversely affect world food production and huge economic losses [12][39][113][114]. The use of pesticides has contributed immensely to the increase in agricultural productivity; however, these pesticides lead to serious environmental pollution, affecting human health and causing the death of non-target organisms [115]. There is now an increasing trend in the use of botanicals with more than 2400 bioactive medicinal plant species identified for their pesticide and antipathogenic properties [40][116].
    According to Isman and Grieneisen [117], scientists continuously search for novel pest control products from medicinal plants. The rich flora found around the world provides known medicinal plant species that may exert insecticidal properties based on their chemistry and efficacy under laboratory conditions [118][119]. Using medicinal plant extracts for pest control has several advantages in terms of preventing the development of insecticide resistance due to the usual presence of several bioactive compounds, their low persistence in the environment and their generally low cost, particularly for smallholder farmers with limited income [120][121][122].
    Muzemu et al. [90] reported that fever tea (Lippia javanica) leaf powder extract at 12.5% w/v using 0.1% v/v soap could be used against rape aphids and tomato spider mites. Extracts of fever tea leaf powder and bitter apple (Solanum delagoense) ripe fruit pulp were evaluated as alternatives to conventional pesticides against rape aphids and tomato red spider mites under on-station conditions. The fever tea and bitter apple applied at 12.5% and 25% reduced aphids by 83% and 75.9% and mites by 86.5% and 75%, respectively. Both extracts were more effective against aphids than mites while fever tea was more effective than bitter apple on both crop pests. According to Manenzhe et al. [123], the reduced number of aphids and mites could be due to extracts’ repellent, toxic and anti-feedant effects since they contain essential oils and alkaloid constituents with pesticide properties. This shows that fever tea and bitter apple had some insecticidal effects against the vegetable pests (Table 3).
    Mazhawidza et al. [88] did a trial on direct topical and residual sprays of aqueous extracts of jimsonweed (Datura stramonium), snake bean plant (Bobgunnia madagascariensis) and bitter apple against the ladybird beetle on rape (Brassica napus). The crude extracts of jimsonweed fresh leaves, bitter apple fresh fruits and snake bean plant dried pods were applied separately at 5, 10, 15, 20 and 25% w/v under laboratory conditions. Mortality of ladybird beetle in laboratory bioassays increased with an increase in post-exposure time, and snake bean plant (25% w/v) caused the highest mortality. Based on lethal dose at 50% (LD50) values, snake bean plant extracts were most toxic (LD50, 30% w/v) followed by jimsonweed (LD50, 34% w/v) and bitter apple (LD50, 49% w/v) 24 h post-application. Under laboratory conditions, significantly higher ladybird beetle mortality rates from snake bean plant than D. jimsonweed and bitter apple were observed [124]. This observation may have been due to saponins in snake bean plant and other anti-feedant compounds such as quercetin. The results show that jimsonweed and bitter apple extracts at the application rates used in the study were relatively safer to H. variegate than snake bean plant (25% w/v). These ladybird beetles have been reported in horticultural crops such as spinach, tomatoes, cabbage, raddish, etc. [125]. Hence, jimsonweed and bitter apple can be explored for integrated pest management programs of horticultural crops (Table 3).
    Kayange et al. [93] evaluated the effectiveness of fish bean (Tephrosia vogelii) and candida (Tephrosia candida) extracts against green bean aphid (Aphis fabae) at three different dilutions (0.5%, 2% and 5% w/v). According to the authors, there was a high mortality rate of aphid on the plots treated with fish bean compared to plots treated with candida at the same dilution. These plant extracts at 5% significantly controlled the green bean aphid. According to Stevenson and Belmain [98], the presence of isoflavonoids which are toxic substances in fish bean might have reduced the presence and population of aphids. The active components in leaves of fish bean have anti-feedant, insecticidal, acaricidal, ovicidal and ichthyotoxic effects, which act as a stomach poison in insects [126].
    Sharma and Gupta [87] evaluated the biological activity of plant extracts against cabbage moth (Pieris brassicae) (Linn.) on cabbages. Aqueous extracts (10, 5, 2.5 and 1.0%) from leaves of neem (Azadirachta indica), chinaberry tree (Melia azedarach), wild-sage (Lantana camara), hemp (Cannabis sativa), oleander (Nerium indicum), Eucalyptus sp., castor bean (Ricinus communi) and black nightshade (Solanum nigrum) were used as treatments. The protection of cabbage foliage at all the dilutions of M. azedarach was higher when compared to other plant extracts. The maximum protection was provided at 5% of chinaberry tree (88.3%) and neem (82.5%). The minimum (4.6%) protection to the cabbage foliage was observed at 1% of neem. The anti-feedant effect of the different concentrations decreased with a decrease in concentration. Irrespective of plant extract, high doses resulted in maximum mean protection to foliage (68.1%), while the lowest dose, 9.5%, resulted in the least protection (Table 3).

    The entry is from 10.3390/su13115897


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