Millets are the most important cereals food grain crops, especially grown in arid and semi-arid regions of the Asia and Africa. Finger millet (
Eleusine coracana) is popularly known as ragi. Karnataka, Tamil Nadu andhra Pradesh, Orissa, Maharashtra, Uttar Pradesh, Bihar and Gujarat are major finger millet growing states in India and together contribute for more than 95 per cent of the total area under finger millet and more than 98.13 per cent of the total production in the country (
Prajapati, 2020). The outer layer of the grain or the seed coat contains dietary fiber, minerals, phenolics and vitamins hence the use of millets as whole grain provides essential nutrients and also offer health benefits.
Finger millet is a good source of carbohydrates and hence it supplies high amounts of energy. Because of its rich sulphur containing amino acids, proteins due to its low glycemic index with high fibre. Hence, it is recommended for diabetic patients as it is very effective in controlling blood glucose levels of diabetics. Finger millet also contains high calcium, high soluble fibre and low fat. High diastatic power of malted grains makes finger millet unique. Consumption of finger millet prevents cholesterol and constipation (
Panwar, 2016).
About 5-8% of protein is present in finger millet, 65-75% carbohydrates, 15-20% dietary fibre and 2.5-3.5% minerals (
Chethan and Malleshi, 2007). Powder of naked caryopsis is generally used for the preparation of roti, mudde and thin porridge called ambali. Regular consumption of whole grain cereals and their products have shown in epidemiological studies that they can protect against risk of diabetes mellitus, gastrointestinal diseases and cardiovascular risks. Finger millet contains lower levels of protein (6-8%) and fat (1.5-2%), their proteins are unique because of the sulphur rich amino acid contents. Finger millet is the richest source of calcium (300-350 mg/100 g), also rich in phytates 0.48 g/100 g, polyphenols, tannins 0.61%. Finger millet contains 8.71 mg/g dry weight fatty acid and 8.47 g/dry weight protein.
Health benefits of finger millet
Finger millet is also a good source of essential amino acids like arginine, lysine, methionine, lecithin etc. and performs a number of essential health promoting functions including precursor for the synthesis of nitric oxide and it helps in stimulation and release of growth hormone (
Pragya, 2012). It reduces the healing time of bone injuries and helps in increased blood circulation and muscle mass. It also alleviates male infertility, improving sperm production and motility. Finger millet may prevent cardiovascular disease by reducing plasma triglycerides in hyperlipidemic rats (
Lee et al., 2010). Finger millet based diets have shown lower glycemic response due to high fibre content and also alpha amylase inhibition properties which are known to reduce starch digestibility and absorption (
Kumari and Sumathi, 2002).
Diseases of finger millet
Although finger millet is considered a hardy crop, it is affected by more than 20 diseases caused by different pathogens
i.e. Bacteria, fungi and virus. The pathogens infect different parts and stages of the plants from seedling to grain formation result in reduction in total yield and ultimately results in economic loss to farmers. Following are the major diseases of finger millet (Table 1).
Blast disease
The causal agent of blast in finger millet is a heterothallic filamentous fungus pathogenic to almost 50 plant species in 30 genera of Poaceae including
Eleusine (
Rossman et al., 1990). The fungus appears to overwinter as mycelia in the infected living leaves or dead plant debris in the soil (
Uddin, 2003). Among various diseases serious losses in finger millet caused by blast pathogen
Pyricularia grisea (Cooke) Sacc. [Teleomorph:
Magnaporthe grisea (Hebert) Barr.]. The average loss due to blast has been reported to be around 28 to 36 per cent (
Pordel, 2018) and in certain areas, yield losses could be as high as 80 to 90 per cent. The disease affects finger millet at all growth stages but neck and finger blast are the most destructive forms capable of causing up to 100% reduction of biomass and average yield per year
(Takan et al., 2012; Lenne et al., 2007).
The blast disease in finger millet often resulting in more than 50% yield losses (
Shaikh, 2017 and
Esele, 2002) and it is as high as about 80-90% in endemic areas. In India,
Ramappa et al., (2002) recorded upto70 % finger blast and 50 % neck blast during
Shaikh, 2017 in Mandya and Mysore districts of Karnataka. Also affected significant yield loss in other states of India including Andhra Pradesh, Madhya Pradesh, Haryana and Maharashtra.
Pordel et al., (2018) reported that the ultimate loss in yield is due to enhanced spikelet sterility and reduction in grain weight and number.
Symptoms of blast disease
Initial inoculum comes from weeds or collateral hosts; spread by air-borne conidia Blast symptoms can be observed on the seedling, leaf, leaf sheath, rachis, nodes, glumes, peduncle and finger, depending on the stage of the crop (Fig 1 and 2). Elliptical or diamond shaped lesions on leaves with grey centers, water-soaking and chlorotic halo surrounding the lesions are characteristic symptoms. It is characterized by typical spindle shaped spots on leaf lamina. Under highly congenial conditions such spots enlarge, coalesce and leaf blades especially from the tip towards base give a blasted appearance. Finger blast symptom starts at the tip and proceeds toward the base of the finger, which becomes brown. Neck infection is the most serious phase of the disease that causes major loss in grain number, grain weight and increase in spikelet sterility. The pathogen also attacks fingers. Pathogen Minimum temperature of 15-25°C and relative humidity of more than 85 % with intermittent rainfall are congenial climatic conditions for disease development. Environmental factors including high temperature, high relative humidity and leaf wetness influence disease development (
Ruiz, 2003,
Sekar 2014 and
18).
Management strategies
Eco-friendly management strategies are important in achieving agriculture sustainability with high quality, disease free crops. Blast pathogen is the most damaging disease of finger millet, affecting the finger millet production and causing massive yield loss of up to 80% of finger millet yield per annum globally
(Mgonja et al., 2013). In this review an attempt has been made to report the utilization of various bio-agents in controlling the blast pathogen and its adverse effects in finger millet.
Finger millet blast infection primarily comes from the seeds because the disease is externally seed borne. The most efficient, feasible, ecofriendly and cheapest way to control the plant diseases is the host plant resistance. Efforts are being made to develop finger millet resistance lines to understand inheritance of resistance to
Pyricularia grisea (Patro et al., 2013; Patro and Madhuri, 2014).
Chemical methods
Since, the primary infection comes from seed-borne inoculum, treating seeds with various kinds of fungicides has been carried out by many research groups.
Ramaswamy (2007) used Tricyclazole (8 g/kg seed) followed by sprays of two plant extracts of notchi and
Prosopohis sp. have been found effective. During later stages of the crop two sprays of Ediphenphos or Kitazin or propiconazole (0.1%) or Carbendazim or tricyclazole (0.05%) (
Palanna, 2021), first at the time of ear emergence and second after 10 days or an initial spray of 0.05% Carbendazim or tricyclazole followed by a spray of Mancozeb (0.2%) 10 days later are effective (
Palanna, 2021). Two sprays of Saaf (0.2%) first at 50 per cent flowering and again 10 days later were also found effective in reducing the blast and increasing the yield
(Ramappa et al., 2002).
Management through bio-agents
It involves the use of naturally occurring non-pathogenic microorganisms that are able to reduce the activity of plant pathogens and thereby suppress diseases. Hence, controlling this pathogen using bio-agents will help in enhancing the yield of the crop. Among different biological approaches, use of the microbial antagonists like yeasts, fungi and bacteria is promising and gaining popularity. In addition biological control using PGPR represent a potentially attractive, alternative disease management approach, which are known for growth promotion and disease protection against several plant pathogens.
Prajapati et al., (2020) studied the management of finger millet blast disease with fungicides and bioagents as seed treatment as well as foliar spray. Among all the tested fungicides and bioagents, seed treatment with carbendazim, 2 g/kg seed + 2 sprays of tricyclazole, seed treatment with carbendazim, 2 g/kg seed + 2 sprays of tebuconazole and seed treatment with
Pseudomonas fluorescens, 10 g/kg seed + 2 sprays of
P. fluorescens significantly control the blast disease of finger mill
et also promoted grain and fodder yield. These findings are in support of
Madhukeshwara et al., (2005) who found that seed treatment with carbendazim @ 2 g/kg seed and tricyclazole @ 2 g/kg seed gave lowest leaf blast, neck blast and finger blast incidence and enhanced the yield of finger millet.
A study carried out by
Patil and Kumudini (2019) evidenced the induction of resistance in finger millet plants by seed priming against blast pathogen
M. grisea, which has become the most effective approaches in induction of resistance. The greenhouse studies revealed that seed priming with plant growth promoting bacteria (PGPR) fluorescent
Pseudomonas isolates JUPC113 and JUPW121 from cotton and wheat rhizosphere enhanced the rate of disease protection by fungal inhibition compared to that of control plants.
Netam et al., (2016) conducted a field experiment to evaluate the efficacy of bioagents spray combination to control blast disease in finger millet caused by
Pyricularia grisea. In addition to significant increase in grain yield, lowest neck blast incidence, leaf blast severity and finger blast incidence was effectively controlled by three sprays of
Pseudomonas fluorescens, followed by two sprays of
P. fluorescens.
Negi et al., (2017) identified competent rhizobacteria having promising biocontrol activity against
M. grisea infecting ragi. Among 70 rhizospheric pseudomonads isolated from various annual plants, of which 10 isolates showed maximum inhibition of two test isolates of
M. grisea. Pseudomonas fluorescens isolate Pf-30 exhibited maximum inhibition (81.25 and 88.43%) against the test pathogen, followed by Pf-53. Of all, 19 isolates were prominent in chitinase production with Pf-30 showing maximum efficiency (218.18%) for enzyme production. Implant evaluation in polyhouse revealed that different pseudomonads could suppress the disease significantly when given as seed treatment and foliar spray. Maximum disease suppression was exhibited by Pf-47 and Pf-53 (82.77 and 82.06%, respectively).
In vitro evaluation of the effectiveness of biological agents and fungicides against the mycelial growth of
P.grisea isolates was studied by
Gashaw et al., (2014). They collected a total of 42 isolates of
P.grisea were collected and isolated from infected finger millet plants and wild relatives from five agro-ecological zones of Ethiopia. The highest percent of mycelial growth inhibition of
P. grisea isolates was observed by
T. harzianum,
T. viride and
P. fluorescens respectively
. Sancozeb was the most effective fungicide than ridomil, bayleton and curate in significant inhibition of
P. Grisea mycelial growth.
Sekar et al., (2014) demonstrated the potential of the indigenous rhizospheric
Pseudomonas sp. strain MSSRFD41 to control blast disease and promote the growth of finger millet. In dual plate culture assays, strain MSSRFD41 significantly suppressed mycelial growth of
P. grisea consistent with antifungal action through the production of secondary metabolites, antibiotics, volatiles and hydrolytic enzymes. Further research
Sekar et al., (2018) suggested MSSRFD41 exhibited multiple beneficial traits to the host plants and results of compatibility and survival assessment revealed that MSSRFD41 could be a substitute and sustainable resource to reduce the usage of chemical fungicides for control of blast disease on finger millet.
Kumar and Kumar (2011) reported that seed treatment with Pseudomonas fluorescens @ 0.6% along with two sprays @ 0.6% were significantly most effective in reducing blast disease of finger millet. They carried out seed treatment with
Pseudomonas fluorescens Pf 2 (@ 0.6%)+ its two foliar sprays (@ 0.6%) followed by two sprays of hinosan @ 0.1% were significantly most effective in reducing blast disease. Similar finding was reported by
Patro et al., (2008). Several other workers
i.e.
Krishnamurthy and Gnanamanickam (1998);
Vidhyasekaran et al., (1997) and
Ramappa et al., (2002) were also reported the effectives of
P. fluorescens applied as foliar spray.
Seed treatment with
P. fluorescens @ 0.6% followed by two sprays of same, first at 50% flowering followed by second spray 10 days later was found to be the best for the management of blast disease
(Patro et al., 2008 and
13). Under
in vitro conditions a synthetic product of garlic oil and neem oil called allitin (0.1%), were also found effective against blast pathogen,
P. grisea. Similar results were reported by
Mannuramath (2015) on which
Pseudomonas fluorescens was shown to effectively inhibit
Rhizoctonia solani and
Pyricularia oryzae by agar plate method. These might be due to producing secondary metabolites, which inhibited the mycelial growth of
P. grisea isolates.
Two sprays of
P. fluorescens against
Pyricularia grisea; first spray at 50% flowering followed by second spray 10 days later are recommended for the control of neck and finger blasts
(Ramappa et al., 2002). Palanna, 2021 reported the effectiveness of six strains of
Pseudomonas fluorescens for their ability to inhibit the blast fungus. The application of bioagents in reducing the blast disease of finger was also reported by
(Senthil et al., 2012).
