Blast Disease of Millets: Present Status and Future Perspectives

Abstract

Millet crops are affected by various biotic and abiotic stresses. Among biotic stresses, blast disease caused by Pyricularia grisea (finger, pearl and proso millets) and Pyricularia setariae (foxtail millet) is the most devastating and widespread disease that causes substantial grain and forage yield losses and is a key constraint to pearl millet, finger millet and foxtail millet production in most of finger millet growing areas, and recently, it is also reported in barnyard millet in few locations. This book chapter emphasizes mainly on occurrence, distribution, symptoms, yield loss, etiology, genetic diversity, mode of spread of the pathogen and survival and integrated disease management approaches for mitigating of disease. This information will be highly helpful for better understanding of the disease. Further, it will be useful to enhance production and productivity of millets and to reinforce the food and nutritional security in the developing countries of Asia and Africa continents where the millets are mainly grown as staple food crops.

1. Introduction

Millets are small-seeded cereal crops widely known for its nutraceutical importance as well as food and fodder. The most frequently cultivated millets are sorghum (Sorghum bicolor L. Moench), finger millet (Eleusine coracana (L.) Gaertn.), foxtail millet (Setaria italica (L.) P. Beauvois), pearl millet (Cenchrus americanus (L.) Morrone.), kodo millet (Paspalum scrobiculatum L.), little millet (Panicum sumatrense Roth ex Roem. and Schult.), proso millet (Panicum miliaceum L.), browntop millet (Brachiaria ramosa (L.) Stapf) and barnyard millet (Echinochloa crusgalli (L.) P. Beauvois). In India, millets are cultivated for both grain and fodder. Though millets are regarded as hardy crops, present day climate change has rendered most of them susceptible to many pathogens.

Many diseases of millets regularly appear in a severe form under different climatic conditions and cause considerable economic loss, while others appear sporadically in specific climatic situations and have less damaging effect to the crop. Fungal diseases are more than bacterial and viral diseases. Important fungal diseases of millets are grain mold, ergot, smut, anthracnose, downy mildew, blast, rust, charcoal rot, foot rot, banded sheath blight and sheath rot. The diseases infect different plant parts including root, stem, leaves, peduncle or grain and adversely affect yield and quality of the produce. These diseases assume different significances for seed production, certifications and marketing of millets.

Among all these diseases, blast is a serious disease of millets especially in finger millet, pearl millet and foxtail millet. It is caused by the fungi, Pyricularia grisea (finger, pearl and proso millets) and Pyricularia setariae (foxtail millet). In finger millet crop is affected at all stages of its growth, and the disease is diagnosed by the production of elliptical- or diamond-shaped lesions on the leaf, peduncle and fingers, depending on the stage of the crop. The most damaging stage is neck blast, followed by finger and leaf blast. In foxtail millet, barnyard millet, proso millet and pearl millet symptoms are confined to only leaf. Moderate temperature, high humidity, cloudy days with intermittent rainfall conditions are ideal for the quick disease spread. The disease occurs almost every year in most of the finger millet and foxtail millet growing areas during rainy season, and in other millets, the disease is confined to specific location. The yield loss varies depending on the time of onset of the disease, severity, plant variety and prevailing weather.

1.1 Blast

Blast caused by Pyricularia spp. is one of the serious threats and most destructive disease that occurs widely in major millet growing regions of world. It is the major production constraints under natural conditions especially in finger, pearl and foxtail millet cultivation causing considerable economic losses with varying degrees of damage. In India, the finger millet blast was first reported from Tanjore delta of Tamil Nadu [1]. While foxtail millet blast was recorded in 1917 by Nishikado from Japan [2], but in India, it was reported in 1919 from Tamil Nadu [1], which further has also been recorded from Maharashtra, Andhra Pradesh [3] and Uttarakhand [4]. Since 1970, blast disease in pearl millet has been prevalent in major growing states of India; increased incidence has been reported recently in most pearl millet growing states like Gujarat, Uttar Pradesh, Madhya Pradesh, Rajasthan, Delhi, Maharashtra and Karnataka [5]. The disease is prevalent in all the major millet growing areas and spreading to new location as well with emerging pathotypes showing varying intensities depending on the cultivar, favorable conditions and production techniques.

1.2 Etiology

Pyricularia grisea (Cooke.) Sacc. [Perfect stage: Magnaporthe grisea (Herbert) Barr] causing blast in finger and proso millet whereas Pyricularia setariae Y. Nisik. infects foxtail millet. Kulkarni and Patel [6] grouped P. setariae into four physiological races on the basis of physiological, cultural, morphological characters and pathogenic ability of the fungus. However, Gaikwad and D′ Souza [7] determined that the isolates of P. setariae that infect foxtail millet differ from those that infect rice, finger millet and pearl millet. In case of pearl millet, Pyricularia grisea is known to cause blast disease in pearl millet. However, recently Singh et al. [8] reported that the foliar blast of pearl millet in western arid Rajasthan, India, is caused by Pyricularia pennisetigena.

1.3 Diagnostic symptoms

Blast pathogen can infect all the stages of plant in both finger and foxtail millet, the young seedlings are more prone for the attack and showed burnt appearance in nursery under severe infection [9]. In finger millet, P. grisea attacks at different growth stages of the crop and leads to formation of typical symptoms like leaf blast, neck blast and finger blast while in case of foxtail millet, P. setariae attacks the leaf lamina producing leaf blast symptoms [10, 11, 12, 13].

On leaf lamina, the pathogen produces typical symptoms of water-soaked, spindle- or diamond-shaped lesions which are initially surrounded by chlorotic halo. Typical leaf blast symptoms are the formation of elliptical- or diamond-shaped lesions containing grayish center with dark brown margins. Under severe infection, adjacent lesions enlarge and may coalesce to form large necrotic areas which gives the crop burnt appearance from far. The pathogen infects and develops lesions on the leaf, peduncle and finger depending on the stage of the crop. The most devastating stage of finger millet blast is neck blast, in which the pathogen targets the neck region, reducing the number and weight of grain per earhead and leading in earhead sterility [14]. In this, neck portion of 2–4 inches below the ear immediately turns initially brown and later to black, where olive gray fungal growth can be observed in the blackened portion under high humid climate. In finger blast where the pathogen attacks fingers, i.e., attacks usually the apical portions running towards the base (Figure 1). Infection of finger blast results in shriveled and blackened seeds which makes unfit for seed purpose and human consumption because of loss of minerals and vitamins. Ramakrishnan [15] observed spindle-shaped dark brown leaf spots 1–3 mm in length with grayish center and brownish periphery on finger millet, rice and Digitaria spp. leaves.

Figure 1.

Typical blast disease symptoms of millets a&b) leaf blast of finger millet, c&d) neck blast of finger millet, e & f) finger blast of finger millet, g&h) blast of foxtail millet, i) blast of barnyard millet and blast of pearl millet.

Symptoms of foxtail millet leaves mainly developed as from a small water-soaked yellowish dot, which later turned circular to an oval spot with a grayish center surrounded by a brown margin. Spots measured an average 2–5 mm in diameter within 2–3 days. The spots then coalesce and resulted in drying of leaves. The disease starts with the lower leaves and extend to upper leaves. No symptoms were observed on neck of foxtail millet [16]. Sharma et al. [17] observed blast disease symptoms on foxtail millet leaves as tiny circular spots with gray-colored centers measuring 3–5 mm in diameter surrounded by a brown margin and also observed high disease severity in dense plant stand with moist condition.

In barnyard millet, the symptoms appear on the young seedlings under the field conditions. The spots are spindle to circular shaped with varying sizes. Initially the spots showed yellowish margin with grayish center. Later, the centers turned ash colored. Fungus develops an olive-gray overgrowth at the center of the spots under humid conditions [10].

1.4 Mode of spread and survival

The blast fungus, Pyricularia, can invade the host either by piercing the epidermal cells directly or through stomatal opening. Pyriform air borne conidia serves as both primary and secondary source of inoculum. The pathogen survives on infected host species or on weed hosts.

1.5 Host range

Finger millet, proso millet, foxtail millet, pearl millet, rice and wheat, etc., are infected with the pathogen. Nagaraja et al. [18] described that P. grisea isolated from finger millet possess the potential to infect rice crop but not vice-versa. Likewise, P. setariae isolated from foxtail millet shows the ability to infect finger millet, pearl millet, wheat and Dactyloctaenium aegyptium [19].

Mackill and Bonman [20] proposed that diverse weed hosts growing adjacent to the cultivated plants could serve as possible sources of inoculum for the disease, providing the fungus with an alternate method of survival. Despite the fact that blast infects a wide variety of sympatric flora Hamer et al. [21] and Valent et al. [22] determined that M. grisea populations are strongly confined by host range. Under experimental conditions, inoculations of rice with isolates of from weeds resulted in successful [20] and unsuccessful [23] cross-inoculations. Viji et al. [24] reported that in the laboratory, ten isolates of M. grisea from rice did not infect finger millet and vice versa, confirming that the M. grisea populations infecting rice and fibger millet in India were distinct. Similar results were reported by Kato et al. [25] and Todman et al. [26], who found that Magnaporthe isolates from Eleusine coracana failed to incite disease on rice and vice versa. Contradictory results were reported by Kumar and Singh [27] which could be attributed to prevailing environmental conditions during the experiments and the soil’s nutritional level [28, 29].

Pennisetum is a diverse genus with over 100 species [30]. Susceptibility of all the species of Pennisetum to Magnaporthe grisea infection is not yet clear. The available information indicates that the pathogen infects principally Pennisetum glaucum, P. macroforum, P. squamulatum, P. pedicellatum [31], P. ciliare [32], P. purpureum [33]. Other graminaceous hosts such as Agrostis palustris, Brachiaria mutica, Eleusine indica, Cyperus rotundus, Eragrostis sp., Panicum miliaceum serve as collateral hosts for the pathogen [34].

1.6 Epidemiology

The crop is susceptible to the blast disease during all stages of its growth, i.e., seedling (vegetative) to grain formation (reproductive) stage. Especially, young seedlings more prone to the blast both in the nursery and field conditions with favorable weather [35]. Moderate temperature (25–30°C) with high relative humidity (>90%) and cloudy days coupled with intermittent rainfall creating continuous leaf wetness for more than 10 hr. are congenial for rapid development and spread of the disease. Continuous rains during heading lead to the occurrence of finger blast, resulting in massive production losses in both finger and foxtail millet. Also, high nitrogen fertilizer application is reported to increase blast disease [36].

1.7 Economic importance

Finger millet blast is economically one of the most important diseases, while blast of proso and foxtail millet are relatively of minor occurrence. The disease occurs almost every year in finger millet during rainy season, and losses vary with the time of onset of the disease, severity, cultivar and climatic conditions. During late 1970s to 80s, 1% incidence of finger and neck blast by M. grisea resulted in a corresponding enhancement of yield losses by 0.32 and 0.084% for neck and finger blast, respectively. Grain yield losses in finger millet, on the other hand, ranged from 6.75 to 87.5% [37]. In its severe form, foxtail millet blast can lead up to 30–40% loss of economic yield [10] while mean yield loss of finger millet blast ranged from 28 to 36% and may go up to 90% in endemic areas with frequent disease [38]. In pearl millet also the blast disease causes considerable yield loss under favorable environmental conditions (Table 1).

Crop	Yield loss	Place	Ref
Pearl millet	13.3–14.9	India	[39]
Finger millet	42%	Ethiopia	[40]
	50 to 100%	TN, India	[41]
	35.78%	Gujarat, India	[42]
	22.57 to 56.67%	Karnataka, India	[43]
Foxtail millet	30–40%	India	[10]
	60%	India	[44]
	40%	India	[45]

Table 1.

Yield loss caused by blast disease in different crops.

1.8 Disease cycle

The pathogen harbors in glumes, straw as well as on some graminaceous weeds. The blast pathogen is seed-borne with presence of inoculum in the pericarp and endosperm [19]. Blast fungal life cycle is complex due to its nature of disease which shows sensitivity to the weather conditions, survival and spread inoculum in different ways. During off-season, i.e., in the absence main host, it survives on the graminaceous weeds as collateral hosts who provides the primary inoculum for onset of infection. Further, the fungus spreads mainly by airborne conidia and occasionally through seeds.

1.9 Characterization of the pathogen

For proper diagnosis of the disease, the understanding of the pathogenic characteristics is needed as much of knowing symptomatology and disease cycle. Blast caused by the Pyricularia spp. is identified based on its above-described symptoms in the field while in vitro, pathogen characterized based on cultural-morphological and molecular attributes. Morphological characterization includes studying mycelial features on agar plates, viz. appearance, color and amount of melanin pigment produced as well as the microscopic conidial characters. Molecular characterization of pathogen includes amplification of targeted genomic regions with fungal universal primers (ITS) as well as secondary barcoding regions such as beta tubulin, TEF and LSU and also by studying the DNA polymorphism using various molecular markers [46].

1.10 Morphology

M. grisea is a haploid, filamentous Ascomycete with morphological traits such as three-septate fusiform ascospores and black nonstromatic perithecia (ascocarp) with long hairy necks. The asexual stage Pyricularia grisea produces Conidia which are pyriform to obclavate, narrowed towards tip, rounded at the base, solitary, 2-septate, hyaline to pale brown, with a distinct basal hilum, sometimes with marginal frill. Studies on growth of P. penniseti on different media by Lukose et al. [47] indicated medium containing pearl millet leaf extract enriched with dextrose supported maximum growth of the pathogen. Light brown submerged growth was observed in potato dextrose agar medium, while pearl millet leaf extract medium showed grayish white superficial growth. Konda [48] tested effect of ten different solid media on growth of P. setariae and reported that maximum radial growth was observed in oat meal agar, PDA and malt extract agar followed by host leaf decoction +2 per cent sucrose agar medium. M. oryzae isolates were producing dull white to grayish-black colonies with regular margins. Conidia were pyriform, hyaline to pale olive and measured 16–23 x 4–7 μm in size [49].

Based on cultural and conidial variation, Viji et al. [24] differentiated Pyricularia isolates from different hosts. Sonah et al. [50] investigated that the cultural morphological variability of M. grisea isolates isolated from rice and other hosts and discovered that isolates with fast vegetative growth have gray-green or gray-white producing more spores than those with slower vegetative growth (submerged or subdued growth patterns). Isolates from non-rice hosts also have aberrant spore morphology, with longer, cylindrical and obpyriform spores. They also noticed a fair to good diversity in cultural and conidial characteristics among 17 field isolates of pearl millet [51].

1.11 Genetic diversity

Lot of information is available on variation among isolates of Pyricularia infecting various hosts like cereals and grasses. The pathogen is well studied in rice and is unfathomed in millets. The Magnaporthe grisea repeat sequence MGR586 was commonly used for studying population genetics of rice. Similarly fungal repetitive DNA or transposable elements are widely used for the purpose. The molecular level studies indicate the presence of variations among the isolates within or across the hosts. Several researchers have used molecular markers like RFLP [52] and SSR [53] and grouped the Indian isolates of M. grisea into two distinct populations—one finger millet group and other foxtail and rice group. Shivakantkumar et al. [51]. reported significant genetic variation among 17 M. grisea isolates infecting pearl millet with ITS, inter simple sequence repeats (ISSR) and simple sequence repeats (SSR) markers.

1.12 Mating type

Sexual reproduction is known to be a significant source of genetic variation in many fungi. Sexual compatibility in M. grisea is determined by the presence of two alleles (idiomorphs) at a single mating-type locus designated MAT1.1 and MAT1.2. MAT1.1 and MAT1.2 of M. grisea have been cloned and sequenced using a genomic subtraction strategy. The perfect stage of P. grisea was first described by Hebert [54] in crosses between isolates from cereals and wild grasses. Since then, efforts have been made to produce perithecia successfully on artificial media under controlled conditions using hermaphroditic tester isolates from finger millet and rice. Although both mating types have been found in the same field at the same time. However, it has not yet been possible to observe the perfect state in nature. The mating type assay Magnaporthe population infecting millets revealed that the mating-types, male fertile, female fertile and hermaphrodite nature of fertility existed finger, foxtail and branyard millet in the country. Which indicates the possibility of sexual recombination in field level and which may lead to high variability in pathogenicity and diversity in Magnaporthe population adapted to millets in India. All the tested isolates of pearl millet showed unknown fertility in PCR assay with MAT primers. It indicates fertility of pearl millet isolates has to be confirmed using range of tester isolates [55].

2. Management strategy

The integrated management strategies as well as novel approaches need to be adopted for the management of blast disease in millets. The integrated disease management strategies (Figure 2) for mitigating of millet blast are as follows.

Figure 2.

Integrated disease management strategies of millet blast.

2.1 Cultural practices

Several agricultural practices such as timely sowing, maintaining optimum plant populations and spacing, timely weeding, balanced use of fertilizers, crop rotation, deep plowing during summer season, removal of crop residues from the field, cleaning of field bunds after crop season, uprooting the diseased plant from the field and burning, regulating irrigation water from entering into other field, etc., will help in reducing chances of disease occurrence.

2.1.1 Adjustment of date of sowing

The choice of sowing date in relation to crop disease has one principal aim to reduce to a minimum the period over which infective agent meets the susceptible stage of the host. Early sowing reduces blast severity.

2.1.2 Optimization of plant population

Due to high plant density, humidity in the field is always high, temperature is low, there is lack of aeration, and pathogen grows rapidly. Maintaining optimal plant population in the field to reduce the relative humidity build up in the field help in reducing disease severity.

2.1.3 Use of disease-free seeds

Use of pathogen free seeds is a pre-requisite of eco-friendly control of plant disease because numerous plant pathogens are transmitted to the field via infected or contaminated seeds and seedlings.

2.1.4 Sanitation

Field sanitation is another important measure through cultural practices for preventing spread of plant disease and their management. Plant and plant parts are some of the best reservoirs of disease organisms. The inoculum present on few plants in the field may multiply on the plant and in due course of time may appear to cause epidemic in next season.

2.1.5 Eradication of alternate and collateral hosts

Plant pathogens usually have a wide host range and they used weeds, wild host plants or self-sown host plants as means of their active survival during absence of main crop. Therefore, keeping the field free from additional host of pathogen is a major sanitary cultural practice. Their timely removal helps to control blast disease.

2.1.6 Fertilizer management

A properly nourished plant is able to withstand or tolerate the attack of pathogens much better than a plant that has either nutrient deficiencies or excesses. A nutrient-deficient plant will be stressed and therefore more prone to disease attack. Excessive fertilizer applications can also cause plants to be more susceptible to disease. Nutrient can affect the relationship between crop and pathogen in many ways. Regulating the amount of nitrogenous fertilizer reduces incidence of blast and other diseases.

2.2 Host plant resistance

Exploiting host resistance to control disease is not only economical but also a practical necessity in a low value crop like millets where there is a limitation for any additional cash inputs such as fungicides, etc. Development of resistance varieties is the best way of combating the disease, which is primarily grown by resource-poor and marginal farmers. Disease resistant varieties identified and released for the different millets growing areas of India are tabulated in Table 2.

S.No.	Crop	Disease resistant/moderately resistant varieties	Reference
1	Pearl millet	BHB-1202, (MH 1831), Central Pearl Millet Hybrid RHB 223, JKBH 1008, MPMH 21, HHB 272, PB 1852, DHBH 1397, PROAGRO 9450, XMT 1497, Bio 8145, 86 M84 (MH 1890), KBH 108, 86 M88 (MH 1816) and ABV 04 (MP 552)	[56]
2	Finger millet	GPU 26, GPU 45, Chilika (OEB 10), VL 315, GPU 48, PRM 1, Bharathi (VR 762), Srichaitanya, KMR 301, KOPN 235, OEB 526, OEB 532, PPR 2700 (Vakula), VL 352, GNN - 6, GN - 5, VL Mandua - 348, KMR 340, Dapoli – 2 (SCN – 6), CO 15	[56]
3	Foxtail millet	RAU (Rajendra Kauni 1–2) and SiA 3085	[56]

Table 2.

Blast disease resistant varieties identified and released for the different growing areas of India (2000–2018).

2.3 Biological control

Biological control is an alternative to synthetic chemical pesticides and having several benefits to human beings and ecosystem; they can ensure the protection of plants against biotic and abiotic stresses, production of good quality grains, improve soil fertility, sustainable and safety of environment. The demand for development and application of indigenous bioinoculant products has increased among researchers because of their role in plant growth promotion and crop protection in sustainable farming systems and also for their economic value. Bio-control agents especially strains of Trichoderma and Pseudomonas are useful for seed and soil borne diseases of millets.

In finger millet seed treatment with P. fluorescens @ 6 g/Kg seed and spray P. fluorescens formulations at 2 g/lit of water. First spray immediately after noticing the symptom. Second and Third sprays at flowering stage at 15 days interval was found effective for blast disease management.

2.4 Chemical control

Chemicals are not normally used for disease management in millet, because of involvement of high cost of chemical and labor. However, sometimes its use in combination with resistant cultivar becomes necessary. Fungicides are mostly used either as seed treatment or foliar spray. However, combination of them gives better management.

Seed treatment with carbendazim @ 1 gm/Kg of seed. Spray any one of the fungicides viz., Carbendazim (0.2%) or Iprobenphos (IBP) (0.1%) or premixture fungicide (Carbendazim+Mancozeb) (0.1%), Ediphenphos (0.1%) or propiconazole (0.1%) or Tricyclazole (0.1%). First spray immediately after noticing the symptoms. Need-based second and third sprays at flowering stage at 15 days interval to control neck and finger infection in finger millet. Similarly in rice several fungicides like Tricyclazole have been extensively tested and recommended [57], Probenazole [58], Isoprothiolane [59], Azoxystrobin, etc., are recommend to control blast disease. Many fungicides are used against blast disease, including benomyl, iprobenfos, pyroquilon, felimzone, diclocymet, carpropamid and metominostrobin [60].

3. Future perspectives

With the everyday increasing population of world demands not only the food security but also the nutritional security which combined to form agricultural sustainability. Updated reports show that the agricultural production needs to be increased 50% by 2050 to meet the growing food and nutritional demand [61]. Nutritional security is as much important as food security for better quality of life which can be fulfilled by the cultivation of millets on large scale. Erstwhile commercial crops like rice, wheat, sugarcane, maize, etc., have been given more importance owing to their wide distribution and acceptance as daily food. In recent times, millets especially small millets gaining huge attention with growing health concerns which are fulfilling by their nutraceutical properties. This results in increasing area of cultivation, and further pathogen gets better opportunity for survival and spread; changes in virulence spectrum; emergence of new pathotypes/races, etc., which intern results in breakdown of resistance as well as expansion of host rage, etc., may lead to drastic reduction in production and productivity.

Therefore, systemic research in small millets on many aspects like breeding of new varieties, sequencing of genome, etc., are still underway has to be initiated on priority. With the advent of advanced genomic approaches like next generation sequencing (NGS), genome editing techniques, etc., made identification, cloning and transfer of resistant (R) genes easy. Using of such approaches in millets aid in better understanding of the crops and pave way for possible manipulation of crop genome to generate disease resistant crops which is an eco-friendly perspective. This will make the millets possibly the eco-friendly alternative for nutraceutical supplement, cost effective due to no use of pesticides and farmer friendly. Also, surveillance of the established diseases and regular monitoring of new diseases aid in achieving the food and nutritional security.

4. Novel/alternative strategies for enhancing blast disease resistances in millets

Millets are known to show exceptional tolerance to both biotic and abiotic stress in comparison with the popularly grown cereals. The stress resilience is due to various morpho physiological, biochemical and molecular mechanisms. Yet, in the recent era of drastic climate change, there is an increasing report of occurrence of blast disease in millets. The scenario of changing disease severity emphasizes the need to explore into novel, alternative strategies for disease management (Figure 3).

Figure 3.

Novel/alternative strategies for enhancing blast disease resistances in millets.

4.1 Exploitation of germplasm collection

Germplasm collections aid in the preservation of genetic resources for use in agricultural research and crop improvement programs around the world. Various millet germplasm has been preserved in various national and international gene banks. International Crop Research Institute for Semi-Arid Tropics (ICRISAT), Patancheru, in collaboration with different national and international organizations has enormous collections of accessions of millets with various traits of interest from across the world. India holds largest collection of millets at ICAR-National Bureau of Plant Genetic Resources, New Delhi, followed by All India Coordinated Small Millets Improvement Project (AICSMIP) at Bangalore and IIMR, Hyderabad. Germplasm could be a fantastic low-cost resource for allele mining and genotypic variant identification for blast disease resistance and other essential agronomic features. Mapping with genetic markers, identifying trait-specific germplasm and identifying candidate genes have a wide range of applications in millet improvement programmes. Lack of genetic resources hampers improvement of millets. Identifying and use new genes for host plant resistance in order to generate cultivars resistant to biotic stresses is critical.

4.2 Genomic assisted/physiological traits-based resistance breeding approaches

How plants adapt to external conditions determines plants functional trait (morphological, phenological, physiological and nutritional). Variation in these features results in one genotype being superior to others. Automation, imaging and software solutions have opened the way for many high-throughput phenotyping research in recent years. To conduct genetic studies or marker-aided breeding in any crop, DNA-based molecular markers, genetic linkage maps and sequence information are required genomic resources. Early diversity studies utilizing first generation molecular markers (RAPD, RFLP and ISSR) revealed limited sequence diversity in most millet populations [62], limiting their use in analyzing genetic variations in millet populations. Germplasm collections are used to select ideal mating parents for hybrid breeding, study population structure and analyze QTLs. On the other hand, adaptation of millets to diverse climate makes it impossible to rule out the possibility of the existence of considerable genetic diversity among the millets germplasms. The creation of relevant SNPs, as well as the mapping and tagging of QTLs related with blast resistance, would aid in the cloning of important disease resistance genes and the generation of resistant cultivars through a marker-assisted breeding programme [63].

4.3 Application of PGPR/microbe-mediated mitigation

Plant growth promoting bacteria (PGPB) promote growth and regulate plant development via cell proliferation and elongation, and development of lateral and adventitious roots by IAA production and ACC deamination. The various PGPB studies in millets include Pseudomonas sp., Florescent pseudomonads, Enterobacter sp. PR14, Sphingomonas faenimutants, Acinetobacter calcoaceticus and Bacillus amyloliquefaciens EPP90. These bacteria help in mitigating the effects of various abiotic stresses by an increased phosphate solubilization and antioxidant activity of enzymes and accumulation of osmo-protectants and a decreased lipid peroxidation [64]. Isolation and identification of such PGPR/ microbes with antimicrobial properties against blast disease would be the breakthrough in eco-friendly management of the blast disease in millets.

4.4 Application of biotic stress mitigants (chemical/biological)

Rice blast disease, caused by Magnaporthe oryzae, is one of the most devastating diseases worldwide. Many excellent chemicals for this particular disease viz., blasticidin S, kasugamycin, iprobenphos (IBP), edifenphos (EDDP), isoprothiolane, ferimzone and metominostrobin have been developed. Present reports of resistance towards fungicides prioritize now as the high time to focus our interest in non-fungicidal disease controlling agents since they are supposedly specific to target organisms along with least resistance development problems. Actually, two groups of non-fungicidal rice blast chemicals are currently on the market; melanin biosynthesis inhibitors (e.g., fthalide, tricyclazole, pyroquilon, carpropamid, diclocymet and fenoxanil) and the so-called priming effectors or plant defense activators (probenazole, acibenzolar-S-methyl and tiadinil) which induce host resistance against the pathogen’s attack [65]. Helvolic acid, a terpenoid molecule extracted from the yeast Pichia guilliermondii. Isolated from the medicinal plant Paris polyphylla, was found to strongly inhibit M. oryzae spore germination. Cryptocin, isolated from the fungal endophyte Cryptosporiopsis quercina, which colonizes the inner stem bark tissue of Tripterygium wilfordii, similarly affects M. oryzae [66]. In comparison with the synthetic fungicides now in use, these plant extracts are non-hazardous, ecologically safer, locally available, renewable and easily accessible when used to manage rice blast disease.

4.5 Application of genome editing tools to improve disease resistance in millets

Of late development of cultivars employing genetic engineering technologies is gaining importance in plant biology and stress physiology. Understanding the mechanisms that regulate gene expression and the ability to transfer essential genes from other organisms into plants will broaden the ways in which plants can be utilized. In the near future, the employment of innovative approaches combining physiological, biochemical, molecular and genetic techniques could yield great results. Considering the geographical area of cultivation, available resources and the expertise of native researchers, millet genome editing is expected to progress at a slow pace. Due to limited expertise and infrastructure among millet research labs in developing countries the reach of modern tools like genome editing is delayed. Among the available genome editing tools, the CRISPR/Cas system may play a key role for genome editing in millets due to it user friendly and cost-effective construct design [67].

4.6 Multi-omics approaches

Most of the present information regarding the genome of millets is obtained through comparative genomics application with rice. With the availability of Eleusine coracana’s high-quality genome sequence, it will be possible to locate new selection targets and utilize genomic selection and prediction approaches. This will expedite the breeding process and will allow simultaneous selection for yield, quality and disease resistance. Up to date only draft genome sequences are released for finger millet [68], pearl millet [69] and proso millet [70]. Fully annotated genome sequences are not yet available for these millets. Not even a draft genome sequence is reported for other millets (little millet, barnyard millet and kodo millet) up to now. The cross genera transferability of the reported DNA markers demonstrates their usability in understanding phylogenetics. However, molecular breeding efforts utilizing omics tools and translational research are lagging in most of the millet crops [71]. Using transgenic technology or molecular-assisted breeding, the possible candidate genes found through various transcriptome and proteomic investigations could be used to generate cultivars with better adaptability to endure harsh climatic conditions.

4.7 Monitoring evolving races of the pathogen

In several Indian states, host-directed evolution of pathogenic variation has resulted in severe disease. As a result, understanding pathogen diversity, mode of action and genetics of host plant resistance is critical for developing successful crop improvement methods against biotic stressors. Despite significant progress in managing various fungal diseases in millet, there is still much room for improvement in developing disease management strategies, with a primary focus on virulence monitoring, identification and characterization of newer isolates and development of resistant hybrids/cultivars for commercial cultivation [72].

It is summarized that the blast disease poses a significant challenge to the millets production, now and in the future as evidenced by increase in disease incidence on pearl millet, finger millet and foxtail millet as well as the blast incidence is also observed on other millets viz., barnyard millet and brown top millet at few locations in moderate to severe form. In this chapter, attempts have been made to briefly summarize the key aspects of blast disease caused by Magnaporthe spp. and novel strategies for enhancing Blast disease resistances in millets. This chapter serves as a reference point for pathologists and breeders accompanied in field study in non-exhaustive manner to comprehend the complexity of diseases and to contemplate them in a more holistic manner.