Keywords

3.1 Introduction

Finger millet [Eleusine coracana (L.) Gaertn.] originated in East Africa and is a nutrient-rich staple food for millions of people in the semiarid tropics of East Africa. It plays a crucial role in the subsistence farmer’s economy and diets. It is world’s fourth-ranked millet crop after sorghum, pearl millet, and foxtail millet. Finger millet generates sustainable income for millions of poor people in the semiarid regions of Eastern and Southern Africa, as well as South Asia. Finger millet is estimated to occupy 12 % of global millet area and accounts for cultivation in more than 25 countries in Africa and Asia. Uganda, India, Nepal, and China were stood as major producers. In the western Uganda and Ethiopian highlands, the crop was known to be domesticated around 5000 years BC and from there around 3000 BC, it reached to the west cost of India (Hilu et al. 1979). Finger millet is an essential food especially for the rural populations of Southern India and East and Central Africa. It can be grown under a wide range of climatic conditions ranging from plain areas to hilly regions of Himalayas but the crop performs better under well-drained, loamy soil. It is a highly productive crop that can survive well under various abrupt climatic conditions, and it can also grow as an organic crop. It has the capability to grow on low-fertile soils and is less dependent on the use of chemical fertilizers; hence, it is a boon for the huge arid and semiarid regions which can be grown by resource-poor farmers (Gull et al. 2014). Karnataka, Odisha, Maharashtra, Tamil Nadu, Andhra Pradesh, Uttarakhand, Uttar Pradesh, and Bihar are the major finger millet-growing states in India. It is cultivated at a wide range of altitudes in parts of Andhra Pradesh and Tamil Nadu to about 2400 m above sea level in hilly areas in northern India (Upadhyaya et al. 2007). Among the minor millet produce, finger millet constitutes about 81 % in India (Latha et al. 2005). About 60 % of finger millet is produced by the state of Karnataka which constitutes about 34 % of global production. Global warming has the major impact to cause a vicious cycle of disturbance in global ecosystems which have mostly negative effects on local natural fauna and flora. Impact of climate change has the potential to influence disease management aspects in various ways.

Finger millet is affected by various biotic and abiotic constraints. Among biotic constraints, blast disease caused by Pyricularia grisea Sacc. (teleomorph: Magnaporthe grisea (T. T. Hebert) M. E. Barr) is the prime catastrophic disease that causes substantial grain and forage yield losses. It is a serious problem in major finger millet-growing areas of Madhya Pradesh, Tamil Nadu. Karnataka, Kerala, and Maharashtra which causes heavy losses to the crop almost every year. It has also been recorded from other countries like Uganda, Tanganyika, and Malaya. The yield loss was reported by different workers from 50 to 100 % (McRae 1922; Venkatarayan 1946; Jegan et al. 2018). Rath and Mishra (1975) reported that neck infection causes great loss in grain number and grain weight and also spikelet sterility was increased significantly. Pall (1977) reported that neck infection resulted in considerable loss in panicle length, grain number, and grain weight. Management of blast disease is very challenging depending mainly on chemical fungicides like organophosphorus fungicides which are appreciatively effective (Kumar and Kumar 2011; Magar et al. 2015). However, excessive use of chemical fungicides has showed the development of pesticide-resistant fungal pathogens, with negative effects on the ecosystem like soil fertility and water quality and leading to dangerous health problems including birth defects (Hawkins et al. 2014; Hollomon 2016).

3.2 Distribution and Its Occurrence

Blast disease was spread to almost all the finger millet-growing regions of the world affecting different aerial parts of the plant starting from seedling till maturity. The disease is known to appear in India (Mc Rae 1920), Srilanka (Park 1932), Nepal (Thompson 1941), Malaya (Burnett 1949), Tanzania (Kuwite and Shao 1992), Somalia (Mohamed 1980), Zambia (Muyanga and Danial 1995), Ethiopia, Kenya, and Uganda (Dunbar 1969; Adipala 1992). In India the disease is prevalent in most of the finger millet-growing areas, viz. Karnataka, Tamil Nadu, Maharashtra, Andhra Pradesh, Orissa, Bihar, and Uttaranchal. The disease was first time reported in India from Tanjore delta of Tamil Nadu by Mc Rae (1920). It was subsequently reported from Karnataka (Venkatarayan 1937), Andhra Pradesh, Orissa, Bihar (Thirumalachar and Mishra 1953), Assam (Roy 1989), etc. Ramappa et al. (2002) registered up to 50 % neck blast and 70 % finger blast in Mandya and Mysore districts. The major blast hot-spot locations identified from India are shown in Table 3.1 (Anon 2018, 2019).

Table 3.1 Major finger millet blast hot-spot locations of India
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3.3 Pathogen Variability

The development of disease-resistant varieties/cultivars and knowledge on the pathogen population structure, such as the type of variants/haplotypes present in a location and the extent of variation, are very much essential for plant breeders to develop suitable resistant variety at a specific location. Therefore, specific delineation of pathogenic variability in the target production area is a primary factor for identifying finger millet genotypes with a stable resistance to the highly variable pathogen populations. From an ecological, epidemiological, and breeding perspective it is important to know how genetic diversity is maintained and how new, well-adapted highly virulent races evolve in the pathogen population. The frequent resistance breakdown mechanism of blast-resistant cultivars and studies on the extent of genetic diversity present in the population of M. grisea in a particular geographical region are effective (Levy et al. 1993). There is insufficient information available (Kumar et al. 2007) on the development of an uncertain set of differentials for assessing the racial differentiation for finger millet blast pathogen. Comprehensive work has been carried out with rice blast and detailed pathogenic variation has been described from single spores originating from single lesions and monoconidal subcultures (Ou and Ayad 1968; Ou et al. 1970).

Earlier evaluation of genetic diversity of M. grisea from various crops mostly relied on MGR-based restriction fragment length polymorphism (RFLP), which is a costly and time-consuming technique. The most commonly used DNA-based markers include randomly amplified polymorphic DNA (Williams et al. 1990; Welsh and McClelland 1990), amplified fragment length polymorphism (Vos et al. 1995), and sequence characterized amplified region (SCAR) markers (Soubabere et al. 2001). These markers are PCR based and any sequence information is not required; it is a speedy means to generate molecular markers but provides several genomic fragments with a marker in the single experiment (Varshney et al. 2007). However, these molecular markers are not locus specific, whereas RAPDs suffer with reproducibility. Microsatellites or SSR markers are random repeat DNA sequences present throughout the eukaryotic genome and on the other hand represent the locus-specific, highly polymorphic, multi-allelic, and codominant marker systems which have been proved to be the markers of choice in plant genetics and breeding applications (Gupta and Varshney 2000). Generation of SSR markers is a time-consuming, cumbersome, and expensive task. Several SSR (Brondani et al. 2000; Kim et al. 2000; Kaye et al. 2003; Suzuki et al. 2009) and minisatellite markers (Li et al. 2007) have already been developed for M. grisea. Dobinson et al. (1993) recognized a retro element in strains of M. grisea that infects finger millet and entitled it as grasshopper (grh). M. grisea isolates of rice and finger millet gathered from southern parts of India were characterized by MGR-DNA fingerprinting (Viji et al. 2000) and they also reported that the blast fungus did not cross-infect which was collected from these two hosts and also exhibited different fingerprint patterns. Takan et al. (2004) described that isolates causing leaf, neck, and panicle blast on finger millet compared by AFLP analysis were genetically similar indicating the same strains having the ability to cause different expressions of blast under suitable conditions. Degree of sexual compatibility that exists between rice and finger millet strains of M. grisea is high and there is great possibility of gene flow among these two host-limited populations of the pathogen (Rathour et al. 2004).

The population structure of rice, finger millet, jungle rice, goosegrass, and crabgrass infecting isolates of M. grisea from the north-western Himalayan region of India was analyzed using isozyme and native protein. Among different host-limited pathogen populations including those infecting rice and clustered in accordance with their host specificity, it showed a high level of genetic diversity. Subpopulations of the pathogen attacking rice and weeds were in the same field and were genetically distinct and there was no gene flow among rice and non-rice isolates of the pathogen (Rathour et al. 2006). Zheng et al. (2008), based on the released complete genome sequence data of M. grisea, developed polymorphic SSR markers (Dean et al. 2005) and constructed a genetic map consisting of 176 SSR markers. Sonah et al. (2009) noticed the high level of genetic variability through PCR-based RAPD analysis of M. grisea isolates from different non-rice and rice hosts and isolates from same location were grouped together irrespective of the crop from which infected samples were collected. Tanaka et al. (2009) examined the population structure of Eleusine isolates of M. oryzae by DNA fingerprinting with three repetitive elements, MGR586, MGR583, and grasshopper, which revealed that the isolates collected just after an outbreak of finger millet blast in Japan during 1970s had almost identical fingerprint profiles although they were collected in distant prefectures, and supported the idea that the outbreak was caused by seed transmission of a particular strain of Eleusine isolates. Takan et al. (2011) reported stable genetic variation pattern and lack of clonal lineages, with a broad range of haplotypes in 328 isolates of M. grisea from finger millet, rice, and Dactylaria spp. in East Africa.

3.4 Symptoms

The plant is susceptible at all growth stages starting from seedling till the time of grain formation. Based on the stage/plant parts affected the symptoms categorized three types, first symptom include leaf blast, as it is more severe in the tillering stage of the crop. The disease is diagnosed by spindle-shaped spots on the leaves with gray center surrounded by reddish brown margins as the disease progresses; there will be complete drying of foliage showing burnt appearance. The second symptom includes neck blast, as it appears at the time of flowering stage; the typical symptoms appear in the neck region just below the ear head which turns sooty black in color and usually breaks at the point of infection. In early neck infections, the entire ear head becomes chaffy and there is no grain formation. If grain setting occurs, they are shriveled and reduced in size. Neck infection results in great loss in grain number and grain weight and also spikelet sterility increases significantly (Rath and Mishra 1975). Third symptom represents finger blast; it shows infection on individual fingers during flowering stage, infected fingers turn black at the point of infection, the entire finger becomes chaffy, and there is no grain formation (Fig. 3.1). The finger-infected plants mature early compared to normal plants under field conditions.

Fig. 3.1
figure 1

Blast symptoms and conidia of M. grisea (a leaf blast; b finger blast; c neck blast; d, e conidia)

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3.5 Etiology

Blast is caused due to the Ascomycetes fungus P. grisea (Cooke.) Sacc. (formerly P. oryzae Cavara.) anamorph of Magnaporthe grisea (Hebert) Brar. It is a heterothallic, filamentous fungus pathogenic to almost 40 plant species belonging to 30 genera of Poaceae family (Ou 1980; Murakami et al. 2000; Inukai et al. 2006) including Eleusine. Initially, the opinion with regard to the nomenclature of the pathogen was different. The correct identity of the pathogen is still uncertain. Park (1932) reported P. oryzae on ragi in Uganda. In Malaya also P. oryzae has been recorded on this host. The isolate from ragi does not infect rice. Ramakrishnan (1948) recorded it to be a race of P. oryzae. Wallace and Wallace (1948) also reported P. oryzae from Tanganyika on ragi. Wallace and Wallace (1948) classified it to be as P. setariae. Morphologically it is very similar to P. oryzae (Ramakrishnan 1948). The perfect stage of Pyricularia grisea was earlier named as Ceratosphaeria grisea (Hebert 1971). Later Yaegashi and Nishihara (1976) suggested the genus Magnaporthe. Yaegashi and Udagawa (1978) finally gave M. grisea as the perfect stage of P. grisea (Cke.) Sacc. instead of Ceratosphaeria grisea. Chauhan and Varma (1981) reported P. grisea on Eleusine indica from Kanpur, India.

Young hyphae are hyaline and septate, and older hyphae are brown colored. Numerous conidiophores and conidia are formed in the middle portions of the lesions under humid conditions. The upper surface is darker than lower surface. The conidiophores emerge through epidermal cells or stomata. They are septate straight, subhyaline at the tip, and dark colored at the base. The conidia are hyaline, thin walled, and subpyriform. The size of the conidia varies, being 19–31μ × 10–15μ. Each pyriform spore is 2–3 celled, the middle cell being darker and broader than others (Fig. 3.1). They are formed acrogenously one after another, by the sympodial growth of the conidiophore. Conidiophores are simple and septate, with basal portion being comparatively darker. The pathogen produces fertile perithecia under laboratory conditions (Viji and Gnanamanikam 1998).

3.6 Mode of Spread and Survival of the Pathogen

The fungus enters the host tissue by piercing through the epidermal cells or through stomata. The incubation period varies from 4 to 6 days. Intensity of the disease depends on the weather conditions prevailing in a given season. The initial inoculum of pathogen comes from alternate hosts like weeds, collateral hosts, plant debris, and shriveled seeds. Kato and Nishihara (1977) also recorded the survival of fungus on seeds. The pathogen readily infects foxtail millet, bajra, ragi, wheat, barley, oats, maize, and crowfoot grass. Kato and Nishihara (1977) also reported that Pyricularia isolates from E. coracana, E. indica, E. africana, and E. floccifolia were pathogenic to ragi. Isolates from E. coracana, E. indica, Setaria viridis var. minor were pathogenic to Lolium multiflorum, Festuca elatior var. arundinacea, Phalaris arundinacea, Anthoxanthum odoratum, maize, barley and oats. One infected seed could be able to cause an epidemic (Pall 1988).

3.7 Yield Loses

Several workers have investigated on the adverse effect of blast disease on the yield of ragi; immediately after its first report in 1920, McRae (1922) recorded that the loss of grain may amount to over 50 % and subsequently Venkatarayan (1946) recorded 80–90 % loss in yield in erstwhile Mysore state (Venkatarayan 1946); elsewhere blast was observed to destroy more than 10 % ear heads (Anon 1959). Disease survey in Andhra Pradesh, Delhi, Haryana, Madhya Pradesh, Maharashtra, and Karnataka showed that blast causes a serious loss in finger millet crop (Sundaram et al. 1972). Neck infection causes significant loss in grain number and grain weight in different varieties accompanied by significant increase in spikelet sterility. Reduction in spikelet number is less consistent than other characteristics (Rath and Mishra 1975). Pall (1977) reported neck infection root to be the cause for considerable yield loss in panicle length, grain number, and grain weight. The mean yield loss is 46 % in infected ears compared to the healthy ones and there was a loss of 20.9 % in processing, resulting in an effective loss of 63.72 % (Rao 1981).

Rao and Hegde (1987) reported that when the average disease incidence was 7.69 % in the neck and 9.97 % in the finger, the loss due to blast was 29.51 %. In further studies, Rao (1990) observed that the loss in ragi ranged from 6.75 to 87.5 %. A rise of 1 % infection in the neck and finger resulted in a similar increase of 0.32 and 0.084 % in yield losses. In India, the average loss due to blast has been reported to be around 28–36 % (Vishwanath et al. 1986; Nagaraja et al. 2007), and in endemic areas, yield loss can be as high as 80–90 % (Vishwanath et al. 1986; Bisht 1987; Rao 1990). Ragi blast in Himalayan region appears at lower elevation and it was recorded at <1600 m and caused 25–40 % yield loss (Bisht et al. 1997). Cent percent yield reduction was recorded at Rampur, Nepal (Batsa and Tamang 1983). Grain yield losses associated to blast were predicted to be between 10 and 50 % in Kenya. In Uganda, blast incidence (13–50 %) and severity (24–68 %) varied significantly across main finger millet-cultivated areas in the North and East (Takan et al. 2004). Gupta (1997) reported a grain yield loss of 56 % due to blast. Vishwanath et al. (1997) reported the average annual loss due to blast in finger millet to be around 28 % with a range of 15–36 %. Ramappa et al. (2006) reported that different dates of sowing have showed different levels of yield loss. Dagnachew et al. (2014) reported 42 %; Prajapati et al. (2013) 35.78 %; Jegan et al. (2018) 50–100 %; Rao (1990) 6.75–87.5 %; and Kumar et al. (2005) up to 28 % colossal loss annually.

3.8 Disease Forecasting and Epidemiology

Forecasting of the disease occurrence is a very effective tool in the management of several economically important plant diseases. Knowledge on relationships between weather variables and blast disease could be used to strengthen techniques to screen for resistance and to design effective strategies for management of disease. For example, mist was used to provide high relative humidity and leaf wetness that are ideal for initial infection development which is already being used for screening pearl millet for blast resistance (Thakur et al. 2009). In general, favorable conditions for blast disease development were long periods of leaf wetness, high relative humidity, and temperature range of 17–28 °C. These factors are more appropriate with a polycyclic, airborne pathogen like Pyricularia spp.

No serious attempts have been made with regard to forecasting of blast of finger millet. Thomas (1940) observed high incidence of blast on crops sown in June, July, and August; less incidence was observed in May- and September-sown crops and negligible incidence in the crops sown in the remaining months. Fortnightly sowing of susceptible varieties, recording the onset of blast on each sowing, and relating it to weather variables are being done under All India Coordinated Research Project (AICRP) on small millets at Bengaluru center for the past several years. Blast incidence was high on crops sown in August and less severe on crops sown during second fortnight of June. The average minimum and maximum temperatures were 22 °C and 29 °C, respectively, with 85–99 % RH during the growth period (Patel and Tripathi 1998). Kumar et al. (2005) stated that increased neck and finger blast incidence was due to decreased temperature and increased relative humidity; opposite trends were recorded for low blast disease development. September and October months resulted in more than 50 % leaf blast severity in the nursery at 20 days after sowing. During these months, maximum rainfall, higher number of rainy days, high relative humidity, and low minimum temperature were recorded (Ramappa et al. 2006). Low temperature, high RH (>80 %), and high rainfall support blast development (Nagaraja et al. 2010).

The climatic conditions that were more favorable for blast disease development prevailed from first fortnight of July with an average minimum and maximum temperature of around 20 °C and 30 °C, respectively, and relative humidity of >80 % (Bisht et al. 1984). The temperature range of 18–24 °C was more congenial for the development of neck and finger blast in ragi than at other temperature ranges (Chaudhary and Vishwadhara 1988; Gowda and Gowda 1995; Kumar et al. 2005). Ramappa et al. (2006) reported that highest leaf blast severity over 50 % in finger millet was observed in nursery raised in October month probably due to availability of high inoculum pressure coincided with favorable weather conditions, i.e., more number of rainy days, high relative humidity, and low night temperature recorded during the month of October.

3.9 Mechanism of Resistance

The mechanism of resistance in plants towards blast disease is not clearly understood. Susceptibility to blast was found to be positively correlated with protein content and most of the high-yielding varieties were low in proteins (Dineshkumar et al. 1985). Pyricularia infection resulted in increase in protein content of seed and reduced starch and ash. β-Glucosidase activity was greater and glucose content lesser in disease portion of the neck than in healthy tissue (Pall 1992). There is a relationship between total phenols, total tannins, and level of susceptibility to blast fungus (Kumar and Singh 1995). In fact, the brown grain types are resistant to blast compared to white grain (Ravikumar and Seetharam 1993). The total protein and reducing sugar content was more in susceptible variety compared to that in resistant variety, and total phenol and tannin content was high in the resistant variety compared to that in susceptible variety (Somappa 1999). Byregowda et al. (1998) revealed that the resistance/susceptibility appears to be the result of multiple biochemical compounds present in plant and no single mechanism solely accounted for disease resistance. The resistant genotypes consistently had higher levels of phenols and tannins. Interestingly the attributes like phenol, tannin content, and grain yield showed high heritability and high genetic advance representing the role of additive genes (Byregowda et al. 1999a). The resistant genotypes were found to possess thicker leaves, significantly thicker upper epidermis, and a less pronounced reduction in thickness consequent to infection compared to susceptible varieties (Somappa 1999).

The resistance mechanism of leaf, neck, and finger blast has showed that less leaf area, narrow leaf angle, less number of stomata, short plant with better conversion efficiency of photosynthates from source to sink (harvest index), thick epidermis and cuticle on the leaf and neck, fewer chlorenchymatous strands, higher total phenols, and low quantities of total and reducing sugars contributed towards blast resistance in finger millet (Jain and Yadava 2004). Blast disease recorded low heritability and moderate genetic advances indicating the role of nonadditive gene effect (Byregowda et al. 1999b). Neck blast and finger blast were positively correlated with glume cover, seed protein content, and peduncle length and negatively correlated with seed calcium content, days to flowering, and yield, and there was no relationship between grain color and blast resistance (Nagaraja et al. 2010).

3.10 Integrated Disease Management

Ever since the first report on the distribution and existence of blast disease by Mc Rae in 1922, various workers are making concerted efforts from time to time to manage the disease. Many strategies were adopted like resistant genotypes coupled with good agronomic traits. Likewise, there are several highly effective fungicides and biocontrol agents were also identified for management of disease.

3.10.1 Cultural Methods

The incidence of blast is much high in direct-seeded ragi than in the transplanted crop. This may be due to thick plant population in direct seedling which alters the microclimate which is favorable to multiplication and rapid spread of the pathogen (Mishra et al. 1985). Application of increased levels of potassium has decreased blast severity while nitrogen application enhances the incidence of blast severity. Calcium silicate seems superior over sodium silicate in reducing neck blast and finger blast on the test genotypes (Krishnappa et al. 2013). Change in sowing time was useful in avoiding/escaping the incidence of neck and finger blast infection and in turn getting higher grain yields (Nagaraja et al. 2007). Seed treatment gave good control of leaf blast regardless of the cultivar used (Madhukeshwara et al. 2005).

3.10.2 Host Plant Resistance (HPR)

Exploiting host resistance to control disease is not only economical but also a practical necessity in a low-value crop like finger millet where there is a limitation for any additional cash inputs such as fungicides. Development of resistance varieties is the best means of combating the disease, which is predominantly grown by resource-poor and marginal farmers. The success of such programs depends on the identification of durable resistant sources and its subsequent utilization in breeding. The search still continues for sources of high levels of host-plant resistance (HPR). However, large-scale evaluation of germplasm collections against various biotic or abiotic stresses is resource consuming and time consuming. Blast-resistant varieties identified and released for the different finger millet-growing areas of India are tabulated in Table 3.2 (www.aicrpsm.res.in).

Table 3.2 Blast-resistant varieties identified and released for the different finger millet-growing areas of India (Source adopted from www.aicrpsm.res.in)
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3.10.3 Biological Control

Biological control is an alternative to synthetic chemical pesticides and has several benefits to human beings and ecosystem; they can ensure the protection of plants against biotic and abiotic stresses, ensure production of good-quality grains, improve soil fertility, and assure sustainable and safe 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 (Schreiter et al. 2014; Santhanam et al. 2015; Sekar et al. 2016; Cai et al. 2017). However, the performance of bioinoculants, viz. Pseudomonas, Trichoderma, and Bacillus, in the field highly depends on their survival ability to express key traits in the soil without adversely affecting the native soil microbes (Gupta et al. 2015; Thomas and Sekhar 2016; Sharma et al. 2017).

There has been concerted effort to test various biocontrol agents and compounds in the management of blast of finger millet. Gliocladium virens and Trichoderma viride were tested as seed dressers to see their efficacy on blast. Both reduced the leaf blast significantly and were on par with standard seed dressing fungicide, carbendazim (Somappa 1999). Pseudomonas sp. (strain MSSRFD41) showed a 22.35 mm zone of inhibition against P. grisea; produced antifungal siderophores, metabolites, IAA, and hydrolytic enzymes; and solubilized phosphate. Environmental SEM analysis indicated the potential of MSSRFD41 to inhibit the growth of P. grisea by affecting cellular functions, which caused distortion in fungal hyphae. Bio-primed finger millet seeds showed significantly higher levels of germination and seedling vigor index and enhanced shoot and root length compared to check seeds. Cross streaking and RAPD analysis showed that MSSRFD41 is companionable with different sets of rhizobacteria and lived in the rhizosphere. In addition, PLFA analysis showed no significant variation in microbial biomass between the treated and control rhizosphere samples. Field trials showed that MSSRFD41 treatment significantly decreased blast infestation and improved plant growth compared to other treatments. A liquid-formulated MSSRFD41 product maintained shelf life at an average of 108 CFU mL−1 over 150 days of storage at 25 °C. Overall, results from this study revealed that Pseudomonas sp. MSSRFD41, an indigenous rhizobacterial strain, is an effective, alternative, and sustainable resource for the control of P. grisea infestation and growth promotion of finger millet (Jegan et al. 2018). Several workers have reported pseudomonas as bioinoculants with the potential to control phytopathogens and promote the growth of crops cultivated under varied agroclimatic conditions (Yin et al. 2013; Selvaraj et al. 2014; Wang et al. 2015; Jegan et al. 2018).

The influence of bioinoculants for the management of blast disease in the finger millet has shown disease reduction in the range of 16–54 % (Radjacommare et al. 2004; Kumar and Kumar 2011; Waghunde et al. 2013; Negi et al. 2015). Inoculation with the native finger millet strain MSSRFD41 resulted in 8.39 % disease incidence, which was considerably better than other treatments including a chemical fungicide. Many studies have reported that foliar application of pseudomonads has an ability to act at the site of pathogenic infestation by damaging the fungal cell wall and stopped the growth through a network of interconnecting signal resulting in accumulation of defense-related enzymes and proteins via induced systemic resistance (ISR) and systemic acquired resistance (SAR) systems (Bahadur et al. 2007; Vleesschauwer et al. 2008; Spence et al. 2014; Negi et al. 2015; Fatima and Anjum 2017; Yasmin et al. 2017).

Chitinolytic enzyme production by rhizobacteria has been attributed to antagonism against various fungal phytopathogens (Frandberg and Schnurer 1998; Vishwanathan and Samiyappan 1999). These enzymes attack on fungal cell wall and cause lysis by degrading chitin and therefore are considered as one of the most important mechanisms of biocontrol of pathogens. Fluorescent pseudomonads have also been reported to produce antifungal enzymes (Chang et al. 2003; Kohli et al. 2006; Pankaj et al. 2012). Antifungal activity of chitinase and antifungal metabolites produced by P. fluorescens against P. grisea has also been reported (Radjacommare et al. 2004; Ayyadurai et al. 2007). The suppression of ragi blast disease is done by seed treatment and foliar sprays of P. fluorescens even under field conditions (Patro et al. 2008; Kumar and Kumar 2011). Similarly, Karthikeyan and Gnanamanickam (2008) found that fluorescent pseudomonads could suppress 88 % of setaria blast disease under field conditions.

Blast disease was controlled by using biocontrol agents like Trichoderma harzianum (Gouramanis 1997) and Pseudomonas fluorescens. PGPR strains like Bacillus subtilis and B. pumilus have been found to control blast pathogen both via biocontrol and induction of resistance. Streptomyces species were also found to be promising for the management of blast disease (Krishnamurthy and Gnanamanickam 1998). Watanabe (1985) evaluated different Trichoderma sp. against 24 airborne plant pathogens including M. oryzae and found that isolates of T. harzianum and T. viride showed severe antagonism against M. oryzae, and he also found that T. polysporum was a weaker antagonist. The P. fluorescens strains showed inhibitory activity against P. oryzae with 47 and 59 % of mycelial inhibition (Gnanamanickam and Mew 1992). Gouramanis (1997) reported that antagonistic bioagents such as T. harzianum and Chaetomium globosum gave 70–88 % mycelial growth inhibition of P. oryzae. Fengycin produced by Bacillus subtilis was found to produce inhibitory activity against fungi P. oryzae (Joshi and Gardener 2006). Karthikeyan and Gnanamanickam (2008) reported effective strains of bacterial antagonists for M. grisea through laboratory dual-culture method. Goud and Muralikrishnan (2009) studied the antifungal activity of P. fluorescens against Macrophomina phaseolina, Pythium ultimum, and P. oryzae. All three fungi were inhibited by P. fluorescens with inhibitory activities ranging from 50 to 80 %. Hassanein et al. (2009) described that Pseudomonas sp. had the ability to produce secondary metabolites such as antibiotics, ammonia, and cyanide. Hajano et al. (2012) tested six biocontrol agents, viz. T. harzianum, T. polysporum, T. pseudokoningii, Gliocladium virens, Paecilomyces variotii, and P. lilacinus, against M. oryzae. Maximum mycelial inhibition was observed in P. lilacinus followed by Trichoderma spp. The efficacy of T. viride and P. fluorescens against P. oryzae showed growth inhibition of 72 and 78 %, respectively (Arumugam et al. 2013). Bacillus firmus E65 was found to be highly effective in controlling P. oryzae with 53.32 % while P. aeruginosa C32b showed 33.65 % of inhibition of the test pathogen (Suryadi et al. 2013). Pandey and Chandel (2014) studied the efficacy of P. fluorescens against P. oryzae. Maximum per cent inhibition in colony diameter was observed in P. oryzae. Ali and Nadarajah (2014) collected 22 Trichoderma isolates from soil and examined their efficacy against M. grisea of rice. Among the 22 isolates, 9 isolates inhibited the growth of M. grisea by causing 100 % coverage/overgrowth of the 9 cm plates.

3.10.4 Fungicidal Control

Ever since the first report of occurrence of blast of finger millet in India was published in 1920, a number of attempts have been made to control this disease by use of fungicides. The first authentic report of finger millet blast chemical control was by Raju and Rao (1961) who tested five fungicides and stated that Bordeaux mixture (1 %) and copper oxychloride gave best control. Subsequently, Shanmugam et al. (1962) evaluated 15 fungicides who claimed that ceresin lime dust, Dithane Z-78, flit 406 Bordeaux mixture (1 %), wettable sulfur, and zineb were used as foliar sprays. Out of the nine fungicides tested, ceresin lime mixture spray reduced blast and increased yield by 20.5 % (Vijayan and Natarajan 1967). Desh and Mohanty (1969) related the relative efficacy of different fungicides and antibiotics. Keshi and Mohanty (1970) evaluated fungicides for their efficacy to control blast and found brestonol to be effective. Benlate was found to be highly efficient in checking neck infection (Deshkar et al. 1973). Sprays with ceresin lime dust, Bla-s, zineb, and edifenphos resulted in a corresponding yield increase of 36.8, 26.3, 23.5, and 16.9 % (Sivaprakasam et al. 1974). Further on, in their continued study on control of blast of ragi they found miltox and zineb to be highly efficient (Sivaprakasam et al. 1975).

Among several chemicals, iprobenfos (IBP) was best in both controlling disease and enhancing the yield (Mohan and Jairajan 1986). With the arrival of fungicides with indirect effect like tricyclazole, the disease control was much more effective. P. grisea was highly sensitive to carbendazim followed by thiophanate methyl, edifenphos, kitazin, mancozeb, etc. (Kumar and Singh 1995). Many fungicides are used against blast disease, including benomyl, iprobenfos, pyroquilon, ferimzone, diclocymet, carpropamid, and metominostrobin (Kato 2001). Viswanathan and Narayanasamy (1991) reported that tricyclazole was effective at 200 mg/L in vitro against P. oryzae Cav. Anwar et al. (2002) observed that mancozeb exhibited excellent control of rice blast disease caused by M. oryzae. Mohan et al. (2011) evaluated different fungicides against P. grisea. Among those, tebuconazole, propiconazole, difenoconazole, tricyclazole, and azoxystrobin + difenoconazole were found significantly effective over others. Among the five fungicides, viz. thiophanate-methyl, carbendazim, fosetyl aluminum, mancozeb, and copper oxychloride, used against M. oryzae, only mancozeb was a highly effective fungicide that completely inhibited the mycelial growth of the pathogen at 1000 and 10,000 ppm (Hajano et al. 2012).

3.11 Conclusions and Future Prospects

From the previous conversation it is apparent that finger millet is grown in a variety of agroecological situations and it is known for resilience and drought-enduring capacity. It is relatively less prone to pests and diseases. However, climate change has impacted various biotic and abiotic constraints that limit production and productivity of small millets as well. Among the various biotic constraints, blast caused by M. grisea is widespread and devastating. Several researchers worked on identification of potential resistance source, effective biocontrol agents, and promising fungicides and other aspects of the disease. However, to deal with the impact of climate change on crop production, more emphasis could be given on developing cultivars tolerant to disease through marker-assisted selection (MAS), heat and salinity stress, resistance to flood and drought, modifying crop management practices, adapting new farm techniques such as resource conservation technologies, crop diversification, integrated disease management, better weather forecasting, crop insurance, and harnessing the indigenous technical knowledge of farmers which in turn results in increased production and productivity of the crop.