Keywords

4.1 Origin, Distribution and Diversity

Finger millet blast disease is caused by the haploid, filamentous, ascomyceteous fungus M. grisea (anamorph Pyricularia grisea). Blast disease has emerged as an explosive threat to many of the landraces and high-yielding varieties of finger millet and can be able to cause more than 80% yield losses under congenial environmental conditions (Vishwanath et al. 1986). The disease has been prevalent in semiarid regions of Africa including Kenya, Uganda, Tanzania, Rwanda, Ethiopia, Zambia, and Nigeria. In Asia the disease has been reported from India, China, Nepal, Sri Lanka, and Saudi Arabia. Pyricularia grisea was first isolated from crabgrass (Digitaria sanguinalis) almost two centuries ago (Saccardo 1880). In India, finger millet blast was reported for the first time from Tanjore delta of Tamil Nadu (McRae 1920). During 1982, the pathogen was also isolated from rice and named as Pyricularia oryzae (Cavara, Fungi Longobardiae #49). Despite few morphological dissimilarities between them, these scanty variations were not considered sufficient to differentiate them. However, M. grisea was considered to use as a name to represent the Mg. complex as per the rules of nomenclature.

The pathogen can infect more than 50 host species in the family Poaceae, including rice, wheat, pearl millet, foxtail millet, and finger millet (Ou 1985; Rossman et al. 1990). In spite of having a wide host range, pathogen populations exist to adapt to their specific hosts and are capable of infecting a single host (Todman et al. 1994; Viji et al. 2000). However, some researchers have reported that few isolates were successful in cross infection under experimental conditions (Mackill and Bonman 1986; Kumar and Singh 1995) while others failed to confirm the results (Todman et al. 1994). The genus Magnaporthe comprises five different species, viz. M. grisea, M. oryzae, M. poae, M. rhizophila, and M. salvinii, which shared common morphological features such as three-septate spindle-shaped ascospores and black ascoma with long hairy necks. Nevertheless, the phylogenetic analyses using molecular sequence data of actin, calmodulin, and beta-tubulin genes resolved the isolates from crabgrass as well-defined phylogenetic group from rice and other grass isolates. The crabgrass isolates were named as M. grisea and the isolates from rice and other grasses were described as M. oryzae (Couch and Kohn 2002).

4.2 Taxonomy and Biology of Pathogen

Magnaporthe grisea belongs to the family Magnaporthaceae and is an ascomyceteous fungus because it produces ascospores in a sexual spore-bearing cell called asci. The asci are produced within the specialised fruiting structures known as perithecia (Fig. 4.1). The fungi are haploid; mycelium is septate having nuclei within the mycelium.

Fig. 4.1
figure 1

Life cycle of Magnaporthe grisea

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4.2.1 Taxonomic Position

Kingdom: Fungi

Phylum: Ascomycota

Class: Sordariomycetes

Subclass: Sordariomycetidae

Family: Magnaporthaceae

Genus: Magnaporthe

Species: M. grisea

4.2.2 Sexual Reproduction

The teleomorphic stage of the fungi is generally produced if opposite mating type is paired, but the rate of occurrence of sexual reproduction is uncommon. The fungus produces spindle-shaped sexual spores with three septa. The asci are unitunicate and the fungus is heterothallic with a bipolar mating system (single-mating-type locus exists with two alleles). However, the alleles include genes encoding for completely distinct proteins; hence, they are technically not alleles, and are described as idiomorphs (Glass et al. 1990).

4.2.3 Asexual Reproduction

The conidia of the pathogen are pyriform, three celled, and hyaline and are produced on the top of conidiophores. The conidia germinate, producing a thin germ tube, which elongates and differentiates into an appressorium. A slender penetration peg forms at the base, which enters the cuticle and manifests within the host tissue.

4.3 Disease Symptoms and Losses

Finger millet blast is a devastating disease and impacts finger millet production drastically. The significance of this disease is obtained from the fact that the pathogen can minimise yield and grain quality. Maximum losses occur when finger infection starts during flowering or initial stage of grain formation.

Disease is marked by the appearance of tiny lesions on leaves, neck, and fingers. On leaves, typical eyespot-shaped lesions are formed, which are broadened in the centre and tapered at both ends of the spot. These lesions are usually greyish in the centre having dark brown margin. Initially the leaves show chlorosis; as the disease advances these lesions enlarge rapidly and coalesce together leading to drying of leaves. The pathogen is also known to infect neck region resulting in neck rot. If the neck is infected, the above parts of infected neck may become dry resulting in total death of the plant (Sreenivasaprasad 2004). This may cause yield losses up to 90% (Ekwamu 1991). In case the fingers are infected, the seeds get shrivelled and deformed and become chaffy (Fig. 4.2).

Fig. 4.2
figure 2

Disease symptoms of finger millet blast: (a) leaf blast, (b) neck blast, (c) finger blast

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4.4 Disease Epidemiology

Blast disease severity greatly depends upon weather situations, cultivar and infected plant part. A combination of cloudy weather and frequent drizzling, which supports leaf wetness for a longer time with an optimum temperature of 25–28 °C, favours disease development. The presence of blast spores in the air throughout the year, especially in the tropical conditions, favours the occurrence of disease. The pathogen can also survive in soils and establishes better in the soils having high N2 content (Sreenivasaprasad 2004; Hayden 1999).

The primary source of inoculum includes grasses, infected plant debris and infested seeds on the soil. The leftover infested seeds on the soil can produce spores abundantly during the initial stage of the crop which are easily disseminated and deposited on healthy leaves especially under windy conditions. These spores germinate and invade the leaf tissues. Disease extremity is often correlated with the amount of primary inoculum available. The amount of disease at the reproductive stage is influenced by the quantum of disease at the end of vegetative phase of the crop. The secondary spores produced at the end of vegetative phase may infect the neck and cause neck blast as the disease advances; the pathogen may also infect the panicles, leading to finger blast. Panicle blast phase of the disease is considered as the most destructive stage, since the infection at this stage can affect the entire panicle resulting in poor seed setting and seeds can also be infected.

4.5 Genetics of Disease Resistance

Genetic resistance is the best possible way to combat the disease because the crop is largely cultivated by subsistence farmers who cannot afford the disease management through expensive chemicals and fungicides that have shown limited efficacy. Disease resistance is generally governed by specific interaction between resistance (R) gene in the host and corresponding gene which conditions avirulence (Avr) in the pathogen. Approximately, 48 R-genes have been cloned from various plant species including rice, wheat, etc. Among them, a large number of R-genes share maximum similarities with nucleotide-binding site and leucine-rich repeat (NBS–LRR) domain protein sequences.

There is very limited information available on genetics of resistance to finger millet blast disease. However, efforts are being made to identify the markers linked to R-genes/QTL conferring resistance to blast disease in finger millet. Panwar et al. (2011) identified NBS-09711, NBS-07688, NBS-05504, NBS-03509 and EST-SSR-04241 markers which are potentially related to blast resistance gene from resistant finger millet genotypes. Studies have also reported that the genotypes VHC3997, VHC3996 and VHC3930 were found to be highly resistant. The molecular markers linked to R-genes/QTLs can be used for cloning of complete gene, which can be used in the marker-assisted breeding for introgression of the blast resistance alleles in finger millet breeding programmes (Babu et al. 2014).

4.6 Disease Management

4.6.1 Management Through Cultural Approaches

Sanitation of field by removing infected straw and crop debris will not only help to reduce the disease propagules but also avoid the spread of disease. Use of disease-free or certified seeds will reduce the source of primary inoculum which helps to decrease the disease level. Planting time is more important because sowing seeds with the onset of rainy season significantly reduces the early infection of seedlings. Avoid excessive use of fertiliser as it increases the disease. Maintaining optimum plant density is highly recommended, since high-density planting may increase disease development. Weed management plays a crucial role in disease management, since presence of weeds deteriorates crop growth which helps the pathogen for easy infection and frequent weeding may also eliminate alternate hosts of blast pathogen. Intercropping of finger millet with cowpea, groundnut and pigeon pea effectively reduces the disease. Planting of improved varieties such as GPU 28, IE 2911, IE 2957, VHC 3997, VHC 3996 and VHC 3930 with good agronomic practices will significantly reduce the blast disease.

4.6.2 Management Through Bioagents

Biological control is an important component in plant disease management which involves suppression of plant pathogens using beneficial microorganisms without harming the environment. These bioagents exhibit a number of mechanisms such as antibiosis, induced resistance, competition, production of lytic enzymes, HCN and siderophore, which not only suppresses pathogens but also promotes better plant growth. Application of Pseudomonas fluorescens isolate Pf-30 showed more than 80% inhibition of Magnaporthe grisea (Negi et al. 2015). Application of P. fluorescens and Trichoderma harzianum in combination resulted in significant reduction of blast disease incidence caused by Magnaporthe grisea (Netam et al. 2016). The use of bioagents has been considered as an alternative to chemical fungicides; it aims to reduce the dependence on plant protection chemicals and their hazardous effects on ecosystem.

4.6.3 Management Through Fungicides

Seed treatment with tricyclazole at 1 g/kg of seeds will inhibit spore germination and mycelial growth and reduce the spread of disease. Systemic fungicides such as pyroquilon and tricyclazole were found to be the most ideal and effective chemicals to reduce both leaf blast and neck blast under field conditions.

4.7 Conclusion and Future Prospects

Blast disease remains a threat to the production of finger millet worldwide, due to the variability of the pathogen and its capability to overcome the host resistance. The biggest challenge for the finger millet blast disease is lack of identified R-genes and inefficiency of the fungicides when the disease pressure is too high with favourable weather conditions for the development of the disease. This requires proper phenotyping facilities, identification of resistance sources and integrated disease management approaches to reduce yield losses. Besides, there is a need for collaborative research and exchange of germplasm resources throughout the globe to combat this disease.