1.7.1 Blast pathogen variability and distribution
Out of the sixteen fungal, three viral, and one bacterial pathogen reported to infect finger millet (Rachie and Peters 1977) blast caused by Magnaporthe grisea (Herbert) Barr is the most destructive (Pande, 1992). Blast is endemic to all finger millet growing sub-regions of Asia and Africa (Seetharam and Ravikumar, 1994). Roumen et al. (1997) and Takan et al. (2012) have reported on the genetic variability of the blast pathogen isolates from both rice and finger millet. Studies by Takan et al. (2012) on genetic diversity of East African blast populations in finger millet found a wide range of haplotypes with a continuous genetic variation pattern and a strong sexual reproductive potential. Srivastava et al. (2009) found high probability for male and female sterile M. grisea isolates in pathogen populations in finger millet from southern and northern India but detected probability for sexual reproduction in the populations of M. grisea from central Himalayas. Earlier reports by Uddin (2000) had found sexual reproduction to be rare in the blast pathogen in rice. Takan et al. (2012) also found common haplotypes in Kenya and Tanzania. According to McDonald and Linde (2002), pathogens with a mixed reproductive system as reported in finger millet pose the greatest risk of breaking down resistance genes. On the basis
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of presence or absence of the grasshopper DNA repeat element (grh) found only in Asian haplotypes, Tanaka et al. (2009) suggested that Eleusine isolates could be divided into two genetically distinct subgroups viz. African and Asian types. However, Takan et al. (2012) detected a few haplotypes in East Africa carrying the grh element and they attributed this to recent germplasm exchanges. Pande (1992) and Takan et al. (2004) also found isolates causing leaf, neck and finger blast to be genetically similar suggesting the role of the same strain in different blast types. In India Ramakrishnan (1948) found similarity in morphological characters among four isolates of M. grisea from rice, finger millet and Digitaria indiginata. Pande et al. (1995) reported M. grisea to be seed borne but the pathogen was confined to the pericarp and not in the embryo. The findings by Takan et al. (2012) on the predominance of sexual reproduction provides a new dimension and calls for a rethink on breeding strategies to counter the rapid evolution of the pathogen that would otherwise lead to quicker breakdown of host resistance.
Breeding for blast resistance would therefore have to focus on horizontal resistance to counter pathogen variability. Success in managing blast using genetic resistance will also have to be complimented by effective control of blast pathogen host weeds, debris management and appropriate finger millet seed treatment.
1.7.2 Conditions for blast disease development and disease symptoms
The source of blast infection in the field is from seed and previous seasons’ crop debris. Many weedy relatives like E. indica and E. africana, Digitaria spp., Setaria spp. and Doctylocterium spp. are alternate hosts of the blast pathogen and play an important role in disease epidemiology since these serve as primary sources of inoculum (Sreenivasprasad et al., 2007). The seed borne nature of the pathogen is largely confined to the pericarp (Pande, 1992; Hayden 1999). Blast infection is promoted by cloudy skies, frequent rain and drizzles, which support accumulation of dew on leaves for a long time. The rate of sporulation increases with increasing relative humidity >89% and for pathogen germination, the optimum temperature should be 25-28 °C. The fungus also establishes better on plants grown in soils with high levels of nitrogen (Sreenivasaprasad et al., 2007, Hayden, 1999). Nitrogen supply influences branching and leaf expansion leading to a large canopy that is conducive to spore transfer and pathogen infection (Kurschner et al., 1992). Finger millet blast is characterized by the appearance of lesions on the leaves, nodes and heads. On the leaves, lesions are typically spindle-shaped, wide in the centre and pointed towards either end [Figure 1.7-1 (i)]. Large lesions usually develop a grayish centre, with a brown margin on older lesions. Under blast disease-conducive conditions, lesions on the leaves of susceptible lines expand rapidly and tend to coalesce, leading to complete drying of infected leaves. Resistant plants may develop minute brown specks, indicative of a hypersensitive reaction. When the area between the leaf
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blade and leaf sheath (leaf collar) is infected, the collar turns brown and dies. The fungus also attacks the neck (section between the ligule of flag leaf and the base of the inflorescence) causing neck rot [Figure 1.7-1 (ii)]. When the neck is infected, all parts above the infected section may die (Sreenivasaprasad 2004). When this occurs, yield losses may be large because grain formation is inhibited and/or formed grains may be shriveled. In such cases yield losses may be as high as 90% (Ekwamu, 1991). The panicle phase of the disease is the most destructive causing non-formation of grain or poorly filled shriveled grain (Pande, 1992; Takan et al., 2012). One, several or all fingers could be affected [Figure 1.7-1 (iii)]. The fungus infects the panicle as seeds form causing gray brown lesions on the fingers.
(ii)
Figure 1.7-1: (i) Leaf blast (ii) Neck blast and (iii) Finger blast symptoms
The major finger millet production agro-ecologies in East Africa fall within the sub-humid environments where conditions favour blast pathogen development and wild relatives of finger millet are present as alternate hosts of the pathogen. Several options are available for the management of blast among them:
use of clean seed; use of optimum plant populations (very high densities enhance blast development due to high dew accumulation on leaves); weed management to eliminate alternate hosts; planting resistant cultivars; and chemical control using fungicides e.g. Benlate, Bavistin, Dithane M45 and Mancozeb (Prqdhanang and Abington, 1993; Bua and Adipala, 2008). Chemical blast control, though effective to a reasonable level, is a very expensive option for the resource poor farmer and use of resistant cultivars (where available) is the most viable and cost effective approach for blast control in finger millet. So far a few cultivars with blast resistance have been identified through selection and released in the region and more need to be developed to manage the pathogen variability as reported by Takan et al. (2012).
Screening the sub-regions germplasm needs to be a priority to identify blast resistant sources for breeding and direct utilization.
(i) (iii)
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1.7.3 Blast screening
The key to disease resistance breeding is dependent on availability of sources of resistance to the disease.
These sources could be cultivated genotypes or wild relatives of the crop. Blast resistance is often evaluated in the field in hot spot areas under natural infection (Nagaraja, 2010). This sometimes provides opportunity for escapes leading to spurious resistance being identified (Thakur et al., 2009). To avoid disease escape artificial inoculation either in the field or in the greenhouse is carried out. However field inoculation is more appropriate as it helps the breeder know whether the results from artificial inoculation are consistent with those obtained under natural infection conditions (Lübberstedt et al., 1999). Blast research through host resistance in East Africa was reported in Uganda as early as 1958 when a finger millet line, Mozambique 359 was used as a source of resistance in a breeding program to transfer resistance to local Uganda lines (Rachie and Peters, 1977). Subsequent research activities have seen several blast resistant lines developed for example Gulu E, Seremi 1, Seremi 2, Pese 1, SX 8, and SEC 915 and subsequently released in East Africa (Lenné et al., 2007; ICRISAT, 2013). Several authors have also reported on blast resistant sources in finger millet germplasm evaluations. Somasekhara et al. (1991) found no cultivar to be resistant to leaf blast but identified line IE 1012 to be immune to neck and finger blast under field screening. Fakrudin et al. (2000) identified lines IE 2912, IE 2885 and IE 2912 to be resistant to both leaf and finger blast. Partial or slow blasting resistance has also been reported in both finger millet and rice (Parlevliet, 1988; Pande, 1992; Sunil and Anilkumar, 2003; Wu et al., 2005). Slow blasting cultivars were found to have low levels of neck and finger blast. Partial resistance has been reported to be horizontal thus long lasting and more stable (van Der Plank, 1963). Several plant qualitative traits have also been associated with blast resistance. Pigmented plants with compact panicles and dark seed have been associated with blast resistance (Pande, 1992; Takan, 2004; Krishnappa, 2009a).
Reports by Takan et al. (2004) and Obilana and Manyasa (2002b) from surveys in Uganda and western Kenya indicated that cultivars with dark coloured seeds and compact heads had less blast than lighter coloured and open headed cultivars. Plant pigmentation (reddish brown or brown pigments) has been useful in crossing as a marker in identification of F1 plants (Krishnappa et al., 2009b). The identification of blast resistance through germplasm screening in Uganda is an indication of the existing potential of the region’s finger millet germplasm which needs to be exploited for enhanced finger millet productivity.
More blast resistant sources need to be identified to counter chances of resistance breakdown in the few improved resistant cultivars due to blast pathogen variability.
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