Spot blotch - an emerging issue in South Asia
Sundeep Kumar 1, Rakesh Singh1, Amit K. Singh1, Satish K. Yadav1, Monendra Grover1, Pooja Sharma2, Sunil Kumar1 and Rajesh Kumar1
1: National Bureau of Plant Genetic Resources, New Delhi, India
2: Shri Vankateshwara University, Vankateshwara Nagar, Gajraula, J.P. Nagar – 244236, India
Corresponding author: Sundeep Kumar
Wheat is the most widely grown and consumed food crop of the world. Importance of wheat as staple food is well known as nearly 35% of the world population depends on wheat and demand for wheat is expected to grow faster than for any other major crop (Pingali et al. 1999). It is estimated that to maintain self-sufficiency in the availability of wheat in India, annual production of wheat and rice together should increase by 2 mt every year. The demand of wheat in 2020 is estimated to be ~87.5 mt (Joshi et al. 2007) to ~109 mt (Sharma et al. 2002; Nagarajan 2005).
During the last 50 years, significant improvement in wheat production and productivity was achieved through exploitation of major genes for traits like dwarfness, photoperiod insensitivity and resistance to biotic stresses (Reynolds and Borlaug 2006). In India, the productivity of wheat has improved, tremendously in the past three decades with the expansion of high yielding dwarf varieties and better use of inputs. However, a quantitative jump in wheat production and productivity is still needed to feed the fast expanding human population despite of drastic changes in world climatic conditions. New biotic stresses like foliar blight has emerged a big constraint for the successful production of wheat in South East Asia (Wiese 1987; Mathur and Coufer 1993). This disease is now expending towards non-traditional cooler regions like North West Plain Zone (NWPZ) of India, which is considered as major contributor of wheat in South Asia.
Main concerns about spot blotch
In the last three decades, there has been a remarkable progress in breeding for rust resistant varieties, which in turn prevented the calamity of rust epidemic in different epidemiological sub-zones of the country and reduced the competition among pathogens (Nagarajan et al. 1984; Bahadur et al. 1994). However, very less progress has been made for other diseases such as foliar blight (Joshi et al. 1983; Singh et al. 1986).
Occurrence: Spot blotch is a disease of importance mainly in warm, humid wheat growing areas where the mean temperature of the coolest month is higher than 17.5˚C (Dubin et al., 1998). It causes serious yield losses to wheat crop in South East Asia (Saari 1998), North and Latin America, Africa (Duczek and Jones-Flors 1993), India (Joshi et al. 2002), China (Chang and Wn 1998) and Brazil (Mehta et al. 1993). More recently, spot blotch has also expanded into the cooler, non-traditional irrigated rice-wheat production areas (ME-1) (Dubin and Van Ginkel 1991; Duveiller and Gilchrist 1994; Van Ginkel and Rajaram 1998).
Host range: Sprague (1950) reported that Bipolaris sorokiniana has a large host range and almost all the plants belonging to family Poaceae come under its host range. Apart from wheat, it infects oat, barley, rye, Phylaris, Agropyron, Pennisetumm, Lollium, Poae, Secale, Setaria etc. (Bakonyi et al. 1998). Some reports indicate its existence in dicot crops such as alfalfa, red and yellow clover (Gourley 1968) and buckwheat (Renfro 1963). The host range of the pathogen is reported to differ from isolate to isolate; some isolates had broader, while other had a narrow range (Misra 1981).
Nature of the pathogen: Foliar blight is reported to be a complex of many pathogenic fungi occurring together or simultaneously at different growth stages of wheat crop (Singh 1993). Collectively the disease is called Helminthosporium Leaf Blight (HBL). The three important leaf blight diseases reported in South Asia are Spot blotch (B. sorokiniana) (Sacc. In sorok) shoem, Tan spot (Pyrenophora tritici var. repentis (Died.) Drechs.) and Alternaria blight (Alterania tritici) (Pras and Prab). However, spot blotch (B. sorokiniana) is of increasing concern in South East Asia (Chaurasia et al. 1999; Joshi et al. 2007; Kumar et al. 2010). In addition, it occurs in warm humid wheat growing environments of Latin America, Africa and China. The symptoms of spot blotch appear as small, light brown lesions which are scattered throughout the leaves and increase in size with stage advancement. Later, these lesions coalesce and change to large spots after a week of infection (oval to oblong and measuring 0.5 to 10 mm long and 3 to 5 mm wide) (Chand et al. 2002).
Variation in Spot Blotch Pathogen B. sorokiniana: Variabilities in the isolates of B. sorokiniana have been morphologically (Maraite et al. 1998) and pathologically (Nelson 1960; Hetzler et al. 1991; Maraite et al. 1998) reported. However, very little information is available on the aggressive pattern of the B. sorokiniana isolates. Pathogenic variability is of crucial significance in disease management where host resistance is the major component. The spot blotch resistance gene(s) in wheat are not known to interact in gene for gene manner, but resistant genotypes are known to show significant reduction in disease development as compared to the susceptible cultivars (Joshi et al. 2002). Hetzler et al. (1991) found differences in disease causing ability of different strains on a set of wheat genotypes, which was further confirmed by Maraite et al. (1998). Recently, Chand et al. (2003) reported that the isolates of B. sorokiniana could be grouped in to five categories based on their morphological and pathological traits.
Aggressiveness: In addition to morphological traits, B. sorokiniana varies in its pathogenicity on gramineous hosts. However, little information is available about aggressiveness of the B. sorokiniana. Sexual reproduction in B. sorokiniana is rare and reported only from Zambia (Raemaekers 1987). As far as variability in asexual population is concerned, it is due to para-sexual recombination (Tinline 1962). However, these results were not verified and other means of variability were not explored. Hetzler et al. (1991) found differences in disease causing ability of different strain on a set of wheat genotypes, which was further confirmed by Maraite et al. (1998). Other workers also showed that isolates of B. sorokiniana possessed pathological variability (Nelson 1960; Hetzler et al. 1991; Maraite et al. 1998). Ten isolates of B. sorokiniana (Cochliobolus sativus) from different geographical regions of Brazil were analyzed for their virulence on wheat cultivars, morphological characteristics, and growth rate on PDA. Variability in cultural characteristics was observed in the morphology and growth rate between the original isolates and the re-isolates. However, no relationship between morphological variability and virulence was observed among the ten-origional isolates (Oliveira et al. 1998). Chand et al. (2003) demonstrated that the five groups of the isolates of B. sorokiniana differed for their morphological appearance.
Variability at molecular level: Traditional methods used to study variability in pathogens were based on morphological, pathogenicity or biochemical tests. These methods distinguish pathogen isolates on the basis of their physiological characters i.e., pathogenicity and growth behaviors. However, these are highly influenced by the host age, inoculum quality and environmental conditions. DNA markers are not influenced by any such conditions and therefore provide more reliable results. Kumar (2003) for the first time established variability among the isolates of B. sorokiniana by using RAPD markers. Jaiswal et al. (2007) identified molecular markers to assist the aggressiveness for different groups of B. sorokiniana isolates.
Yield Losses: Yield losses due to leaf blight are variable, but are important in fields with low inputs and under late sown conditions. The average yield losses due to leaf blight for South Asia have been estimated to be 19.6% (Saari 1998). Yield losses between 20-80% have been reported by Duveiller et al. (1998). The losses due to foliar blight may be to the tune of 100% under most severe conditions of infection (Srivastava 1982; Mehta 1994). The pathogen not only reduces yield, but also reduces germination, seedling emergence and decrease the intensity of roots in the subsequent crops (Joshi 1986). Further, ear length has been observed considerably reduced, which in turn, affected the number of grains, which neither were shriveled nor was the test weight much affected. However, when the disease occurred only at flag leaf, the losses of grain/ear were up to 24.2% (Table 1).
Sources for spot blotch resistance have been identified over the years and broadly their origin fall into three categories: Latin America, China and Wild relatives of the wheat or alien species (Van Ginkel and Rajaram 1998). The Latin American sources are mainly derived from Brazil and may trace back to Italian ancestry. Older resistant Brazilian commercial varieties are BH-1146 and CNT-11 (Mehta 1985). The Chinese sources are mostly materials from the Yangtze River Basin. Early Chinese sources of resistance used at CIMMYT include Sanghai#4, Suzhoe#8 and Yangmai#6 (Van Ginkel and Rajaram 1998).
Thinophyrum curvifolium is used as alien resistance against spot blotch (Villareal et al. 1992; Mujeeb-Kazi et al. 1996). Using these alien sources in combination with Chinese resistance sources, outstanding lines such as Mayoor and the Chirya series were developed. More than 14000 entries of wheat and related wild species, representing 13 genera and 136 species have been studied at PAU, Ludhiana, Punjab (India) where, Aegilops triuncialis, Ae. speltoides, Ae. squarrosa, Ae. triaristata, Ae. cylindrica, Triticum dicoccoides, and T. boeoticum were found promising (Dhaliwal et al. 1993; Singh and Dhaliwal 1993). Chaurasia et al. (1999) evaluated 1387 spring wheat germplasm for tolerance to foliar blight disease and observed that none of the genotype was immune to the disease. However, CIMMYT lines showed more tolerance than Indian lines. Joshi and Chand (2002) evaluated 1407 spring wheat lines for their leaf angle and resistance to spot blotch in F3, F4 and F5 generations and revealed that lines homozygous for erect leaf posture showed significantly lower mean AUDPC than those with drooping leaves. Joshi et al. (2002) observed a wide range of plant height and days to maturity when evaluated for resistance to spot blotch in two crosses of wheat, and indicated that resistance to spot blotch severity was independent of plant height and days to maturity.
Variability for resistance in wheat genotypes: Wheat genotypes have been investigated by wheat workers from time to time and some sources of resistance have been established. Chaurasia et al. (1999) evaluated spring wheat lines belonging to Indian and CIMMYT gene pool and found 43 lines as resistant; CIMMYT lines showed better tolerance than Indian lines. Sources of spot blotch resistance that have been identified over the years broadly falls into three categories; Latin American, Chinese and wild relatives of wheat or alien species (Van Ginkel and Rajaram 1998). The Latin American sources are mainly derived from Brazil and may trace back to Italian ancestry. Older resistant Brazilian commercial varieties are BH-1146 and CNT-1. Chinese source of resistance used at CIMMYT include Shanghai#4, Suzhoe#8 and Yangmai#6. Thinophyrum curvifolium has also been used as alien resistance sources at CIMMYT.
Inheritance studies on resistance to spot blotch are limited and nature of inheritance is still debatable. Reports indicate presence of both monogenic (Srivastava et al. 1971; Srivastava 1982; Adlakha et al. 1984) and polygenic (Velazquez Cruz 1994) types of resistance. However, the experience of wheat workers to achieve partial resistance in breeding populations (Mehta 1985; Dubin and Rajaram 1996) suggested polygenic type of resistance.
Genetics of Resistance: Little is known about the inheritance of resistance to spot blotch and is still debatable but seems to be polygenic with additive effect (Mehta 1985; Velazquez Cruz 1994). Indian resistance sources have shown one or two genes to be involved as previously reported (Srivastava et al. 1971; Srivastava 1982; Adlekha et al. 1984). Velazquez Cruz (1994) found six genes to be segregating in four moderately resistant to resistant lines (Ginuz, Cugap, Chirya and Subaf) developed at CIMMYT with two or three genes providing good resistance level. The evaluation of large number of genotypes (T. aestivum and T. durum) and triticale over the year has clearly established that majority of the durum wheat genotypes (AB genome) are highly susceptible to leaf blight pathogen. On the other hand, the common wheat genotypes (ABD genome) fall under the category of susceptible to moderately resistant. Most of the triticale genotypes (ABR genome) seem to be moderately resistant to resistant. These data indicated that in common wheat, resistance is probably located on D genome and in case of triticale on the R genome (Chand et al. 2002).
Singh et al. (1986) studied 102 genotypes for resistance against spot blotch. Out of 102, 13-showed resistance and the segregation in all crosses followed 1 (resistant): 15 (susceptible) ratio indicating that inheritance of resistance to foliar blight was controlled by two major recessive genes.
Information on the inheritance of spot blotch resistance in wheat was determined in field studies conducted in four wheat crosses, each involving a Chinese hexaploid parent with high levels of resistance and a commercial cultivar with low to intermediate levels of resistance (Sharma et al. 1997). Data were recorded in the F2, F3 and F4 generations to estimate heritability, involving 150 lines in each cross. Heritability (h2) estimates for spot blotch resistance was intermediate to high when measured in terms of HDS (Highest disease score) (0.47 < h2 < 0.67) and AUDPC (0.58 < h2 < 0.77) in both the F3 and F4 generations in each of the four crosses. Heritability values were somewhat higher for AUDPC than HDS. The results suggest that selection for resistance to spot blotch could be effective in the segregating populations generated from hexaploid wheat parents having different level of resistance (Sharma et al. 1997).
Breeding Progress: Several attempts have been made to check the spot blotch disease but no single effective control measure has been worked out so far. An integrated approach is considered necessary with host resistance as a major component to control the disease (Joshi and Chand 2002). However, in some instances resistant varieties are the only means of controlling this disease.
The slow progress in breeding for resistance to foliar blight has been due to several reasons. The pathogen is highly variable and aggressiveness seems to increase over time (Maraite et al. 1998); no immunity is known against spot blotch in wheat (Van Ginkel and Rajaram 1998) and quantitative and qualitative resistance genes are involved (Sharma et al. 1997), making it difficult to assess disease levels effectively except under strictly standardized experimental conditions (Duveiller et al. 1998). Even the sources of resistance against foliar blight are limited (Dubin and Rajaram 1996; Chaurasia et al. 1999).
However, enhancement of genetic resistance to foliar blight in breeding population remained slow and unsatisfactory (Rajaram 1988; Duveiller et al. 1998). This slow progress is generally attributed to high variability in the pathogen originating from diverse sources (Wood 1962; Kleine and Nelson 1963), their shift towards aggressiveness (El Nashaar and Stack 1989) and quantitative nature of inheritance (Sharma et al. 1997). Nevertheless, there are reports concerning differential interactions between the pathogen population and wheat genotypes (Mehta 1981; Hetzler et al. 1991). Geographically distinct racial patterns in pathogen populations (Hetzler et al. 1991; Hossain and Azad 1992) indicated a strong influence of specific environmental conditions on the expression of resistance and/or susceptibility. These studies suggested that breeding programs should be planned at regional levels, where environmental conditions are likely to be homogenous.
Sharma el al. (1997) investigated four wheat populations involving different Chinese hexaploid parents with high level of resistance and a commercial cultivar moderately resistant to spot blotch to observe the response to selection for low and high area under disease progress curve (AUDPC). Selections were made in the F3 generation for law and high AUDPC of spot blotch and selected progenies evaluated in a replicated field test. Low AUDPC resulted in higher biomass and grain yield, higher harvest index, and higher 1000-grain weight. Results indicated that selection for low audpc of spot blotch in segregating generations would be effective in identifying wheat lines with high levels of resistance and would have positive effects on other characters.
Beek (1986) reported that it is possible to increase quantitative resistance to foliar blight and yield potential simultaneously. Transgressive segregants for spot blotch resistance due to the recombination of diverse resistance genes of the parents have been reported in crosses involving moderately resistant genotypes (Sharma et al. 1997; Joshi and Chand 2002), which could be utilized in breeding program.
Joshi et al. (2002) reported that resistance to spot blotch severity was independent of plant height and days to maturity. Therefore, they suggested that for better evaluation of resistance, breeding population should have susceptible checks of different maturity and height groups. They also suggested that if scoring for disease severity is combined with growth stage, selection for resistance in the segregating generations will be more effective. Joshi and Chand (2002) concluded that genes for spot blotch resistance must be combined with those for erect leaf morphology for better control of spot blotch of wheat.
In the last few years, genotypes having good resistance have been obtained through recurrent selection and recombination of the best genotypes (Dubin and Van Ginkel 1991; Duveiller and Gilchrist 1994; Dubin and Rajaram 1996). CIMMYT strategy for spot blotch is to breed for general resistance based on historically proven stable genes. This non specific resistance can be further diversified by accumulating several minor genes and combine them with different specific genes to provide a certain degree of additional genetic diversity (Van Ginkel and Rajaram 1998).
Marker assisted selection for spot blotch resistance: The association of leaf tip necrosis with spot blotch resistance was established by Joshi et al. (2004). Later, stay green trait was also showed positive linkage with spot blotch resistance (Joshi et al. 2007). These morphological markers are successfully being used in markers assisted selection of spot blotch resistance germplasm lines.
QTLs for spot blotch: Although, many reports of tagging and mapping of several disease resistance genes and QTLs are available in wheat (Langridge et al. 2001) however, not many reports are available for spot blotch. For this disease, the association of resistance with microsatellite markers in bulks of susceptible and resistant progeny lines was reported (Sharma et al. 2007). The QTLs for spot blotch resistance in the Chinese wheat variety, ‘Yangmai 6’ were mapped on chromosome 2A, 2B, 5B and 6D (Kumar et al. 2009). However, more information with respect to the identification of QTLs in different genetic background was generated when QTLs were mapped in two other resistance sources (‘Ning 8201’ and ‘Chirya 3’) and to compare the chromosomal locations of QTLs with ‘Yangmai 6’ to identify diagnostic markers that can be used for marker assisted selection and to make an effective breeding program (Table 2).
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