In this article we will discuss about:- 1. Introduction to Spot Blotch Disease 2. Occurrence of Spot Blotch Disease 3. Yield Losses due to Spot Blotch Disease 4. Taxonomy of Spot Blotch Disease Pathogen 5. Symptoms 6. Host Range 7. Sporulation 8. Association of Resistance 9. Spread.
Contents:
- Introduction to Spot Blotch Disease
- Occurrence of Spot Blotch Disease
- Yield Losses Due to Spot Blotch Disease
- Taxonomy of Spot Blotch Disease Pathogen
- Symptoms of Spot Blotch Disease
- Host Range of Spot Blotch Disease
- Sporulation of Spot Blotch Disease
- Association of Resistance to Spot Blotch Disease
- Spread of Spot Blotch Disease
1. Introduction to Spot Blotch Disease:
Spot blotch incited by haploid hemibiotrophic ascomycetous fungal pathogen Bipolaris sorokiniana (Sacc.) Shoemaker (teleomorph: Cochliobolus sativus), is one of the most destructive foliar disease of wheat causing severe yield and grain quality reductions during epidemic years.
The spot blotch pathogen affects all above ground plant parts causing enormous losses particularly in areas characterized by high temperature and humidity at the late growth stage such as Eastern India, South East Asia, Latin America, the tarai of Nepal, China and Africa.
Spot blotch has been under investigation since it was first recorded in 1914 by Mohy, but it has recognized as a major concern only recently. Severity of spot blotch has increased many fold after green revolution in many countries of tropical and sub tropical climate where rice-wheat cropping system is commonly practiced.
Due to wide spread losses caused by spot blotch, this disease is considered as number one wheat disease in North-Eastern Plain Zone of India.
2. Occurrence of Spot Blotch Disease:
Spot blotch is common in the Mega Environment 5 (ME5) where humid and warm weather prevails during the growth of wheat crop. Surveys indicated that spot blotch has become a serious disease of wheat in several parts of the world, particularly in those areas characterized by moderate temperature and high humidity during the late growth stage such as eastern India, Bangladesh, Tarai of Nepal and Brazil.
Besides B. sorokiniana, other species may also cause leaf blight of wheat. Mitra (1931) recorded the occurrence of H. bicolor on wheat and observed that H. sativum was the most common parasite of wheat and H. tritici-repentis was associated.
Patel and Kamat (1953) described a new strain H. nodulosum var tritici, causing foliar disease in wheat. Deshpande and Deshpande (1966) observed that, H. atypicum could also cause leaf spot of wheat.
The leaf blight have been recorded in most wheat growing areas of India as reported by Joshi (1978) Nema (1986) and Sharma (1996). H. sativum (B. sorokiniana) has been reported as the most frequently isolated fungus from diseased samples.
The air spores study was done by Nema (1969), at four locations i.e. Gurudaspur, Hyderabad, Kalyani and Pusa revealed that the cumulative counts for H. tritici-repentis, at Pusa, were much higher than H. sativum.
Joshi (1970) analyzed 412 foliar blight samples from different growth stages and observed that, Helminthosporium sativum and Alternaria triticina were the two major pathogens isolated from 125 and 82 samples respectively.
McRae (1992) reported that H. tritici-repentis was relatively more common on wheat in Pusa (Bihar) than H. sativum. Singh and Mukerjee (1981) reported. H. tetramera from U.P. that caused foliar blight of wheat. Chaurasia (1999) reported B. sorokiniana and A. triticina from foliar blight of wheat in North Eastern Plains Zone of India and B. sorokiniana being the major one.
3. Yield Losses Due to Spot Blotch Disease:
Spot blotch of wheat, caused by Bipolaris sorokiniana affects all aerial plant parts and may cause 19.6% and 15.5% yield losses in South Asia and India respectively. In the eastern part of the Indian subcontinent, on-farm studies indicated crop losses up to 16% in Nepal and 15% in Bangladesh.
Grain yield losses between 20 and 80% have been reported by Duveiller and Gilchrist (1994) however some workers reported the losses due to spot blotch may be as high as 100% under most severe conditions of infection. Villareal (1995) reported that diseased plots yielded 43% less than fungicide protected plots in Mexico. Numerous trials estimated the losses to be in excess of 20%, depending on cultivar susceptibility and climate.
Yield losses upto 34.4% in variety Sonalika has been reported by Bhatta (1997). Nema and Joshi (1971), in pot experiments, correlated the reduction in grain weight to the number of spots incited by H. sativum, on the basis of disease intensity per unit area of flag leaf. Prabhu and Singh (1974), using the cultivar NP 830, estimated approximately 60% yield reduction due to the combined infection of A. triticina and H. sativum.
According to Sokhi and Joshi (1974), extent of damage on account of A. triticina may be upto 35% when flag, penultimate and third leaves were badly blighted. Ram and Joshi (1979), under artificial conditions of infection reported the loss in average number of grains per ear upto 26.4% when infection appeared at the tillering stage.
Ear length was considerably reduced, which in turn, affected the number of grains which were neither shriveled nor affected test weight. However, when the disease occurred only at flag leaf stage, the losses of grain/ear were upto 24.2%. Severe infection incited by H. sativum could affect the grain colour and made the grains unacceptable to millers.
4. Taxonomy of Spot Blotch Pathogen Disease:
The spot blotch pathogen was initially named as Helmithosporium sorokinianum Sacc. in Sorokin, Trans. Soc. Nat. Univ. Kazan 22:15 (1890). The name Helminthosporium sativum was given by Pammel King and Blakke (1910) without taking in to account the earlier description of H. sorokiniana sacc. in Sorokin, Trans Soc. Nat. Univ. Kazan 22:15 (1890).
Shoemaker (1959) proposed the generic name Bipolaris for the Helminthosporium spices with fusoid, straight, or curved conidia, germinating by one germ tube from each end (bipolar germination) and renamed the spot blotch pathogen as Bipolaris sorokiniana (sacc.) shoem syn. Drechslera sorokiniana (Sacc.) Subrm and Jain.
Currently the Bipolaris sorokiniana is the valid name of the pathogen and the names H. acrothecioides Lindfons, H. californicum Mackie and Paxton, and Drechslera sorokiniana (Sacc.) Subram. and Jain are synonymous.
The mycelium of B. sorokiniana is usually deep olive-brown. B. sorokiniana produce conidiophores, which may be single or clustered and measure 6-10 x 110-220 µm with septations. The walls of conidia are smooth and noticeably thickened at the septa. Conidiophores bear 1-6 conidia with rounded end and a prominent basal scar.
Conidia are dark olive brown, ellipsoid, mostly straight to slightly curved having 5-9 cells measuring 60-120 x 12-20 µm (Zillinsky,1983). The ascigereous stage (teleomorph) was first observed in the laboratory on the natural media in the presence of opposite mating types and described as Opliioholus sativus by Ito and Kurib.
It was later renamed as Cochliobolus sativus (Ito and Kurib) Drechsler ex Dastur (1942). Teleomorph is characterized by globose ascomata usually with long cylindrical neck.
The teleomorph of B. sorokiniana was reported from Zambia on wheat under field condition. Presence of two different mating types is essential for sexual reproduction. The sexual state formed in culture, was in the form of black, globose pseudothecia (300-400 mm in diameter), with erect beaks (50-200 im long).
Asci were clavate and measure 20-35 x 150-250 µm. Ascospores were hyaline, uniformly filamentous, and spirally flexed within asci. The size of ascospores were 5-10 x 200-250 µm and were 4-10-septate.
5. Symptoms of Spot Blotch Disease:
The spot blotch symptoms typically appears as small light brown lesions mostly oval to oblong to somewhat elliptical measuring 0.5 to 10 mm long and 3 to 5 mm wide. These lesions scattered through out the leaves and gradually increase in size and coalesce and form larger necrotic patches.
Well developed lesions show dark brown margin with light brown center. In susceptible genotypes, spot blotch lesions increase very fast, coalesce and causing the death of the entire leaves.
Symptoms of spot blotch are commonly appears on leaf, sheath, node and glumes, but under most severe conditions of infections, B. sorokiniana also attacks the spikes and produce dark brown to black discoloration around the germinating point of seed, called black point.
Severity of spot blotch is modulated by the abiotic factors such as soil fertility, moisture and moderate temperature. Biotic factors such as virulence of pathogen, leafage and growth stages of the plant also influence the symptoms.
6. Host Range of Spot Blotch Disease:
Bipolaris sorokiniana infects a wide range of hosts belonging to family Poaceae. However, it has also been reported on dicot crops including beans, alfalfa, red and yellow clover and Buck wheat. The host range of B. sorokiniana differs from isolate to isolate.
Some isolates had broader and some had narrow host range. B. sorokiniana infects barley, oat, rye, Triticale, Agropyron, Pennisetumm, Lolium, Poae, Secale, Setaria, Bromus, Festuca, etc. Nema (1986) reported Phalaris minor as a host for spot blotch pathogen.
Chinese isolates were able to infect more than 29 graminaceous hosts. Ma Qixiang and He Jabi (1987) reported 65 graminaceous hosts infected by the B. sorokiniana. Chang Naitao and Wu Yousan (1987) reported Saccharum officinarum, Zizania caduciflora, Paspulam thymbergii, Ischaemum ciliare and Apluda mutica as new host.
7. Sporulation of Spot Blotch Disease:
Duczek (1996) reported that sporulation of B. sorokiniana varied from year to year. They observed that production of conidia by B. sorokiniana was determined on crowns of field grown annual crops. Sporulation was highest on crowns of the annual cereal crops, wheat, barley, rye and triticale.
8. Resistance to Spot Blotch Disease:
I. Association of Resistance to Spot Blotch
i. Resistance with Host Growth:
Spot blotch may occur at any growth stage of the crop therefore it is essential to assess resistance at important growth stage. This will help to simplify the resistant breeding programme by a correlated selection between the respective plant growth stages. The resistant at seedling stage could not be correlated with adult plant.
Therefore, early growth stage test is required to those areas where disease starts from seedling stage e.g. Brazil and Bolivia. In Indian subcontinent foliar blight is problem on wheat onwards tillering. However, the genotypes showed resistant after flowering were also found resistant to seedling stage.
ii. Resistance with Morphological Traits:
Studies on association of spot blotch resistance with morphological or other traits that can help in selection of resistant genotypes is lacking. The most useful parameters for selection are yield and grain weight. Tall and late genotypes appear less diseased.
However, this association was due to the late appearance of susceptible growth stage in the late duration genotypes. Joshi (2002) studied the relationship of plant height and days of maturity with resistance to spot blotch using 1407 wheat genotypes of India and CIMMYT and found that the distribution of disease score ignored the growth stages differed from the distribution in which disease score was assessed on a similar growth stage.
They made the crosses each between tall resistant x dwarf susceptible and late resistant x early susceptible genotypes. The evaluation of homozygous resistant lines in F3, F4 and F5 generations of both crosses showed a wide range of plant height and days to maturity but did not show a significant difference for AUDPC values of spot blotch.
The correlation coefficients for AUDPC verses plant height or days to maturity were wee lie,-0336 and 0.061 respectively. Based on this study they concluded that resistant to spot bloth severity was independent of plant height and days to maturity in progenies from these crosses.
Joshi and Chand (2002) studied the variation and inheritance of leaf angle and its association with spot blotch severity in wheat. They made three crosses (M3109 x Sonalika, HP1808 x K9006 and HD2662 x K9006) between erect and drooping leaves. M3109 was resistant, Sokalika susceptible while the other three lines possessed moderate resistance to spot blotch. Leaf erectness in generation showed partial dominance.
Evaluation of F3, F4 and F5 generations revealed that leaf angle was under the control of few genes that appeared to b close to three. Germplasm lines with erect and semi -erect leaves displayed lower spot blotch severity than those having drooping and semi drooping leaves.
Homozygous erect leaf posture genotypes lines in F3, F4 and F5 generations showed significantly lower mean AUDPC than those with drooping leaves. A positive correlation (0.58) between leaf angle and AUDPC further indicated a positive influence of leaf erectness on severity of spot blotch disease.
Joshi (2004) studied the association of leaf tip necrosis (Ltn) with resistance to spot blotch. A total of 1407 spring wheat genotypes were evaluated for Ltn and resistance to spot blotch for three seasons (1994-1995, 1995-1996, and 1996-1997) under field conditions.
Disease severity of spot blotch at six growth stages under artificially created epidemics showed that 75% of the genotypes showing Ltn (Ltn+) were resistant or moderately resistant, whereas 82% not showing Ltn_were moderately susceptible or susceptible.
Mean spot blotch rating of the Ltn_ genotypes was significantly lower than the (Ltn -)genotypes at all growth stages and the genotype x environment interaction was non significant. To confirm the association of Ltn with resistance, individual F2-derived F3, F4, F5, and F6 progenies from the cross ofthe ‘HUW234’ near-isogenic pair for Ltn were evaluated for spot blotch severity.
In each generation, the Ltn_ homozygous progenie had significantly less disease than those homozygous Ltn. These results confirm that leaf tip necrosis is associated with moderate resistance to spot blotch and can be used as a morphological marker to facilitate selection for resistance.
Joshi (2007a) studied the association of stay green trait with resistance to spot blotch. Mean spot blotch severity ratings of stay green genotypes were significantly lower than those of non stay green genotypes at all growth stages. To prove this trait, two spot blotch resistant genotypes possessing strong expression of stay green trait were crossed with non stay green trait susceptible genotype Sonalika.
Stay green trait in the F1 generation was intermediate and showed absence of dominance. Lines homozygous for stay green trait in F4 F5, F6 and F6–7 generation showed significantly lower mean area under disease progress curve for spot blotch than those with non stay green expression. This study clearly established the positive association of stay green trait with resistance to spot blotch.
II. Genotypic Variability of Resistance to Spot Blotch:
Reaction of wheat genotypes to spot blotch caused by H. sativum showed all the genotypes including the dwarfs were susceptible. Pant (1982), reported 130 resistant lines, to H. sativum out of 171. Chaurasia (1999) screened 1387 spring wheat lines belonging to Indian and CIMMYT gene pool and found that 43 lines showed resistant reactions; CIMMYT lines had better tolerance than Indian Lines.
III. Genetics of Resistance to Spot Blotch:
Inheritance studies on resistance to spot blotch are limited and little is known on the genetics of host response to B. sorokiniana. Reports indicate both monogenic and polygenic type of resistance.
Polygenic control for spot blotch resistance in wheat has been reported by some workers. Indian resistant sources have shown one or two genes to be involved as reported by Srivastava (1971), Srivastava (1982), Adlakha (1984) and Sharma (1986).
Velazquez Cruz, (1994) found six genes of which two to three genes providing good resistance levels, to segregate in four moderately resistant to resistant lines developed at CIMMYT. Srivastava (1981), demonstrated that a single ‘a’ factor was involved in controlling resistance to foliar blight.
The genes carried by the parents were nonallelic. Resistance utilized in Queensland (Australia) has been found to be recessive and usually conditioned by several genes. They showed through F3 populations of some resistant x susceptible crosses that resistance is determined by at least three to four recessive complementary genes.
Indian resistant sources have shown one or two genes to be involved as reported by Srivastava (1971); Srivastava (1982) and Adlakha (1984). Joshi (2004) studied the inheritance of resistance to spot blotch of wheat caused by B.sorokiniana by using three F1 progenies and their families in the segregating generations (F1, F3, F5 and F6) obtained after crossing resistant x susceptible wheat genotypes.
They reported that resistant to spot blotch was controlled by the additive interaction of more than two genes, possible only three.
IV. Heritability of Resistance to Spot Blotch:
Heritability of resistance tend to be low and influenced by the changing environment, as determined by temperature, humidity, inoculum pressure, planting date, etc. Low heritability of resistance and substantial influence of the growing environment along with the unknown variability in the pathogen make breeding for spot blotch resistance a difficult task.
V. Combining Superior Agronomic Performance and Terminal Heat Tolerance with Resistance to Spot Blotch:
Joshi (2007c) evaluated 729 diverse wheat germplasm lines for agronomic performance and tolerance to spot blotch in eight locations of India, Nepal and Bangladesh for 5 years (1999-2000 to 2003-2004). Each year, the number of lines represented a new set of 150 lines that included six common checks and a different local check at each of the eight locations.
One hundred and five lines, 21 in each year, advanced from EGPSN were also tested for 5 years (2000-2001 to 2004-2005) in five locations of South Asia through Eastern Gangetic Plains Yield Trials (EGPYT) to verify spot blotch tolerance and superior yield performance of the selected germplasm.
Many lines yielded significantly more than the best check and possessed high levels of spot blotch resistance under warm humid environments of South Asia. The most promising 25 lines have been listed as sources of strong resistance, with 9 lines better yielding than the best resistant check PBW 343 in fewer days to maturity.
Most of these superior lines represented elite CIMMYT germplasm and around half were derived from Kauz and Veery. The line EGPYT 67, Kauz//Kauz/Star/ 3/Prinia/4/Milan/Kauz, was the best for spot blotch resistance, yield, days to maturity, and 1000 grain weight (TKW).The next two lines in the order of merit were EGPYT 84 (Mrng/ Buc//Blo/Pvn/3/Pjb 81) and EGPYT 69 (Chirya3/Pastor).
The results demonstrate that additional spot blotch resistant wheat genotypes with high grain yield and TKW, and early maturity, have become available as a result of the regional and international collaboration in South Asia.
9. Spread of Spot Blotch Disease:
Several attempts have been made in the past to check the spread of spot blotch disease but no single effective control measure has been worked out so far. Hence, an integrated approach is considered necessary with host resistance as a major component to control the disease. Even incomplete resistance supplemented with fungicide/other control measures can makes it more effective.
The rather slow progress in breeding for resistance to foliar blight has been due to several reasons. The major reason of this is the higher variability in the pathogen and aggressiveness seems to increase over time.
The early efforts to breed for resistance to B. sorokiniana were inadequate as variation for resistance in the hexaploid wheat is limited.
Despite lack of good resistance sources in the hexaploid wheat, breeding progress for partial resistance has been achieved through recurrent selection and recombinations of the best genotypes. Beek (1986) showed that, it was possible to increase quantitative resistance to foliar blight and yield potential simultaneously.
The breeding procedures studied were line selection, single plant selection and bulk seed selection, together with natural selection. Line selection and single plant selection proved to be most efficient. Although resistance was increased through bulk seed selection, negative effects occurred on yields. The progress through natural selection was slow.
Selection for spot blotch resistance in F3 and F4 generations, suggested that selection for low AUDPC would be effective in identifying wheat-lines with high level of resistance and would have positive effect on other characters as well. Ginkel and Rajaram (1998) suggested postponement of strong selection pressure to the later generation while Duveiller (1998) suggested that selection for spot blotch resistance must be considered with heat tolerance.
The breeding strategy for spot blotch needs to rely on the infusion of resistance from alien species such as T. taushcii (A. squerrosa) and Thinopyrum curvifolium) and there is need for testing at key sites (at least 5-10) in order to identify suitable parental material and to exchange advanced and segregating materials with good agronomic type and high resistance level.
Association of Spot Blotch Plant Phenotype:
Spot blotch generally influenced by senescent tissue of the plants. Therefore, the late maturing genotypes generally look resistance than early maturing. However, when growth stages were synchronized by staggered sowing and both the genotype gave similar susceptible reaction. Crosses between early susceptible and late resistance revealed random distribution of resistance in both early and late.
The disease severity was less on those genotypes had erect leaf than drooping. Erect leaf creates unfavorable conditions i.e. poor retention of dew, low number of spores deposited and quick drying of leaf. Thus avoid the interaction with the host.
Leaf erectness is controlled by two to three genes with high heritability. Most of the temperature resistance lines were also associated with the spot blotch resistance. Senescence induced due to loss of chlorophyll make plant prone to infection.
Stay green trait was brought through synthetic wheat and offers protection till maturity. Inheritance of stay green traits is controlled by 2 genes. Genotypes with stay green traits are found in all the maturity group. This trait may be very useful for the hot spot of spot blotch.
Markers added Selection:
An intervarietal mapping population in the form of recombinant inbred lines (RILs) developed from a cross between ‘Yangmai 6’ (a Chinese source of resistance) and ‘Sonalika’ (a spot blotch susceptible cultivar) was used for this purpose.
The 139 single seeded descent (SSD) derived F8 lines of Yangmai 6′ x ‘Sonalika’ were evaluated for resistance to spot blotch with 3 replications in different years. Quantitative trait loci (QTL) analysis carried out on spot blotch AUDPC (i.e., mean values across years) revealed three QTLs which altogether explained 56.58 % of the observed phenotypic variance.
Using interval mapping, the QTLs were mapped on chromosome 2BS (R2 = 14.20 %) in the marker interval Xgwml48-Xgwm374, on chromosome 5BL (R2= 30.23 %) in the marker interval Xgwm67- Xgwm371 and on chromosome 6DL (R2 = 12.15 %) in the marker interval Xgwm.732- Xgwm.1103.