In this article we will discuss about:- 1. Source of Inoculum of Bipolaris Sorokiniana 2. Environmental Influence of Bipolaris Sorokiniana 3. Aggressiveness 4. Cytology 5. Mutation 6. Mating Types 7. Population Structure.
Source of Inoculum of Bipolaris Sorokiniana:
1. Seed:
The main source of inoculum of Bipolaris sorokiniana is infected seeds. Shrestha (1977) and Shrestha (1977) reported the presence of Bipolaris sorokiniana in seeds of wheat. Mehta and Igarashi (1981) correlated the high seed infection with higher disease.
Importance of seed transmission was illustrated by Reis and Farcelini, (1993). Reis (1995) reported that seed inoculum alone was sufficient to cause the epidemic within a season depending on climate. Once the spot blotch pathogen has been introduced in to a new wheat growing areas through infected seed, they will survive on wheat residue as long it will be in the field.
2. Crop Residues:
Transmission of Bipolaris sorokiniana through infected seed helps in completion of several secondary cycles that lead to out break of spot blotch. Bipolaris sorokiniana sporulate on necrotic tissues through out the growing season and will reach to the spike and finally move to seeds.
This ensured the most efficient mechanism for survival of pathogen year after year. Sporulation was continued on the residue till it completely decomposed.
Monoculture and zero tillage ensure the presence of stubble for longer period. Pathogen will continue to sporulate on residue and add more inoculum in the soil as free dormant conidia.
Hence, presence of wheat/barley residue is the indication of Bipolaris sorokiniana population. However, in the North Eastern Plain Zone, crop residue exposed to high temperature for 7- 8 hours more than 45 days and after rain it submerged in water.
Survival of Bipolaris sorokiniana could not be detected under NEPZ when it was investigated before sowing wheat. Chand (2002) studied the conidia viability of Bipolaris sorokiniana on wheat straw.
After 4 months, the frequency of individual conidia already present on wheat straw at the time of sampling was reduced and appeared to be progressively replaced by the formation of round structures consisting of conidia aggregates. After 5 months, distinct individual conidia were no longer detected, and only clumps of conidia’ were observed.
These dark black aggregates or clumps of conidia measured 157-170 m min diameter and were grouped into boat-shaped olivacious conidia showing thick wall and measuring 50-82 x 20-30 mm. The germination was unipolar and below 0.5%, suggesting the occurrence of dormancy.
In contrast, individual conidium produced on wheat during the growing season were 96-130 x 16-20 mm, slightly curved, hyaline to light pale, and euseptate with a bipolar germination reaching 98-100%.
Bipolaris sorokiniana conidia produced on PDA were 55-82 x 20-27 mm, tapered at both ends, dark brown to olivacious, distoseptate, showed up to 1% germination, and were predominantly unipolar.
On basis of observations, they suggested that Bipolaris sorokiniana conidia belonged to two different physiological categories corresponding to the pathogen’s infection phase and its survival, respectively. The infection phase is characterized by a high germination percentage as opposed to the survival phase harboring apparent dormancy.
3. Soil:
Bipolaris sorokiniana may also survive in soil as free dormant conidia. Ledingham (1970) reported poor viability of conidia under the higher humidity. Chulkina (1972-1974) reported a higher viability of conidia in the sub soil than in surface soil.
Under trash maintenance cultivation in Russia, there were more conidia on soil surface than in sub surface. Duczek (1981) developed a technique for the quantification of spores in the soil.
The conidia multiplied either on green plant or residue in the soil further transported through wind or rain splash. Free conidia remain viable in the soil up to 37 months. Under such condition, crop rotation with wheat is possible only after 37 month.
These finding indicated the importance of crop rotation in reducing inoculum. Pandey 2005 studied the survival of Bipolaris sorokiniana in soil to know the fate of conidia surviving in the soil on reappearance of spot blotch in rice- wheat cropping region in eastern India where field gets flooded because of the rain.
They tested the viability of conidia of Bipolaris sorokiniana collected from soils during April to October and observed that viability of conidia was sharply declined after flooding in the month of August. In contrast to 82% conidia recovered in April. Viability was 4 and <1% in August and September, respectively.
They also tested the possibility of alternate hosts as green bridge of Bipolaris sorokiniana and concluded that green bridge does not exist and seeds are the most important source of inoculum for reappearance of spot blotch of wheat in rice wheat cropping system in eastern India. Malakar (2007) also studied the off-season survival of Bipolaris sorokiniana in sterilized and unsterilized soils, and in wheat residues spread on soil surface.
They found that the population of Bipolaris sorokiniana increased for two months in both soils and residues and declined thereafter. The decline was very sharp up to four months of survival and then onwards a gradual decline was observed. In case of free residue, a gradual decline in population was observed from the beginning of the pathogen survival.
The pathogen could be recovered up to eight and ten months from the unsterilized and sterilized soil, respectively. It was not possible to recover the pathogen from the residues in unsterilized and sterilized soil after seven and eight months. The pathogen could survive for 12 months in free residue stored at room temperature.
Environmental Influence of Bipolaris Sorokiniana:
Environmental condition particularly moderate to warm temperature (18° to 32°C) and humid weather conditions, influence the severity of spot blotch of wheat. However, it has been increasingly recognized one of the major constraints in optimum and irrigated conditions.
It is also known in ME1 mega-environment characterized by irrigated, low rainfall and temperate growing conditions. Muller (1956) reported that, seed infection by H. sativum was common to those places where rainfall was high. Nema (1969) noted that, foliar blight symptoms in isolated plots under field conditions developed only after rains were received.
According to Joshi (1973), infection on leaf was relatively quicker and more severe at 28°C, whereas the development of lesions were faster at 22°C. Incubation of 24-25 hours in moist chamber was enough to produce the disease. Joshi and Adlakha (1976) reported the foliar blight as endemic in Punjab and other cooler regions of the country had high humidity.
Khetrapal (1981) observed that, high humidity with frequent rains was conducive for the infection of H. sativum.
Couture and Sutton (1978) reported spot blotch infection on leaf when it was wet for more than 18 hrs. at a mean temperature above 18°C. Lapis (1985), Mann (1992), Sarri (1986); Sundars (1988) reported that leaf blight caused by Bipolaris sorokiniana is a major limiting factor to wheat production hot (<22.5°C) and humid climates.
Mehta (1998) also reported that continuous rain for 5-6 days followed by relatively warmer temperatures can develop spot blotch epidemic very quickly and is capable of causing almost total yield loss and poor quality of gains.
Dhene and Oerke (1985), under green house condition, tested the variation in varietal reactions of nine wheat varieties of H. sativum. Leaf inoculation showed that leaf spot formation occurs even at low temperature.
Damage due to chlorosis, was intensified at more than 20°C and high relative humidity. They further opined that, high light intensities also supported leaf spot formation where as, inoculation under temporary low light condition, produced leaf chlorosis and rapid senescence.
Hiremath (1984), repoted that foliar blight disease incidence was reduced by shifting sowing dates from early to late, but increased, when frequent irrigation were given. Bidari and Govindu (1992), observed good germination and appresorium formation in H. sativum on suitable medium and favorable environmental conditions.
Most of the Helminthosporium species was favoured by moderate to warm temperature (18-32°C) and particularly, by humid weather.
They further reported that, leaves must remain wet for 18 hours for disease out break. Numbers of spores in the field were maximum at crop maturity. Triller and Mehta (1997) studied the influence of flag leaf age on the expression of spot blotch resistance in wheat under controlled environmental and found higher infection on older than young leaves.
Chaurasia (1999) reported interaction among the foliar blight pathogen and found higher frequency of A. triticina below 25°C. Bipolaris sorokiniana became dominant pathogen above 25°C.
Joshi (2007c) studied the associations of environments in South Asia based on spot blotch disease of wheat. Division of test sites for this disease into homogenous subregions is expected to contribute to more efficient evaluation and better differentiation of cultivars.
Data from a collaborative regional program of South Asia conducted by CIMMYT were analyzed to group testing sites into relatively homogenous subregions for spot blotch area under the disease progress curve.
Five-year data of eight locations from Eastern Gangetic Plains Nursery (EGPSN) and five location of the Eastern Gangetic Plains Yield Trial (EGPYT) conducted in three countries (India, Nepal, and Bangladesh) of South Asia were used.
A hierarchical cluster analysis was used to group locations on the basis of genotype x location interaction effects for spot blotch AUDPC. Cluster analysis divided South Asia into two broad regions and four subregions. This classification was not entirely consistent with the geographic distribution of locations, but clusters mostly followed general geographic climatic locations.
The locations Varanasi (India) and Bhairahawa (Nepal) were identified as the most suitable sites for evaluation of spot blotch, followed by Rampur (Nepal). The major determinant for the clustering was mean temperature.
The results suggest that the major wheat region of South Asia can be divided into subregions, which may reduce the cost of resistance evaluation and aid in developing wheat with resistance to this disease.
Aggressiveness in Bipolaris Sorokiniana:
Variation for aggressiveness was shown for small number of individual isolate collected from Canada, South Africa, Mexico, Bolivia, India and Bangladesh. Some isolate induced restricted lesions on most susceptible genotype Ciano while other caused severe symptoms on resistant genotype Mayoor.
Significant genotypic variation for aggressiveness was found in the field despite, the difference in mean disease severity across the time and space (years and locations).
Mehta (1981a) also reported difference in pathogenicity and identified 32 races out of 92 isolates from Brazil. Hezler (1992) showed vertical and horizontal system in this pathogen and expressed difficulty in defining large numbers of possible gene for gene interactions.
Variation was found among the isolates (glumes, leaves, stem, sub-crown) from commercial fields and research stations from traditional and non traditional wheat growing areas.
Genetic sub – structuring of population expressed as measures of gene and genotype diversity was very similar. Clustered analysis of virulence pattern indicated geographical subdivision.
Therefore, the resistance test in one region of world may not be useful to other. It can be speculated if migration is less intensive many population that mainly leave on sub ground part of the plant and variation is likely to develop there.
The apparent wide spread-occurrence of strains able to overcome promising sources of resistance under test condition, raises the question of the possible increase in importance of the strains once the area sown to the resistant cultivars. An intensive and systematic monitoring could provide information on possible pathogenic variability in Bipolaris sorokiniana population.
Cytology of Bipolaris Sorokiniana:
The nuclear distribution in conidia and hyphal cells was studied and multinucleate hyphal cells were found in conidial initials. Most of the young conidiophores contained at least two nuclei per cell. He also suggested that heterokaryotic conditions could persist in thalli and be perpetuated by conidia.
After, fusion, nucleus under goes meiotic and subsequent mitotic division and formed eight haploid nuclei. Ascospores are filiform helical coiled and larger than ascus. Hung and Tinline (1974) observed 6 to 7 chromosomes in their study in haploid and diploid somatic nuclei.
Conidia induced by somatic diploids were larger than that of haploid. However, the range of conidial size within isolate appeared sufficiently large to small size as discrete indicator of polyploidy.
Multinucleate conditions lead to the development of different morphological forms i.e black to white and fluffy to adpressed. During the isolation, different type of colonies were recovered with different nuclei. Morphological variation was also correlated with the aggressiveness of the isolates.
Black isolates were more aggressive than others due to higher sporulation capacity. Under natural conditions variation emerged by the sectoring of cell carrying variable number of nuclei that brings the heteroploidy.
Mutation of Bipolaris Sorokiniana:
Some useful markers appeared as spontaneous mutation and others following induced mutation; Tan colour conidia occurred de novo in a monoascocarpic brown spore culture. White-tan and brown-tan coloured isolates have been obtained from infected cereal plants.
Auxotrophic mutations were selected from the population of Bipolaris sorokiniana that were treated with UV light or diethyl sulphate. Albino isolates lack melanin are also reported from the natural population.
Mating Types of Bipolaris Sorokiniana:
Sexual compatibility in Bipolaris sorokiniana appeared to be controlled by a pair of mating type alleles that have been designated as A and a (1958). Mating types are most stable however; there is report of the mutation from, one to other. The two mating types are widely distributed in nature and occurs in the same geographical areas.
In a random survey of mating type frequencies there were 33A and 32 a population of 65 isolate from cereal field. However, this ratio was 38 A 60 a in a population of 98 isolates from single field of Saskatchewan Canada.
Despite the simplicity in the mating system, mature ascomata do not develop in all compatible paring. It is most likely that number of genes controlled the formation of sexual structure and developmental process.
Kleine and Nelson (1968) characterized isolates as weak, intermediate or strong in fertility and suggested genetically controlled reaction. Similarly fertility reactions were also observed by Haring and Tinline (1983) and they postulated that two or more independent genes controlled ascocarp, ascus and ascospore formation.
Heterokaryosis and Parasexuality:
In Bipolaris sorokiniana heterokaryosis arising from anastomosis and nuclear migration between marked strains was reported by Tinline (1962). The heterokaryotic conidium was shown to be perpetuated by less than 10% of the total conidia. On minimal medium heterokaryons grew more slowly and raggedly than parental prototroph suggesting a continual process of association and re-synthesis of heterokaryotic state.
Occasional heterozygotic diploid cultures were obtained from heterokaryotic one. Hetrokaryons appeared as a sector differ in growth form the normal one. Hetrokayosis play important role in the variation of the pathogen especially for the aggressiveness. The auxotrophic requirement and white spore colour were recessive characters.
The isolation of recombinants constituted genetic evidence of somatic heterozygous diploid and parasexuality in the fungus. Supportive evidence of dipoloidy was chromosome number, conidial size and nuclear volume. A difference in ploidy did not had a major effect on pathogenicity. The pathogenic diploid appears quite similar to that of haploid.
Population Structure of Bipolaris Sorokiniana:
Bipolaris sorokiniana showed a good range of morphological variation and large genetic diversity for aggressiveness. A low level pathogenic specialization was recorded among the population. No isolate by isolate interaction was recorded.
Significant, difference in allelic frequencies at the same loci were seldom found among population. RAPD markers differentiated 11 field population sample pooled on the regional basis. The mean total genetic diversity Ht for all 17 loci across all 11 field populations were relatively high. Multiple dimension procedure clearly clustered the Bipolaris sorokiniana population of the same region.
The combination of 17 RAPD markers resulted in the detection of 264 different multilocus haplotype among 799 isolates. In Canada, two different field populations were obtained and it may be interpreted as differential selection.
However, it is unlikely that 6 of 17 markers were directly selected therefore differ in frequencies among two population. If sexual recombination is rare or even missing association among selected and neutral loci may persist an effect commonly reported to as hitch hiking.