Identification of mungbean genotypes with multiple disease resistance

Introduction

Mungbean is an important pulse crop in India and is believed to have originated from the Indo-Burma region. It is short duration legume crop grown mostly as a fallow crop in rotation with rice. Similar to the leguminous pulses, mungbean enriches soil nitrogen content. It is grown mostly in Asian region traditionally, while its cultivation has spread to Africa and America relatively in the recent times. India contributes more than 70% of the world's mungbean production. In the tropics, mungbean acts as a host for pathogens such as bacteria, viruses and fungi. Powdery mildew caused by Erysiphe polygoni is one of the prevalent fungal diseases causing severe yield losses in mungbean. The yield losses due to powdery mildew are estimated to be between 20% and 40% and 100% at seedling stage (Reddy et al., 1994) [16] . In addition, numerous species of the fungus Colletotrichum are responsible for mungbean anthracnose but C. truncatum and C. canescens are the most prevalent pathogens in northern transistional tract of Karnataka (Mandal et al., 2015) [9] . In India, it has been estimated that this disease reduces yield by 30% to 70% (Joshi and Tripathi, 2002) [5] . Mungbean yellow mosaic virus (MYMV) is one of the most destructive viral diseases of mungbean. MYMV is a member of family Geminiviridae and belongs to the genus Begomovirus and is transmitted by whitefly (Bemesia tabaci) as a vector. This disease can cause the yield losses up to 100% (Khattak et al., 2000) [7] . The management of these diseases using chemicals is a costly affair and not environmentally safe. Hence deploying genetically resistant cultivars would be cost-effective, practically feasible, eco-and farmer-friendly and a viable alternative. With this prime concern, the advanced breeding lines of mungbean were screened for their response towards powdery mildew, anthracnose and MYMV to identify the genotypes with multiple disease resistance.

Materials and Methods

Experimental details

The disease reaction of one hundred and thirty advanced breeding lines including three suitable checks was screened for the above mentioned diseases under natural field conditions after ensuring enough load of inoculum using the infector rows. The experiment was conducted using augmented design at E-block, Main Agricultural Research Station (MARS), College of Agriculture, Dharwad during the kharif 2021 and summer 2022. The advanced breeding lines of mungbean utilized in this investigation were obtained from AICRP on MULLaRP, UAS, Dharwad which included the stabilized lines derived from many crosses under the genetic background of diverse parental combinations, mutation per se and mutation breeding followed by recombination for some desirable traits. Apart from advanced breeding lines, the experimental material also included some of the varieties released by different stations across India and a few germplasm lines.

Screening for foliar diseases

During kharif 2021, the test genotypes and the checks were screened for fungal diseases i.e., powdery mildew and anthracnose. One set of experiment was conducted with all the recommended cultivation and disease management practices. Another set of experiment was laid out during the same season in the vicinity of the first experimental plot under unprotected conditions. DGGV-2 was used as a susceptible check whereas TARM1 was used as a tolerant check. For disease reaction, percentage of leaf area covered by the disease was scored manually. The incidence of disease on the leaves of mungbean was scored by using standard scoring procedure given by Mayee and Datar (1986) [10] . The percentage was then converted to the disease score and the% disease index was calculated using the formula given by Wheeler (1969) [23] . Further the% yield reduction of each breeding line due to these diseases was calculated using the yield data from both protected and unprotected conditions.

During summer 2022, the same set of 130 genotypes were screened for their reaction to MYMV since it has been discovered that the MYMV incidence is higher in summer than other seasons. In this season also two experiments (protected and unprotected conditions) were laid using augmented design. DGGV-2 was used as a susceptible check whereas IPM-2-14 was used as resistant check. The% disease incidence was calculated by using the formulae given by Bashir et al. (2006) [2] at 45 DAS, 60 DAS and physiological maturity. Based on the percent disease incidence, the genotypes were categorized into different groups based on the scale given by Bashir et al. (2006) [2] . Furthermore the% yield reduction of each breeding line was calculated.

The PDI of the diseases was further subjected to arc sine transformation and was then analysed using R software version 4.2.5 to know whether the breeding lines differed significantly for their reaction to these diseases.

Results and Discussion

The genotypes showed significant variation for reaction to all the three diseases (Table 1 and2). Amongst all the breeding lines screened for powdery mildew, it has been found that only Vigna trilobata (wild relative) was immune and GPM-19 was resistant for this disease. The similar response of these two genotypes was previously reported by Sarkale (2015) [18] and Pooja and Bhat (2019) [15] . 3.5 df -degrees of freedom, CD-Critical difference, PDI-Per cent disease incidence All the genotypes derived from the cross IPM-2-14 x IPM-2-17 (DGG-215-1, 2, 5, 6) except DGG-251 showed moderate resistance with lesser PDI than that of the parents in some cases. The reason behind this is that, both the parents involved in this cross are moderately resistant to powdery mildew which must have led to same level of resistance or rather increased levels of resistance with low PDI in the progeny. It was also observed that, in the crosses where the moderately resistant line DGG-1 was used as a female parent none of the breeding lines showed the resistance reaction but in the line DGG-216 (DGGV-2 x DGG-1) where DGG-1 was used as a male parent showed moderate resistance towards this disease. It can be reasoned that the pedigree method of breeding involving the parents DGGV-2 x DGG-1 has resulted in a genetic recombination conferring moderate resistance to powdery mildew. Since the pedigree to this segregant was not recorded, it is difficult to conclude about the genetic basis of this resistance (Table 3). Some of the genotypes like DGG-10, DGG-19, DGG-12, DGG-95 derived from the crosses involving TARM-1, which was used as a tolerant check in this study showed moderate resistance. Some of the released varieties (COGG 912, Shikha, Vaibhav, Virat, AKM 8802) which were claimed as resistant in the corresponding locations showed similar reaction in Dharwad (Table 3). Two genotypes viz., COGG 912 and Vaibhav showed similar response in the previous study conducted in the same location by Pooja and Bhat (2019) [15] .

In the present investigation, the PDI of anthracnose ranged from 0.12% to 91.56. The maximum PDI of 91.56 was recorded for the susceptible check, DGGV-2. For anthracnose most of the genotypes screened showed susceptibility except for some of the genotypes derived from mutants of Sonamung (DGG-191, DGG-227), the genotypes derived from the crosses involving TARM-1 and BGS-9 as parents (DGG-10, DGG-21) and the mutants of TARM-1 (DGG-95, DGG-96). This is because in the former case, the parent Sonamung is a photoperiod sensitive and the performance was poor in kharif. However, the mutants of this genotype performed well in kharif and were late maturing type, this might have aided the mutant genotypes to show resistance towards this disease resorting to the mechanism of disease escape. In the latter case, the mutations might have led to resistance in TARM-1 which is otherwise a susceptible variety. The germplasm lines like GPM-19, V-02-709 and V. trilobata showed resistance. One of the released variety, COGG-912 which was claimed to be resistant to foliar diseases was found to show similar reaction in this study. Some of the genotypes such as DGG-191, DGG-12, DGG-59 and DGG-122 screened previously in the same location showed consistently resistant response to anthracnose (Ashritha, 2021) [1] . In this study, the% disease incidence of MYMV ranged from 0% to 85.57%. Most of the breeding lines derived from resistant lines like IPM-2-14 (DGG-215-1, 2, 3, 5, 6, DGG-182-1,2, DGG-251) and IPM-2-03 which is moderately resistant (DGG-184,185, 186, 187, 71, 232) and the mutants of the resistant line LGG-460 (DGG-76, 73, 99) exhibited resistant to moderately resistant reaction. Interestingly some of the mutants of the susceptible line, Sonamung (DGG-190) and the recombinants involving mutant Sonamung (DGG-227,230) showed moderate resistance. The entry COGG 912 which was reported as highly resistant by Mahanta and Sao (2019) [8] was found to show resistant reaction in this study. The genotype, Samrat (PDM 139), which was reported as a resistant variety for use as parent in breeding programmes by Singh (1981) [19] and Paul et al. (2013) [14] has shown similar reaction in the current study. During kharif 2021, the% yield reduction ranged from 3.78% to 86.06%. The breeding line DGG-21 which recorded the least% yield reduction, was resistant to anthracnose but was susceptible to powdery mildew. Followed by DGG-227 which showed 7.41% yield reduction and was resistant to anthracnose and moderately resistant to powdery mildew. The genotypes such as IPM-2-17, V-02-709, DGG-10, DGG-213-1, DGG-215-2, DGG-215-3, DGG-251 showed an yield reduction of less than 20%. This yield reduction may be due to the confounding effect of powdery mildew and anthracnose during kharif 2021. Nonetheless, the powdery mildew symptoms were seen from 45 DAS to 60 DAS and disappeared in later stages due to the rains received during the cropping season whereas the anthracnose symptoms were severe till the end of the cropping season. Therefore it can be inferred that anthracnose had major contribution in the yield reduction in the present study. The similar results were obtained by Vandana et al. (2014) [21] who reported that yield losses due to anthracnose in their experimental material ranged from 24-67%. 4). The wild relative, Vigna trilobata which is considered as the secondary gene pool of this crop (Bisht et al., 2004) [3] has shown multiple disease resistance in the current study. There are two types of population of this species. One is beach population and the other is dry inland population. The beach population with small, thick and entire leaflets were screened in this study. Apart from disease resistance, it has also been observed that the seeds were bruchid resistant during the seed storage. The salt resistance screening experiment by Norihiko et al. (2010) [12] revealed that V. trilobata showed the highest level of resistance even in its seedling stage. Even though this species can serve as an excellent resistance source but the less crossability % of the interspecific cross between trilobata and radiata led to the limited research in this direction. The hybrid seed set is observed to be less due to flower drop after pollination and the hybrid lethality was also reported to be high in these crosses (Pandiyan et al., 2012; Sarkale, 2015) [13,18] . Further some of the advanced breeding lines, such as DGG-10 (MR to powdery mildew and anthracnose), COGG 912 (MR to powdery mildew and anthracnose; R to MYMV), GPM-19 (R to powdery mildew and MYMV; MR to anthracnose); DGG-227 (R to powdery mildew and anthracnose; MR to MYMV) showed tolerance to more than one disease. Some of the previous studies on mungbean for multiple disease resistance were conducted by Singh et al. (1988) [20] . Seventeen genotypes conferring multiple disease resistance were identified by Kaur (2006) [6] . There are instances in which it has been proven that this kind of resistance is governed by polygenes (Vanderplank, 1968) [22] . At the same time, some of the previous studies reported that the horizontal resistance is governed by single gene or oligo genes (Caldwell, 1968) [4] . Since the inheritance studies of the resistant lines identified in this study has not been done, the nature of resistance operating in these lines cannot be contemplated. The resistant genotypes thus identified in the current study can be screened in multiple locations or multiple seasons/ years for identifying genotypes with stable resistance. And also the good yielding resistant genotypes identified can be further tested in different locations to check their suitability for release as varieties. They can also be used as parents in breeding programs. The wild relative V. trilobata, which was found to be resistant to all diseases, can be used as a donor parent in resistance breeding program by employing few simple procedures to overcome the crossability barriers.

Acknowledgement

AcknowledgementThe authors are indebted to the Department of Genetics and Plant Breeding, University of Agricultural Sciences, Dharwad, Karnataka, for providing support for undertaking this experiment and Indian Council of Agricultural Research for providing the financial assistance in the form of NTS fellowship.

References

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