Inheritance and allelism of bacterial wilt resistance in brinjal (Solanum melongena L.)

Introduction

Brinjal (Solanum melongena L.) is an important vegetable crop widely cultivated in India and other parts of the world. It contributes significantly to nutritional security, farm income and employment generation. Despite its importance, brinjal productivity is severely constrained by several biotic stresses, among which bacterial wilt caused by Ralstonia solanacearum is one of the most devastating diseases (Sidhu and Dhatt, 2007) [4] . Bacterial wilt is characterized by sudden wilting of plants without prior yellowing, browning of vascular tissues, and eventual death of the plant. The pathogen survives in soil, crop residues and alternate hosts, making its management extremely difficult. Favorable environmental conditions such as high temperature and soil moisture aggravate disease incidence, resulting in yield losses ranging from moderate to complete crop failure in endemic regions (Manda et al., 2020) [3] . Management of bacterial wilt through chemical, cultural and biological methods has met with limited success. Crop rotation and soil amendments provide only partial control, while chemical treatments are uneconomical and environmentally hazardous. In this context, host plant resistance offers the most effective, sustainable and farmer-friendly solution for managing bacterial wilt in brinjal (Yuliar et al., 2015) [5] . Boukema (1980) [1] investigated for the resistance to pepper strains of TMV in five Capsicum chinense accessions (PI 152225, 159236, 315008, 315023, and 315024) and found that resistance was inherited monogenically and partially dominant. The resistance genes in these accessions are allelic to each other and to the known alleles L 1 and L. Several resistant genotypes have been identified in cultivated brinjal and related Solanum species. However, the inheritance of resistance varies depending on the genetic background, and resistance sources may share common genes. Knowledge of allelic relationships among resistance genes is crucial for breeding programmes, as crossing genotypes with identical resistance genes does not increase genetic diversity. Allelism tests using resistant × resistant crosses help determine whether resistance genes are identical or non-allelic. The present investigation was therefore undertaken to study the inheritance and allelic relationship of bacterial wilt resistance in selected brinjal genotypes.

Materials and Methods

The experimental material consisted of three bacterial wilt resistant brinjal genotypes, namely SM-6-7, IC0598430 and CARI-1. The genotype CARI-1, developed at ICAR-Central Island Agricultural Research Institute, Port Blair, Andaman and Nicobar Islands, was used as a common resistant parent. CARI-1 was crossed individually with SM-6-7 and IC0598430 to generate two resistant × resistant crosses. The F₁ plants obtained from each cross were self-pollinated to produce respective F₂ populations. The parents, F₁s and F₂ populations were evaluated for bacterial wilt resistance under sick plot conditions where the soil was naturally infested with Ralstonia solanacearum. Standard agronomic practices were followed uniformly for all genotypes. Plants were regularly observed for wilt symptoms throughout the crop growth period. Plants showing no wilting symptoms were classified as resistant, while plants exhibiting typical bacterial wilt symptoms were scored as susceptible. Data on resistant and susceptible plants were recorded and used to study segregation patterns and allelic relationships.

Results and Discussion

All parental genotypes evaluated in the present study showed complete resistance to bacterial wilt, with no susceptible plants recorded under sick plot conditions. Similarly, the F₁ plants derived from both crosses exhibited complete resistance, indicating dominant expression of resistance. Evaluation of the F₂ populations revealed that all plants were resistant, with no segregation for susceptibility in either cross. The segregation data of parents and F₂ populations are presented in Table 1. In essence, this means that the genetic resistance mechanisms in these three varieties are identical, and the resistance is controlled by the same two complementary recessive genes across all three sources. The consistent resistance in the F 2 generations of these crosses provides strong evidence that the resistance genes in SM 6-7 and IC0598430 are indeed allelic to those in CARI-1, leaving no room for gene diversity among these resistant lines concerning bacterial wilt resistance. These results are in conformity with the earlier investigations of Chen et al.

Conclusion

The present study revealed that bacterial wilt resistance in the brinjal genotypes SM-6-7, IC0598430 and CARI-1 is governed by allelic genes. The complete absence of segregation for susceptibility in the F₂ populations of resistant × resistant crosses confirmed the allelic relationship of resistance genes among the tested genotypes. While these resistant lines are valuable for immediate use in breeding programmes, the limited genetic diversity for resistance underscores the need to identify and incorporate novel, non-allelic resistance sources. Broadening the genetic base of resistance will be essential for developing brinjal varieties with durable and long-lasting resistance to bacterial wilt.

References

1. Boukema IW. Allelism of genes controlling resistance to TMV in Capsicum L. Euphytica. 1980;29:433-439.
2. Chen P, Ma G, Buss GR, Gunduz I, Roane CW, Tolin SA. Inheritance and allelism tests of Raiden soybean for resistance to soybean mosaic virus. J Hered. 2001;92(1):51-55.
3. Manda R, Addanki R, Venkata VA, Sweta. Bacterial wilt of solanaceous crops. Int J Chem Stud. 2020;8(6):1048-1057.
4. Sidhu A, Dhatt A. Current status of brinjal research in India. Acta Hortic. 2007;743:243-248.
5. Yuliar, Nion YA, Toyota K. Recent trends in control methods for bacterial wilt diseases caused by Ralstonia solanacearum. Microbes Environ. 2015;30(1):1-11.