Abstract

Carioca grains are the most cultivated commercial group of common bean and Fusarium oxysporum f. sp. phaseoli (Fop) is an important disease under field conditions. Genetic resistance is the most effective control strategy. Thus, our objective was to study the inheritance of Fop resistance in the carioca grain type common bean. Lines CNFC 11965 (S) and CVIII 8511 (R) were crossed to obtain F₁ and F₂ generations and backcrosses, inoculated with the isolate FOP UFV 01, and severity grades used to estimate genetic parameters. Segregation of resistant and susceptible plants in the F₂ generations and backcrosses resulted in dominant monogenic inheritance of Fop resistance. The high values of heritability emphasized the probability of success in the selection processes because environmental factors were not significant. The carioca grain line CVIII 8511 is promising, and its Fop resistance gene can be transferred to commercial cultivars of this group using the backcross method.

Keywords:
Phaseolus vulgaris L.; Fusarium oxysporum f. sp. phaseoli; plant breeding; genetic control

INTRODUCTION

The common bean (Phaseolus vulgaris L.) is a cultivated species with socioeconomic relevance and is a major source of vegetable proteins and nutrients worldwide, mainly in the Americas and parts of Africa (Ribeiro et al. 2020Ribeiro ND, Steckling SDM, Maziero SM, Klasener GR2020 Higher-precision experimental statistics for the selection of early and upright common bean lines. Acta Scientiarum Agronomy 42:e42725, Zeffa et al. 2020Zeffa DM, Moda-Cirino V, Medeiros IA, Freiria GH, Santos Neto J, Ivamoto-Suzuki ST, Delfini J, Scapim CA, Gonçalves LSA2020 Genetic progress of seed yield and nitrogen use efficiency of Brazilian carioca common bean cultivars using Bayesian approaches. Frontiers in Plant Science 11:1-14, FAOSTAT 2021FAO - Food and Agriculture Organization of the United Nations2021 Faostat: Statistical databases. Available at <Available at https://www.fao.org/faostat/en/#data/QCL >. Accessed on December 02, 2021.
https://www.fao.org/faostat/en/#data/QCL... ). In Brazil, the carioca grain type accounts for approximately 70% of national bean consumption (Pereira et al. 2019Pereira HS, Mota APS, Rodrigues LA, Souza TLPO, Melo LC2019 Genetic diversity among common bean cultivars based on agronomic traits and molecular markers and application to recommendation of parent lines. Euphytica 215:1-16); however, few commercial cultivars of this group exhibit high yield potential, upright plant architecture, and disease resistance when considered simultaneously.

Common bean hosts a wide variety of pathogens that cause significant losses in yield and affect the commercial quality of grains. Fusarium wilt is caused by the soil-borne pathogen Fusarium oxysporum (Schlecht.) f. sp. phaseoli (Kendrick and Snyder 1942Kendrick J, Snyder W1942 Fusarium yellows of beans. Phytopathology 32:1010-1014) (Fop) and is one of the most important diseases of the common bean. Its control depends almost entirely on genetic and cultural management (Batista et al. 2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233, Jha et al. 2020Jha UC, Bohra A, Pandey S, Parida SK2020 Breeding, genetics, and genomics approaches for improving fusarium wilt resistance in major grain legumes. Frontiers in Genetics 11:1001, Sasseron et al. 2020Sasseron GR, Benchimol-Reis LL, Perseguini JMKC, Paulino JFC, Bajay MM, Carbonell SAM, Chiorato AF2020 Fusarium oxysporum f. sp. phaseoli genetic variability assessed by new developed microsatellites. Genetics and Molecular Biology 43:e20190267). Thus, introgression of resistance genes is the most effective and sustainable control strategy, in terms of both economics and ease of adoption by farmers (Miklas et al. 2006Miklas PN, Kelly JD, Beebe SE, Blair MW2006 Common bean breeding for resistance against biotic and abiotic stresses: From classical to MAS breeding. Euphytica 147:105-131, Gonçalves-Vidigal et al. 2013Gonçalves-Vidigal MC, Cruz AS, Lacanallo GF, Vidigal Filho PS, Sousa LL, Pacheco CMNA, Gepts P, Pastor-Corrales MA2013 Co-segregation analysis and mapping of the anthracnose co-10 and angular leaf spot Phg-on disease-resistance genes in the common bean cultivar Ouro Negro. Theoretical and Applied Genetics 126:2245-2255, Batista et al. 2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12, Leitão et al. 2020Leitão ST, Malosetti M, Song Q, Eeuwijk FV, Rubiales D, Patto MCV2020 Natural variation in Portuguese common bean germplasm reveals new sources of resistance against Fusarium oxysporum f. sp. phaseoli and resistance-associated candidate genes. Phytopathology 110 (3): 633-647.).

After the selection and recombination of genitors, the method to conduct segregating populations is a determining factor for when to obtain a new cultivar and the costs of the program (Batista et al. 2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12). Thus, knowledge of genetic parameters, such as inheritance of resistance to Fusarium wilt, contributes significantly to the rapid transfer of resistance genes. In studies with the carioca grain type, Batista et al. (2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233) and Batista et al. (2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12) reported that common bean resistance to Fusarium wilt is dominant and governed by few major genes, and the presence of polygenes was also identified (Batista et al. 2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12). The presence of dominance in the genetic control of this disease was observed in a study involving three resistant genitors from the carioca grain type in different crosses (Pereira et al. 2009Pereira MJZ, Ramalho MAP, Abreu AFB2009 Inheritance of resistance to Fusarium oxysporum f. sp. phaseoli Brazilian race 2 in common bean. Scientia Agricola 66:788-792).

After studying contrasting crosses, Batista et al. (2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233) concluded that the segregating population derived from line CVIII 8511 was promising for bean improvement aimed toward grain yield and resistance to Fusarium wilt, with a lower frequency of alleles for growth reduction caused by the pathogen than the others parents used in the research. Therefore, introgression of resistant alleles from resistant lines into commercial cultivars is an important strategy for common bean breeding. The objective of this study was to investigate the inheritance of Fop resistance in the carioca grain type common bean.

MATERIAL AND METHODS

Cultivars and elite lines of the carioca grain type common bean CNFC 11965, Dama, Pérola, Madrepérola, BRS Estilo, BRS Uai, Agronorte, Star and CVIII 8511 and the black grain type Meia Noite were evaluated for Fop resistance under greenhouse conditions. Isolate FOP UFV 01, which was used in this assay, was collected in Coimbra (Minas Gerais, Brazil) from plants of the black type grain cultivar Meia Noite, which exhibited the typical symptoms of Fusarium wilt (Pereira et al. 2013Pereira AC, Cruz MFA, Paula Junior TJ, Rodrigues FA, Carneiro JES, Vieira RF, Carneiro PCS2013 Infection process of Fusarium oxysporum f. sp. phaseoli on resistant, intermediate and susceptible bean cultivars. Tropical Plant Pathology 38:323-328). This isolate was chosen because it was previously identified using molecular markers (Cruz et al. 2018Cruz AF, Silva LF, Sousa TV, Nicoli A, Paula Junior TJ, Caixeta ET, Zambolim L2018 Molecular diversity in Fusarium oxysporum isolates from common bean fields in Brazil. European Journal of Plant Pathology 152:343-354) and has been widely characterized and applied in common bean breeding programs owing to its aggressiveness (Batista et al. 2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233, Batista et al. 2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12, Cruz et al. 2018Cruz AF, Silva LF, Sousa TV, Nicoli A, Paula Junior TJ, Caixeta ET, Zambolim L2018 Molecular diversity in Fusarium oxysporum isolates from common bean fields in Brazil. European Journal of Plant Pathology 152:343-354, Paulino et al. 2020Paulino JFC, Almeida CP, Gonçalves GMC, Bueno CJ, Carbonell SAM, Chiorato AF, Bechimol-Reis LL2020 Assessment of resistance in common bean to Fusarium oxysporum f. sp. phaseoli using different inoculation and evaluation methods. Crop Breeding and Applied Biotechnology 20:e337620311, Sasseron et al. 2020Sasseron GR, Benchimol-Reis LL, Perseguini JMKC, Paulino JFC, Bajay MM, Carbonell SAM, Chiorato AF2020 Fusarium oxysporum f. sp. phaseoli genetic variability assessed by new developed microsatellites. Genetics and Molecular Biology 43:e20190267, Paulino et al. 2021Paulino JFC, Almeida CP, Bueno CJ, Song Q, Fritsche-Neto R, Carbonell SAM, Chiorato AF, Benchimol-Reis LL2021 Genome-wide association study reveals genomic regions associated with fusarium wilt resistance in common bean. Genes 12:765).

The inoculum was produced on potato dextrose agar discs containing mycelia that were transferred to Petri dishes containing fresh potato dextrose agar medium. The Petri dishes were incubated in a biochemical oxygen demand medium for 14 days at 25 ± 1 °C, under a 12h photoperiod. The spore suspension was prepared 1 h before inoculation at a concentration of 10⁶ conidia mL^-1 as recommended by Pastor-Corrales and Abawi (1987Pastor-Corrales MA, Abawi GS1987 Reactions of selected bean germplasms to infection by Fusarium oxysporum f. sp. phaseoli. Plant Disease 71:990-993).

Inoculation of the genotypes was performed according to the method of immersion of roots in conidial suspension as proposed by Pastor-Corrales and Abawi (1987Pastor-Corrales MA, Abawi GS1987 Reactions of selected bean germplasms to infection by Fusarium oxysporum f. sp. phaseoli. Plant Disease 71:990-993). The genotypes were sown in 128-cell polystyrene trays containing the vegetable substrate Tropstrato^® (Viva Verde Company, SP, Brazil) and maintained in a greenhouse for seedling germination and growth. At the V2 stage (fully expanded primary leaf), the seedlings were carefully removed from the trays, their roots were washed and 1/3 of their length was cut. Immediately after cutting, the seedling roots were immersed in a conidium suspension of the isolate FOP UFV 01 for 5 min, transplanted into plastic pots containing Tropstrato ^® substrate (2.5 L), and maintained in a greenhouse at 25 ± 4 °C. The plants were irrigated daily, and 10 days after inoculation, each pot received 1.0 g of urea as a source of nitrogen.

The reaction of genotypes to Fusarium wilt was evaluated using the scale of disease severity described by Pastor-Corrales and Abawi (1987Pastor-Corrales MA, Abawi GS1987 Reactions of selected bean germplasms to infection by Fusarium oxysporum f. sp. phaseoli. Plant Disease 71:990-993), which is based mainly on disease intensity in the shoots. According to this scale, 1 = no visible symptoms; 3 = 1% to 10% symptomatic leaves (leaves with mild wilting and chlorosis); 5 = 11% to 25% symptomatic leaves (leaves with moderate wilting and chlorosis); 7 = 26% to 50% symptomatic leaves (leaves with severe wilting and chlorosis); and 9 = dead or severely infected plants. The evaluations were performed at 15, 18, and 21 days after inoculation, and the grade of severity of Fusarium wilt (SFW) in the last evaluation was used to classify the genotypes as resistant (mean grade of 1.0 to 3.0), intermediate reaction (mean grade of 3.1 to 6.0), or susceptible (mean grade of 6.1 to 9.0) to Fusarium wilt.

Among the genotypes evaluated, line CVIII 8511 was selected as resistant (R) and line CNFC 11965 as susceptible (S), two contrasting genotypes in their reaction to Fop. The seeds of these genitors were sown in trays containing Tropstrato^® and kept in a greenhouse for germination and seedling growth. At the V2 stage, seedlings were transplanted into 5 L pots containing soil + manure + sand mixture (3:1:1) plus 10 g of N-P-K and kept in the greenhouse at 25 ± 4 °C. The plants were irrigated daily and 20 days after transplanting, each pot received 5.0 g of urea as the nitrogen source. At the reproductive stage R5 (emergence of the first flower bud), manual hybridization without emasculation was performed between the R and S genitors to obtain F₁ seeds, which were harvested at the R9 stage (harvest stage) and stored. To obtain backcrosses with resistant (BC_R) and susceptible (BC_S) genitors, hybridizations were performed between F₁ plants and the R and S genitors, respectively. Some of the flowers were conserved to obtain F₂ generation seeds by self-pollination. The seeds from backcrosses, F₁, and F₂ were harvested at R9 and stored.

Plants of the genitors, F₁ and F₂ generations, and BC_R and BC_S backcrosses were inoculated and evaluated for their reaction to Fop, as previously described. The plot consisted of one pot with three plants, totaling 15 plants for each genitor (R and S), 12 plants for the F₁ generation, 333 plants for the F₂ generation, 24 plants for BC_R, and 25 plants for BC_S. The inoculation methodology and evaluation of disease severity were performed as previously described. The grades of SFW attributed individually to plants of populations P₁, P₂, F₁, F₂, BC_R, and BC_S were used to obtain the means and variances of each population in addition to estimation of other genetic parameters of bean resistance to Fop.

The environmental variance ( ${\mathrm{\sigma }²}_{\mathrm{E}}$ ) was estimated based on the variances in the P₁ ( ${\mathrm{\sigma }²}_{\mathrm{P}1}$ ), P₂ ( ${\mathrm{\sigma }²}_{\mathrm{P}2}$ ), and F₁ ( ${\mathrm{\sigma }²}_{\mathrm{F}1}$ ) generations:

${\mathrm{\sigma }²}_{\mathrm{E}}^{\mathrm{}}=\mathrm{}\frac{{{\mathrm{\sigma }²}_{\mathrm{P}1}+\mathrm{}{\mathrm{\sigma }²}_{\mathrm{P}2}+2{\mathrm{\sigma }²}_{\mathrm{F}1}}_{\mathrm{}}^{\mathrm{}}}{4}$

Genotypic variance ( ${\mathrm{\sigma }²}_{\mathrm{G}}$ ) and its additive ( ${\mathrm{\sigma }²}_{\mathrm{A}}$ ) and dominance ( ${\mathrm{\sigma }²}_{\mathrm{D}}$ ) components were estimated according to Mather and Jinks (1977Mather K, Jinks JL1977 Introduction to biometrical genetics. Cornell University Press, New York, 231p). From these averages, the average degree of dominance was estimated to determine the allelic interaction of resistance. The heritability in the broad ( ${\mathrm{h}²}_{\mathrm{b}}$ ) and narrow ( ${\mathrm{h}²}_{\mathrm{n}}$ ) senses was calculated according to Allard (1960Allard RW1960 Principles of plant breeding. Wiley, New York, 485p). Heterobeltiosis from susceptible and resistant parents were calculated using the following formulas:

${\mathrm{h}}_{\mathrm{b}\mathrm{S}}=\stackrel{-}{\mathrm{F}1}-\mathrm{}\stackrel{-}{\mathrm{P}1}$

${\mathrm{h}}_{\mathrm{b}\mathrm{R}}=\stackrel{-}{\mathrm{F}1}-\mathrm{}\stackrel{-}{\mathrm{P}2}$

The minimum number of genes (n) determining bean resistance to Fop was estimated using the method described by Castle (1921Castle WE1921 An improved method of estimating the number of genetic factors concerned in cases of blending inheritance. Science 54:223):

$\mathrm{n}=\mathrm{}\frac{(\stackrel{-}{\mathrm{P}1}-\mathrm{}\stackrel{-}{\mathrm{P}2})²}{8\mathrm{}{\mathrm{\sigma }²}_{{\mathrm{G}}_{\mathrm{F}2}}}$

The frequencies of resistant, intermediate, and susceptible (R, I, and S) plants of the genitors, hybrids, and F₂ generation were grouped according to the disease severity grade. Based on the frequencies of R, I, and S plants of the F₂ generation, hypotheses of Mendelian inheritance of resistance were tested using the chi-square test at 5% probability with the aid of the software Genética Básica On-Line (GBOL) (Cruz 2011Cruz CD, Viana JMS, Carneiro PCS, Bhering LL2011 GBOL - software para ensino e aprendizagem de genética. Editora UFV, Viçosa, 326p).

RESULTS AND DISCUSSION

The cultivars CVIII 8511, Dama, Pérola, Madrepérola, Star, BRS Uai, Meia Noite, Agronorte, CNFC 11965, and BRS Estilo presented SFW values of 1.0, 1.2, 4.3, 4.6, 5.0, 6.2, 7.1, 8.7, 8.85, and 9.0, respectively. The line CNFC 11965, selected as a susceptible genitor (S), showed an SFW grade between 7 and 9, with an average of 8.85. This line was previously identified as susceptible to Fop with high grades, which resulted in the death of plants (Batista et al. 2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233, Batista et al. 2017). In contrast, most plants of line CVIII 8511, selected as a resistant genitor (R), showed no symptoms of the disease in our study. This line was also characterized by the lowest values of general combining ability considering the severity of Fusarium wilt, indicating a high frequency of Fop resistance alleles to isolate FOP UFV 01 (Batista et al. 2016).

The plants of the F₁ generation from the cross CNFC 11965 (S) × CVIII 8511 (R) were resistant to Fusarium wilt, whereas in the F₂ generation, 249 resistant and 84 susceptible plants were observed. In backcrossing with line CVIII 8511 (BC_R), all plants were resistant, whereas in BCs, 11 resistant and 13 susceptible plants were observed, which revealed dominance towards Fusarium wilt resistance. The frequencies of resistant and susceptible plants in the segregating F₂ generation support the hypothesis that the inheritance of resistance is monogenic and dominant (3:1) (Table 1).

A 3:1 ratio of resistance:susceptibility to Fop, was also observed by Cross et al. (2000) when evaluating the segregating F₂ and F₃ progenies derived from Durango bean. In a study with cowpeas, Musoni et al. (2010Musoni A, Kimani P, Narla RD, Buruchara R, Kelly JD2010 Inheritance of fusarium wilt (Fusarium oxysporum f. sp. phaseoli) resistance in climbing beans. African Journal of Agricultural Research 5:399-404) reported the inheritance of resistance as monogenic dominant to Fop and recommended the use of the backcrossing method for common bean improvement. When evaluating the F₂ generation of the BRSMG Talismã × BRS Estilo cross, 3:1 segregation was identified for carioca bean resistance to Fop (Batista et al. 2017Batista RO, Silva LC, Moura LM, Souza MH, Carneiro PCS, Carvalho Filho JLS, Carneiro JES2017 Inheritance of resistance to fusarium wilt in common bean. Euphytica 213:1-12). However, in other crosses between carioca bean genotypes, the 57:7 hypothesis for Fop resistance was observed (Batista et al. 2017) contrasting with the 3:1 result of the present study. Recently, nine single-nucleotide polymorphism markers associated with oligogenic resistance to Fop have been identified (Leitão et al. 2020Leitão ST, Malosetti M, Song Q, Eeuwijk FV, Rubiales D, Patto MCV2020 Natural variation in Portuguese common bean germplasm reveals new sources of resistance against Fusarium oxysporum f. sp. phaseoli and resistance-associated candidate genes. Phytopathology 110 (3): 633-647.). These data suggest that when studying the same crop, the difference between genotypes confers a variability in resistance genes, and inheritance is essential for the choice of the method of conducting the improvement program.

The SFW grades of CNFC 11965 (S), CVIII 8511 (R), F₁, and BC_R generations (0.29, 0.56, 0.25, and 0.49, respectively) varied minimally in each population, resulting in low variance values, with all plants of P₁ being susceptible, whereas the P₂, F₁, and BC_R generations were resistant (Table 2). In contrast, F₂ and BC_S showed two phenotypes (resistant and susceptible), resulting in increased diversity of means and consequently variances of large magnitude (9.73 and 13.46). The difference between the variances of F₂ and BC_S is owing to the different proportions between them, in which F₂ showed a ratio of 3:1 (–:¼), whereas BC_S showed a proportion of 1:1 (½:½). The lowest environmental variance (0.3361) indicated experimental precision under controlled conditions. Pereira et al. (2009Pereira MJZ, Ramalho MAP, Abreu AFB2009 Inheritance of resistance to Fusarium oxysporum f. sp. phaseoli Brazilian race 2 in common bean. Scientia Agricola 66:788-792) and Cândida et al. (2009Cândida DV, Costa JGC, Rava CA, Carneiro MS2009 Controle genético da murcha do fusário (Fusarium oxysporum) em feijoeiro comum. Tropical Plant Pathology 34:379-384) analyzed the inheritance of resistance to Fop and observed few environmental effects on the characteristics. This implies that the phenotypic variance (9.73) is mostly due to the genotypic variance (9.39) (Table 3).

The variance owing to dominance deviations (3.89) was a considerable part of the genotypic variance, demonstrating the dominance effect verified by the 3:1 hypothesis. It is worth noting that the dominance of this trait is in the sense of conferring resistance, as low grades indicate plants with low disease severity (Cândida et al. 2009Cândida DV, Costa JGC, Rava CA, Carneiro MS2009 Controle genético da murcha do fusário (Fusarium oxysporum) em feijoeiro comum. Tropical Plant Pathology 34:379-384). However, the additive variance (5.50) shows a substantial value of the allele effect in contributing to resistance. In the pathosystem Bean × Fop, several previous studies have reported dominance, but with expressive additive effects (Cândida et al. 2009Cândida DV, Costa JGC, Rava CA, Carneiro MS2009 Controle genético da murcha do fusário (Fusarium oxysporum) em feijoeiro comum. Tropical Plant Pathology 34:379-384, Pereira et al. 2009Pereira MJZ, Ramalho MAP, Abreu AFB2009 Inheritance of resistance to Fusarium oxysporum f. sp. phaseoli Brazilian race 2 in common bean. Scientia Agricola 66:788-792, Batista et al. 2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233).

Heritability is one of the parameters that helps in the selection procedure used by plant breeders, and indicates the proportion of genotypic variance in the total phenotypic variance (Cruz et al. 2012Cruz CD, Regazzi AJ, Carneiro PCS2012 Modelos biométricos aplicados ao melhoramento genético. Editora UFV, Viçosa , 514p, Ramalho et al. 2021Ramalho MAP, Santos JB, Pinto CABP, Souza EA, Gonçalves FMA, Souza JC2021 Genética na Agropecuária. UFLA, Lavras, 508p). The high broad-sense heritability estimates of 96%, as listed in Table 3, suggest a higher proportion of genetic variation in resistance and lower influence of environmental factors on disease severity. In addition to the low evidence of environmental effects (0.34), the divergence between the genitors CNFC 11965 (S) and CVIII 8511 (R) also contributed to an increase in the genotypic variability of the cross and, consequently, the heritability values. Considering the narrow-sense heritability that only includes the effects of additive genetic variance, we observed a value of 56%. Pereira et al. (2009Pereira MJZ, Ramalho MAP, Abreu AFB2009 Inheritance of resistance to Fusarium oxysporum f. sp. phaseoli Brazilian race 2 in common bean. Scientia Agricola 66:788-792) observed broad-sense heritability of 76% and narrow-sense heritability of 34% for resistance to Fop. Higher heritability values indicate a greater possibility of obtaining gains through selection to enhance breeding program success, indicating that backcrossing is an appropriate method.

The heterosis effect, which refers to the superior performance of the average of the F₁ hybrid compared to the average of its genitors, was -3.98, whereas heterobeltiosis, which is manifested when an F₁ is superior to each genitor, was -0.64 and -7.31 in relation to CNFC 11965 (genitor S) and CVIII 8511 (genitor R), respectively. Similarly, an average degree of dominance of -1.19 determined that the average of the heterozygotes was close to the average of the resistant genitor (grade 1). The negative values of heterosis, heterobeltiosis, and average degree of dominance corroborate the effect of resistance alleles, leading to a reduction in the value of disease (disease severity grade) and confirming that the resistance is dominant.

The screening test was relevant to confirm that line CVIII8511 was resistant to Fop isolate FOP UFV 01 and was consequently used in contrasting crosses to study the inheritance of resistance in a new combination with another genitor. To continue the breeding program, especially for the carioca grain type with this line, the selection should start early to advance generation of only resistant plants. The F_2:3 families should be evaluated for agronomic traits such as yield potential, upright plant architecture, and commercial quality of grains. Considering the yield potential evaluated by mean and general combining ability (Batista et al. 2016Batista RO, Oliveira AMC, Silva JLO, Nicoli A, Carneiro PCS, Carneiro JES, Paula Júnior TJ, Queiroz MV2016 Resistance to Fusarium wilt in common bean. Crop Breeding and Applied Biotechnology 16:226-233), the resistance gene of line CVIII 8511 can be easily transferred to other genotypes of the carioca grain type by backcrossing, probably without a negative influence on the frequency of favorable alleles for grain yield.

ACKNOWLEDGMENTS

We would like to thank financial support of CNPq (Process 438698/2018-5) and Editage for English language editing.

REFERENCES

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