Inheritance of Fusarium Root Rot resistance in three common Bean Genotyes
Ongom, Patrick Obia
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Fusarium root rot, caused by the fungus Fusarium solani f. sp Phaseoli (FSP) is an important soil-borne disease affecting common bean (Phaseolus vulgaris L.) production in Uganda and many other countries. This pathogen exists in complexes with other root rots, most importantly, with Pythium ultimum var. ultimum. FSP resistance has been reported in a few bean cultivars and was speculated to be associated with the major gene for resistance to P. ultimum. The objective of this study was to understand the inheritance of FSP resistance in bean varieties RWR 719, NABE 14 and NABE 13, and to determine if this resistance is linked to the Pythium resistance gene and/or to the SCAR marker PYAA19800. A 5 x 5 full-diallel mating design involving the 3 resistant and 2 susceptible genotypes (K132 and NABE 4) was used to generate F1 and F2 populations which together with the parents were artificially inoculated with FSP isolate in a screenhouse. The results indicated that there was significant variation for FSP resistance among genotypes. General combining ability (GCA) and specific combining ability (SCA) effects were significant for FSP resistance, indicating importance of both additive and non-additive effects, respectively. The preponderance of GCA effects (2σ2gca/(2σ2gca + σ2sca) = 85% and 90% at F1 and F2 respectively) suggested that additive genetic effects were more important than non-additive effects. The broad sense coefficient of genetic determination (CGD analogous for fixed genotypes to broad-sense heritability for random genotypes, H) calculated on an entry mean basis, was 0.90 in F1 0.96 in F2, while the narrow-sense coefficient of genetic determination (CGD analogous to narrow-sense heritability, h2) was 0.76 and 0.86 in F1 and F2 respectively. These results suggest that selection and backcrossing would be the best breeding procedures for improving resistance in the popular large-seeded bean varieties in Uganda. Reciprocal effects were not significant (P >0.05), suggesting that cytoplasmic genes did not play a major role in modifying FSP resistance. Heterosis for FSP resistance, relative to mid-parents was significantly negative in several crosses with a range of -4.4% to -49.4%. Two crosses (NABE 4 x NABE 14 and NABE 14 x NABE 13) were more resistant than the better parent, while most crosses were more resistant than the worse parent. Fusarium root rot resistance was conditioned by two or more loci that displayed partial dominance and epistasis, suggesting that inheritance of resistance is complex. Parental lines RWR719 and NABE14 showed highly negative GCA effects qualifying them as suitable parents for transferring resistance genes to their progenies. To determine if FSP resistance is linked to the Pythium resistance gene and/or to the PYAA19800 molecular marker, the F2:3 populations of RWR 719 and K132 cross were screened with the P. ultimum isolate and the SCAR marker PYAA19800. The genetic linkage test showed that resistance to FSP was independent of the major gene for Pythium resistance in RWR 719 (χ2 = 4.96, P = 0.29) and marker-trait association analysis revealed that the Pythium SCAR marker PYAA19 was not associated with FSP resistance in RWR 719 (R2 = 0.005, P ≤ 0.49), but was strongly associated with Pythium resistance (R2 = 0.77, P ≤ 0.001). To the author’s awareness, the lack of genetic association between resistances to FSP and Pythium in a segregating population has not been reported previously, and contradicts the common speculation of possible linkage and or pleitropy of the two resistances that has been based on the joint occurrence of both resistances in the available donor genotypes.