Fusarium root rot (FRR) is a major disease of common bean worldwide. Knowledge of the inheritance of resistance to FRR would be important in devising strategies to breed resistant varieties. Therefore, a 12 9 12 full diallel mating scheme with reciprocal crosses was performed to generate 132 F 1 progenies, which were then advanced to the F 3 . The progenies were evaluated for resistance to FRR under green house conditions in Uganda. General combining ability (GCA) effects were highly significant (P B 0.01) for disease scores. Specific combining ability effects were not significant (P [ 0.05) in the F 1 , but were highly significant (P \ 0.01) in the F 3 generation. These results indicate that resistance to FRR was governed by genes with additive effects in combination with genes with non-additive effects. Reciprocal differences were also significant (P = 0.01) at F 1 and F 3 , primarily reflecting a large influence of maternal effects in both these generations. In fact, susceptible parents did not differ significantly (P [ 0.05) for disease scores when used as paternal parents in the F 3 , but differed strongly as maternal parents (P = 0.0002). Generally, the progenies were distinctly more resistant when the resistant parent was used as the female in crosses, especially as observed in the F 3 . The maternal effects were strong in the F 3 generation, suggesting a complex form of cytoplasmic-genetic interaction. The non-maternal reciprocal effects in the F 3 were significant (P \ 0.05) in both the resistant 9 resistant diallel, and in the resistant 9 susceptible crosses. Mid-parent heterosis (MPH) occurred in most crosses, with average heterosis approximately equal in each of the three generations, indicating that epistasis was probably more influential than dominance of individual genes. Genenumber formulas indicated that several genes were involved in resistant 9 susceptible crosses. Among resistant 9 resistant crosses, many produced continuous distributions of F 1 progeny scores, suggesting polygenic inheritance, while bi-modal distributions were characteristic of the F 3 distributions, and fit expected ratios for two or three loci segregating in each cross. Dominant forms of epistasis favoring resistance were strongly indicated. Parent-offspring heritability estimates were moderate. Overall, the results indicate that resistant parents contain a number of different resistance genes that can be combined with the expectation of producing strong and durable resistance. The lines MLB-49-89A, MLB-48-89, RWR719 and Vuninkingi, with large and negative GCA effects, contributed high levels of resistance in crosses and would be recommended for use in breeding programs.

Root rots cause severe yield losses of up to >70% in common bean (Phaseolus vulgaris L.) in most parts of the world, with devastating effects on the major commercial bean cultivars in East and Central Africa. Increased intensity of droughts and rains, and higher temperatures influence the occurrence and distribution of root rots, resulting in increased epidemics. The causal pathogens tend to occur in a complex, and since most cultivars do not have broad resistance, adverse effects continue to occur. The objective of this study was to evaluate the levels of dual resistance of new breeding lines (BL) developed for root rot resistance, as well as Andean (ADP) genotypes, for resistance to Fusarium (FRR) and Pythium root rots (PRR). Altogether, 316 new BL developed for root rot resistance and 295 ADP bean genotypes were evaluated at Kawanda in Uganda. There were significant differences (P<0.05) among genotypes for both root rots and yield. Thirty and 1.9 percent of the BL and ADP genotypes expressed resistance to both root rots. In addition, more than 80% of ADP genotypes showed susceptibility to both root rots. Yield was generally poor with means of 458 kg ha-1 for ADP, and ranging from 949 to 1075 kg ha-1 for the BL groups. Nonetheless, the 0.3 and 2.4% of the ADP and BL that yielded >2000 kg ha-1 expressed high yield potential, considering that majority of the genotypes yielded below 1000 kg ha-1.

Though Rwanda is among the highest common bean producers and consumers, this crop is still challenged by different factors including root rot diseases resulting in low yield potential. Root rot diseases can cause up to 100% of yield loss. Despite different soil borne pathogens causing bean root rot diseases; there is no recent literature that clearly documents the most aggressive ones affecting common beans in Rwanda. This study aimed at identifying the root rot pathogens affecting bean crop in Rwanda. A survey was conducted to collect bean samples showing root rot symptoms in 12 agro -ecological zones where bean crop is mostly grown. The selection of samples was based on the level of contamination and the purity of cultures. A total number of 132 samples were therefore selected and considered for further analysis. The pathogen isolation was done in Rubona at Rwanda Agriculture and Animal Resources Development Board (RAB) Biotechnology and Pathology laboratory. Structural characterization of soil borne pathogens was also carried out along with pathogenicity evaluation. The growth rate of identified pathogens were significantly different (P<0.001). Significant differences were observed among isolates inoculated on both plantlets and seeds (P<001). Four root rot diseases were identified and characterized including Rhizoctonia spp, Fusarium spp,Sclerotium spp and several Pythium spp. The four pathogens were mostly observed in Northern and Southern parts of Rwanda than East and Western parts of Rwanda. Morphological characterization showed that Fusarium spp, Rhizoctonia spp and Sclerotium spp were predominant than Pythium species. Fusarium spp was identified as the most predominant fungal pathogen while Sclerotium spp was the most virulent pathogen. The present study suggests bean root rot disease control program to reduce losses associated with these diseases especially the use of resistant varieties since the fungi are widespread and survive in soils for a long period of time.

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