Evaluation of effectiveness of rhizobia isolates from Rwandan soils on common bean (Phaseolus vulgaris)

Greenhouse and field experiments were conducted to evaluate the effectiveness of rhizobia isolates from two Rwandan soils and their effectiveness on two types of common bean under different agro ecological zones. A preliminary greenhouse experiment using Leonard Jars in Rubona Station was conducted to evaluate the potential of 174 rhizobia isolates. A selection criterion was based on nodule number, color, size and nodule weight. After 21 days, the best 50 isolates were selected for further evaluation in pots. Highly effective isolates of rhizobia for Common bean were identified through a stepwise approach.. The effectiveness of rhizobia isolates on two types of beans (bush bean (RWR 1667) and climbing bean (Gasilida) was conducted in a pot experiment using CIAT 889, UMR 1597 as standard for comparison and and nitrogen nutrient as control. Both bush and climbing beans were inculated with appropriate rhizobia. tThe five best rhizobial isolates were (NAR 265, NAR 151, NAR 139, NAR75 and NAR 206) were evaluated for their effectiveness in two agro- ecological zones Ruhunde in the North and Rubona in the South. The design was a randomized completed block with nine treatments and three replicates. Before sowing, bean seeds were inoculated with test isolates, commercial strains (CIAT 889 and UMR 1597) while other bean plots were supplied with nitrogen and control plots were not inoculated.

The results showed that beans inoculated with Rwanda isolates had significantly higher number of nodules, plant dry fresh and weight at maturity. The nodules were also large and pink compared to where there no inoculation. These parameters compared well with beans inoculated with standard strains. The analysis of variance on biomass for N total % and P% showed that the difference was highly significant. The study concluded that five rhizobial isolates from Rwanda are highly effective on bush and climbing beans and compared well with rhizobia strains CIAT 899. The results further showed that these strains were not only effective on beans but also able to reduce disease severity on beans thereby boosting bean production in Rwanda.