Soybean germplasm reaction to rust in Uganda, associated yield loss, and rust control using fungicides

Currently, Uganda is experiencing a severe out break of soybean rust (phakopsora pachyrhizi sydow), a disease previously unknown in the country until 1996. As part of national efforts to contain the epidemic, three studies were conducted; 1) assessment of yield loss attributable to rust across four diverse locations 2) screening of 51 introduced soybean lines of different maturation dates against rust, and 3) evaluation of different fungicides and spray regimes for management of the rust epidemic. In the first experiment, five varieties: Nam1, Nam2, Namsoy3 (commercial), and UG-5 and GC-00138-29 9 (promising), were grown in central, Eastern, Western, and Northern parts of the country, for three consecutive seasons. Data was collected on: rust severity at full seed formation, yield from rust protected and rust unprotected plots, which was used to compute yield loss and rust tolerance index. Data was also collected on yield components (seed weight and pods per plant), for both rust protected and rust unprotected plots. Results indicated that commercial varieties (Nam1, Nam2 and Namsoy3)were highly susceptible recording higher rust severities (>40%) and yield losses (26.9-33..2%), while the promising lines were quite resistant recording lower rust severities <30% and yield losses <10%. The Central region registered the highest yield loss(22.9%), while the Northern region registered the lowest (15,%). Yield loss was associated with reduction of filled pods per plant and seed weight. The second experiment was conducted at Namulonge Agricultural and Animal Research Institute (NAARI), Central region for three consecutive seasons. Only two lines TGx 1835-10E (early maturing) and TGx 1838-5E (late maturing) were graded as moderately resistant at full seed formation (R6), the rest were either moderately susceptible or highly susceptible. Significant differences were observed in rust severities atn full pod and full seed formation growth stages, with higher severities at full seed stage. Late maturing lines had higher severities as compared to the early and medium maturing lines.