Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/7/2010
Publication Date: 1/1/2011
Citation: Twizeyimana, M., Ojiambo, P., Hartman, G.L., Badnyopadhyay, R. 2011. Dynamics of soybean rust epidemics in sequential plantings of soybean cultivars in Nigeria. Plant Disease. 95:43-50. Interpretive Summary: Epidemics of soybean rust and the environmental factors that favor the disease were studied under natural infection in Nigerian soybean fields by sequentially planting early and medium maturing soybean cultivars at intervals ranging from 30 to 45 days. Over a three year period, disease severity for both cultivars consistently was greatest on soybeans planted in the month of August. Rust infection and development on both cultivars were highly affected by environmental factors with strong negative correlation of rust severity to evaporation and wind speed. Spore trapping revealed a year-round availability of inoculum in the air above the soybean canopy. The information on environmental conditions conducive to rust infection and development may enabled researchers to develop better forecasting models to provide recommendations for disease management. This information will be useful for epidemiologist and plant pathologist interested in forecasting and modeling disease epidemics.
Technical Abstract: Soybean rust, caused by the fungus Phakopsora pachyrhizi, is an important foliar disease of soybean. The disease intensity is dependent on environmental factors, although the precise conditions of most of these factors is not known. To help understand what environmental factors favor disease development, two soybean cultivars were planted throughout the year at intervals of 30 to 45 days in experimental plots at the International Institute of Tropcial Agricuture in Nigeria. The disease onset varied by planting date; soybeans planted earlier in the calendar year had rust at an earlier growth stage (pre-flowering), while those planted later in the calendar year did not have rust until the plants were developing seed. Rust severity for both cultivars was significanlty (P < 0.05) negatively correlated to evaporation of water(r = -0.73) and to wind speed (r = -0.71). Other environmental parameters including solar radiation and temperature were also negatively correlated to rust severity. Rust severity was significantly (P < 0.05) affected by planting time and soybean genotype, with rust severity being higher on soybeans planted in the first rainy season of the year than those planted in the second rainy season. Within the first rainy season, soybeans planted in May and July had significantly (P < 0.05) lower rust severity than those planted between August and October. Spore trapping revealed a year-round availability of inoculum in the air above the soybean canopy. This study suggests that selection of planting time could be a useful cultural practice for reducing soybean rust.