Epidemiological studies of shading effects on Asian soybean rust
Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi, is considered to be the most threatening disease in major soybean production regions worldwide. Leaf infection usually results in premature defoliation, which reduces soybean yield components related to pod number, seed number, and size. Among other environmental factors, sunlight intensity negatively affects P. pachyrhizi biology with possible effects on disease epidemiology. Field observations suggest that higher disease severity occurs in shaded environments, such as on soybean leaves in the lower canopy and kudzu leaves under trees, compared with open ground. In this thesis, investigations have been carried out with the following three objectives: (1) to experimentally quantify the effects of shade intensity and duration on ASR establishment on soybean; (2) to apply results from shading experiments to parameterize a simulation model for temporal disease development using seasonal cloud cover data; and (3) to characterize and compare the spatial and temporal patterns of regional disease movement in different soybean production regions/seasons and to analyze the impact of solar radiation intensity on spatial and temporal disease dynamics.;Results from 2005 to 2007 support field observations that ASR incidence and severity are greater in the shade compared with no shade. Inoculated soybean plants placed in the shade for at least 2 days had significantly greater disease incidence and severity compared with those without shade. From our experimental results, a simulation model was developed to estimate temporal disease progress regulated by cloud cover conditions. The model was validated with disease data from Brazil and South Africa with correlation coefficients greater than 0.9. On average, severe ASR epidemics developed when 18 cloudy days were observed after disease onset, and mild epidemics occurred when only 8 cloudy days were observed. In four growing seasons in Brazil and two in the United States, the progress of Asian soybean rust epidemics did not follow a wavelike pattern, and it resulted in an exponential distribution of distances to disease locations over time with variable monthly expansion rates. The disease front reached 500 km distance from major inoculum sources after 3 months similarly in both countries. Greater solar radiation intensity was associated with delays in epidemic onset. This was the first study to investigate the effects of solar radiation on ASR development; our results may be useful to improve risk assessments for seasonal ASR epidemics.