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Assessing the rainfall suitability for Asian soybean rust epidemics in the US soybean production regions.

Presenter: Emerson Del Ponte, Department of Plant Pathology, Iowa State University, Ames, IA 50011
Coauthor(s): Xun Li, Department of Plant Pathology, Iowa State University, Ames, IA 50011. X.B. Yang, Department of Plant Pathology, Iowa State University, Ames, IA 50011.

Previous risk assessment studies have demonstrated the suitability of temperature and dew conditions for soybean rust development in US soybean production regions. Earlier work in China and recent observations in Brazil suggested the importance of rainfall in the epidemics. A model was developed based on the relationship between rainfall and final disease severity by using a range of location/year data in Brazil. This model uses rainy days and rain amount in a month after disease detection/onset and was used to quantitatively assess the risk of epidemics. In our assessment, we selected twenty locations arbitrarily across US soybean regions. Two scenarios for earlier (late June) or later (late July) stage of disease onset were used for each location. Thus, rainfall in July or August, the considered time for disease development, was used to estimate severity. Two suitability indices, that is the likelihood of a moderate (severity >25%) or severe (>50%) epidemic to develop, were calculated based on the epidemic frequency for each location/scenario with a historical data of 50 years. Estimated severities were higher for lower latitudes and earlier disease onset. July is more suitable for epidemics than August. The rainfall pattern in most locations seems suitable (index >50%) for light to moderate epidemics for most of years. However, for regions south of Tennessee, the suitability index for a severe epidemic ranges from 18-43% if disease onset occurs before late June or 10-31% if disease onset occurs before late July. In our calculations, rust spores were assumed available locally and temperature effects were not considered.

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