Modeling the effect of El Niño Southern Oscillation on the onset of soybean rust in Southern Brazil

Abstract

Soybean rust (SBR; Phakopsora pachyrhizi) is one of the most damaging fungal diseases of soybean, particularly in Brazil where farmers rely on fungicide sprays. Disease risk is variable among regions, as well as between years; the main drivers include the availability of inoculum early in the season and both pre- and within-season weather conditions. El Niño-southern oscillation (ENSO) is known to affect global climate. In southern Brazil, precipitation levels are above normal during spring and summer in the El Niño phase. Here, we describe the temporal and spatial features of SBR spread across municipalities in Southern Brazil and evaluate the effects of ENSO events and anomalies on the sea surface temperature (SST) in the El Niño 3.4 region in the central Pacific on those features. We used data provided by the 'Consórcio Antiferrugem' on disease reports during 17 crop seasons (2004/05 to 2020/21). The temporal and spatial aspects of SBR epidemics were variable among the years and significantly affected by El Niño events. A spatially structured Cox proportional Bayesian model suggested that the anomalies in the normal SST during May-June-July and October-November-December trimesters increase the risk of early SBR onset in municipalities. In general, epidemics are expected to establish and spread early and faster across the entire region of southern Brazil during the El Niño seasons. Results of this study provide insights into the large-scale spread of soybean rust across southern Brazil, which may be useful for seasonal outlook and decision-making when planning disease management.