Time of Soybean Sudden Death Syndrome Foliar Symptom Onset Influences Final Disease Intensity, Yield, and Yield Components.

Sudden death syndrome (SDS), caused by , causes substantial yield losses in soybean. However, relationships between soybean yield and components of disease progress, including time of disease onset, are poorly understood. Individual soybean plants (2018) and quadrats (2016 to 2018) were monitored in commercial fields and experimental plots in Iowa to quantify the impact of SDS foliar symptom onset on final SDS intensity, soybean yield components, and yield. The date when SDS foliar symptoms were first detected (onset time) and progress of SDS incidence and severity were recorded weekly. Individual soybean plants and quadrats were harvested at the end of each season. Beta-regression showed that date of SDS onset had a consistent and stable effect on final disease intensity both at individual plant and quadrat levels. The slope of the relationship between date of SDS onset and final SDS severity was common across all field sites and years. Weighted linear regression revealed that SDS onset explained 60 to 83% of the variation in number of pods, number of seeds, and total seed weight in individual plants, and 94 to 97% of the variation in seed yield in quadrats. Soybean yield damage functions (slopes) indicated that for each day SDS onset was delayed, soybean yield increased by 30.5 to 31.3 kg/ha. This new quantitative information improves understanding of the impact of SDS on final disease intensity and soybean yield. Further experiments are needed to determine how this relationship is affected by site-specific factors.