How Does Soybean Rust Cause Yield Loss.

Reductions in soybean seed yield due to Soybean rust (SBR) have been attributed to leaf loss by disease-infected plants. The objective of this study was to assess effects of SBR infection on soybean yield and to identify causes for the SBR-induced yield reduction.  Studies were carried out in the 2005/2006 and 2006/2007 growing seasons at Londrina, Paraná, Brazil. Treatments involved ASR infection beginning at either the R1 or the R5 growth stage, manual defoliation to mimic the leaf loss observed with those two treatments, and a control treatment. Control and defoliation treatments were protected against ASR through fungicide applications.  Disease severity, lesion area, and leaf area were monitored from R1 to maturity and biomass and seed yield were measured at maturity. Mean ASR-induced yield reductions were 67% when infection started at R1 and 50% when infection started at R5. Effect of leaf loss on the ASR-induced yield reduction was relatively small and neither the area under the disease progress curve nor leaf area duration after R1 were associated well with seed yield. Besides leaf loss, the ASR-induced yield response resulted from (i) reduction in green leaf area corrected for ASR lesions, (ii) reduction in dry matter accumulation per unit absorbed radiation by green leaf area, and (iii) reduction in the proportion of dry matter allocated to the seed at maturity (i.e., harvest index, HI).  The response of HI was attributable to reduced seed set and seed mass resulting likely from ASR-induced reductions in rate of dry matter accumulation.