Early Season Soybean Diseases: Don’t Rush to Spray

We have started to receive some questions regarding the need for foliar fungicides in soybean. In walking our research fields at Arlington and West Madison this week, we noticed evidence of brown spot, otherwise known as Septoria leaf spot (causal organism: Septoria glycines) and bacterial blight (causal organsim: Pseudomonas syringae). Given the spring conditions we have had, with cooler and wet, and now more warmer and wet conditions, this find was not a surprise. Brown spot typically occurs yearly in soybean fields and has rarely been shown to cause an economic yield reduction. Brown spot can be easily differentiated with bacterial blight in that brown spot is found in the lower and older leaves while bacterial blight can be found in the upper canopy. Research has shown that if a fungicide is applied to slow down the severity of brown spot, applications made from bloom to pod fill are the best timing.

Figure 1. Symptoms of brown spot in the lower soybean canopy at Arlington on June 23, 2008.

Figure 2. Symptoms of bacterial blight in the upper soybean canopy at Arlington on June 23, 2008.