As folks can get in through the muddy fields a couple of foliar diseases are beginning to be spotted.
Septoria brown Spot: This fungus survives on old soybean residue and is splashed onto lower leaves.
The first, Septoria brown spot, is a normal disease found throughout Ohio on the very lower canopy. On unadapted germplasm or old, old varieties this leaf spot would move up through canopy and reach the top leaves and contribute to yield loss. In today’s modern germplasm, this disease is kept in check and is kept in the lower 1/3 of the plant. We completed a study over 3 years and at two locations on four different soybean varieties. We used a special fungicide program to grow soybeans that had healthy leaves with no brown spot and could only measure a 2.9 to 4.3 bushel yield difference. In addition, with fungicide applications at R3 or R4, across studies that I have done but also in collaboration with Laura Lindsey’s lab, we have managed the disease to some extent (average of 1.2% reduction of Septoria in bottom canopy across 12 site-years), but the yields from treated and nontreated plots were similar (average yield increase of 3.7 bu/acre across 12 site-years). So this disease is really not one to worry about as these yield increases do not pay for the cost of application.
Bacterial leaf spot – Note the darker color and limited lesion development.
Bacterial blight is also present in a number of fields this year. This is a bacterium, so a fungicide will not have any effect. The lesions are darker, and a little bit bigger than brown spot lesions. The Bacterial leaf spot is also surrounded by a yellow (chlorotic) border. When several lesions grow together, the leaf will tear and has a shredded appearance. This is a rare disease, but we have picking it up in fields following periods of heavy rains and winds. If it is very severe, that variety needs to be dropped from that company’s line up.
Bacterial leaf spot (left) vs Septoria brown spot
The only reason for a fungicide spray this year in soybeans at that R3/R4 is for Frogeye leaf spot. And that will only be necessary for two reasons, the variety is moderately to highly susceptible AND you can easily find frogeye in the field. In 2015 and 2016, the Lindsey Lab conducted R3 foliar fungicide trials across the state. In 2015, soybean yield increased with fungicide application at three out of six locations by 7.9, 6.3, and 4.4 bu/acre in Clinton County, Marion County, and Preble County, respectively. In 2016, soybean yield increased with fungicide application in Mercer County only by 7.1 bu/acre. (Note: 2015 was a wetter year compared to 2016 creating a favorable environment for frogeye leaf spot.) Soybean yield response was related to a reduction in leaf area affected by frogeye leaf spot. In our research, when there was no frogeye leaf spot (0% leaf area affected) prior to the R3 fungicide application, there was no yield increase after the foliar fungicide application.
We’ve taken a yield hit already with the excessive moisture that has fallen across Ohio so far this season. Unfortunately, the price of soybean is not where we would like to cover all of the inputs we have put in to date. We also have compaction issues – where another trip across fields that are not drying out is going to have some long-term damage. It is always good to know how the varieties are responding to all of the different pathogens out there for future variety selection decisions. But it is also very hard to not do something and save the money.