On your soy plants, look at the lower leaves with yellow discoloration and brown spots. It's a short leap of imagination to see why this fungal disease is called brown spot.
You haven't heard tons about brown spot because it is normally a disease that occurs on lower leaves later in the growing season. Leaves infected with it may die prematurely. However, as long as it is limited to the bottom of the plant, the impact is minimal, if any.
A plant agronomist says you are more likely to see these symptoms already in soybeans that were planted in late April or early may, or on earlier-maturing varieties. As long as the disease is limited to the lower part of the plant you have little to anything to worry about, he believes.
Prolonged periods of wetness and moderate temperatures favor the disease. Its preferred temperature range is 60 to 85 degrees F. It's not hard to understand why you might see some of it in fields and on plants this year, notes Cody Kerr with DeKalb. He's a sales rep with territory in northern Ohio.
Many areas have been wet in both Ohio and Indiana, and temperatures have certainly been moderate for the most part.
Kerr does not worry unless the disease is moving upwards in the canopy. Normally it's limited to lower leaves which aren't as useful to the plant later in the season anyway, he notes. Only limited sunlight typically makes its way down to those lower leaves during the reproductive stage of growth.
The Purdue University Corn and Soybean Field Guide published for 2014, also updated for 2014 as an app for iPad, says you can select varieties with more resistance if you see enough of it and are concerned about it. You could also rotate out of soybeans or return to tillage if it becomes severe for some reason.