Does it pay to scout soybeans in September?
By Justin Petrosino 

As we make the transition from fair season to football season most farmers are busy preparing for harvest. Combines are readied in the barn, commodity prices are constantly checked, and decisions are being made about next year’s crop. However, right not is a great time to be out in the field to find what did and didn’t work for this year’s crop. 

After soybean plants reach the R6 growth stage (full seed) leaves will begin to yellow. How quickly plants yellow depends on field conditions. If yellow patches are visible in a mostly green field this is an indication that something has gone wrong this season. Taking a walk to the impacted areas will help reveal the cause of early senescence. Diseases like Phytophthora, sudden death syndrome, and brown stem rot that have been taking a toll on plants all season may have finally done them in. 

Phytophthora is characterized by a chocolate brown stem lesion from the ground up to the first few nodes with yellowing, wilted leaves. Sudden death and brown stem rot have similar foliar symptoms but brown stem rot hollows out the center of the stem leaving a brown, rotted pith. If a white fungal mass is present on the stems of yellowing plants, parts of the field may have been infested with white mold during flowering. Generalized yellowing in a patch with no outright symptoms of disease may indicate a significant population of soybean cyst nematode. 

Fall testing for soybean cyst nematodes can help identify if they are to blame. If no outright cause is apparent look for patterns like wheel tracks, tillage passes, or poorly drained areas. Soil compaction can wreak havoc on soybean plants. Determining if a disease or nematodes are to blame for early plant senescence will not help this year’s crop but will help with deciding next year’s soybean variety. Picking a high yielding variety that is resistant to the diseases present in your field will help ensure better yields next year. 

While in the field keep an eye out for weeds that may have escaped this year’s program. Constant complaints on marestail, giant ragweed, and waterhemp come into the extension office. A strong spring burndown herbicide program that includes a herbicide with residual activity on problem weeds followed by a postemergence application of a broad spectrum product will help ensure all weeds are controlled. A single pass of a glyphosate product is not effective in soybean weed management. Also keep an eye out for weeds that are just emerging. Marestail will begin to emerge in the next few weeks and if it is present a fall application of glyphosate + 2,4-D may be warranted. 

Finally in double cropped fields and fields planted with a late maturing variety keep an eye out for soybean aphids. Aphids arrived a few weeks ago in west central Ohio. Population growth will decrease with the cooler temperatures of September. However, some fields may still be at risk. If an increasing population of 250 aphids per plant is present across the field an insecticide application may be warranted. Before applying a product make sure to check the preharvest interval as many products may require three to four weeks between application and harvest. 

For more information on the diseases, insects, and weeds that are present in your soybean fields consult the Agronomic Crops website at http://agcrops.osu.edu/. The website is the central hub for agronomic information from The Ohio State University. If you prefer a paper document visit your local Extension Office for a copy of the Ohio Corn, Soybean, and Wheat Field Guide (Bulletin 827) and the 2011 Weed Control Guide.