Fungicides, Pre-Harvest Weed Control, Stinkbugs, and Irrigation

July 14, 2021


Everyone had to watch as Elsa passed over South Carolina over the last few days. Fortunately, for the most part if you were off the coast rainfall and winds stayed in the manageable levels.  However, it very well may have blown spores into our state for several diseases including Southern rust on corn, soybean rust, and possibly frogeye or target spot on soybean.

Dr. Kemerait, the UGA Row Crop Pathologist has reported that Southern rust is now present in Burke County, Georgia, which is located just across the Savannah River from SC.


Here are a few suggestions for dealing with these possibilities and some other developing situations.

Most of our corn is in our entering the R-5, the dent stage. At this point the kernels have accumulated all the dry matter that they will for the year. Although diseases like Southern rust may have blown into your fields and infected your corn, they can be very common on leaves but are unable to affect dry matter accumulation in the kernels. In other words, spraying a fungicide now to control most disease situations is not warranted if the plants are in R-5 or later.

We seem to be very fortunate in that most of our corn is relatively free of foliar diseases.

Late Season and Pre-Harvest Weed Control Options

As corn season starts to wind down and harvest is on the horizon, late season weed growth can be a major impediment during harvest. The crop canopy starts to open and allow light to penetrate to lower parts of the plants during maturity. Small to medium weeds present at the base of the corn plants that were shaded can start rapidly growing. In South Carolina, annual morningglory tends to be our number one harvest impediment weed in corn. These green vines tend to wrap around the knives or other moving parts on the combine head and/or clog the machine if enough green matter is pulled into the feeder. Other weeds of concern include sicklepod, Palmer amaranth, and Texas panicum. There are several herbicide options available to desiccate these weeds prior to harvest. Aim (carfentrazone, 1-2 fl oz/A) and Parazone (paraquat, 0.8-1.3 pt/A) are labeled for ground or aerial applications. Aim provides excellent desiccation of problem weeds like annual morningglory. Glyphosate (22 fl oz/A) is also an option, but its activity on large weeds is marginal. In addition, desiccation or drying of the weed foliage with Glyphosate is minimal. With sensitive crops, such as cotton, still growing during August, 2,4-D is not a recommended pre-harvest option for corn. See product labels for the recommended adjuvants. Apply these herbicides to corn after black layer formation and/or grain moisture is 35% or less.

Corn with Morningglory

Image Credit: Clemson University


Over the last week I have received a few calls about stink bugs in corn and whether or not they should spray. If you are seeing high levels of stinkbugs on field edges or next to small grain fields I encourage you to scout the entire field to ensure that the levels across the field warrant a treatment. Dr. Reay-Jones, has shared a LGPress article about stinkbugs in corn that lists thresholds for treatment.

The link to an ADA article is here:

The table containing the thresholds is based on 100 plants sampled throughout the field. This table is below.


Dr. Kemerait, the UGA Row Crop Pathologist in Georgia, has reported that although rust has been found on kudzu in Georgia, it has not been found on soybean.


The situation in soybean is the opposite of where we are in corn when it comes to spraying foliar fungicides. Most of our crop is still in vegetative stages except for the early planted Maturity Group IV and V soybeans. Barring unforeseen circumstances, we normally try to hold off on spraying soybeans until they reach flowering, or preferably a few days later when they have beginning pod fill. Applying the same logic as with corn, soybeans do not reach optimal grain fill until the end of R-6. So, if we see disease developing in soybean we would spray up until the end of R-6 which is “full sized seed in full sized pods”, i.e. maximum grain fill.  The labels on most fungicides forbid spraying the fungicide after R-6.

I walked some fields at the station today and looked at 100 leaves under a dissecting microscope. NO rust (as of 7/8).  I saw 2 leaves with very early signs of downy mildew.  However normally we do not spray for downy mildew as it causes very little yield loss and goes away when we leave rainy periods or periods with heavy dews.

According to my weather forecaster we appear to be in a typical July thunderstorm pattern. I would say this puts us at a relatively high risk for any foliar diseases your variety is not resistant to. This is especially true in fields which have been in soybeans for multiple years in a row.


According to the USDA crop progress report, 96% of our soybean crop has been planted, 92% has emerged and 12% is beginning to bloom. Water use in early planted soybean reaching reproductive growth (~8-10 weeks after planting) is increasing, a majority of our soybeans are approaching this window. We have been receiving some rainfall around the state especially with the tropical storm that just passed, which has helped meet early season water demands. Use the water use curve shared below to better understand soybean water use when irrigating a soybean crop. At its peak, soybean uses approximately 1.5 inches of water per week or 0.22 inches per day (around R2-R3 growth stage).