Corn Update 6/6 – Nitrogen, Irrigation, and Fungicides

June 7, 2023

Across the state, the earliest planted corn I have looked at is looking very good as we begin to see tassels emerge and pollen shed. In many cases, a lot of our earliest planted corn this year has been under irrigation which sets us up to produce a very good crop. However, we are not home free yet. Tasseling corn has the greatest water use demand out of the entire growing season, so if irrigation is available and you are not receiving 2+ inches of rainfall per week, keep the water to the corn as best you can to meet the water use demand. Water use curve information can be found in the 2022 Corn Production Guide. A table in the guide estimates weekly water use throughout the growing season of corn.

Calls regarding poor-looking corn across the state have been common this spring. Reports of yellow, slow-growing, and variable corn have been the most common. In most cases, pulling soil, tissue, and nematode samples have resulted in nematode issues in some fields. Other instances have resulted in poor-looking corn but sufficient nutrient levels in the soil and tissue. In these situations, we feel confident that the cool temperatures, especially at night, we have had this spring are to blame for slow-growing corn. Limited root development can lead to nutrient-deficient symptoms even though nutrient levels in the field are adequate. Since we are beginning to experience warmer temperatures over the last few weeks, additional rainfall, irrigation, and where side-dress N has been applied, many of these symptomatic fields have cleared up.

Stunted corn roots
Stunted corn roots. Photo Credit: Jay Crouch
Stunted Corn Field
Yellow Stunted Corn. Image Credit: Charles Davis

A few farmers have also inquired about a tassel shot of nitrogen or a very late application of N on good yield potential, irrigated corn. I recommend that you have all of the N you wish to apply to the crop by R1 or silking. After this point, the likelihood of you getting any response from additional N applied is very low. The crop has taken up 80+% of its N by this time. Therefore, tassel applications are often small in quantity and typically only applied in high-yielding environments.

Lastly, fungicide sprays are on everyone’s mind now that we have reached tassel. Before automatically pulling the trigger on applying a fungicide to corn, a few things to consider. First, when was the corn planted? It is not uncommon in SC to “outrun” a lot of our disease, especially southern rust, which is blown in from GA and FL in years we see it, so if the field that you are considering spraying was planted early (before April 1) and your primary concern is southern rust, you could consider holding off for a little while. Second, what kind of disease package does your hybrid(s) have. Many of the commercially available hybrids we grow have good ratings on resistance to northern corn leaf blight, southern corn leaf blight, and grey leaf spot, all of which could be issues in our corn. If you scout your corn and leaves look clean, we continue to have hot sunny weather, and your hybrid has good resistance to many common diseases we face, you may have time to hold off on a spray. Third, what has been your crop rotation? In SC, we typically have a short/no rotation on irrigated ground with corn because many producers want corn under irrigation. In fields of continuous corn, a well-timed fungicide spray will likely be warranted to prevent disease issues. Fourth, scout your fields and know what disease is present, if any at all. We have seen lots of northern corn leaf blight, some southern corn leaf blight, and what we believe to be a touch of Holcus leaf spot in corn around the coastal plain. The NCLB and SCLB are both of concern and can be prevented with fungicide; the Holcus leaf spot, however, does not appear to be a big concern as of now with such low levels where seen (often mistaken for paraquat drift) and cannot be controlled with a fungicide.

Holcus Leaf Spot
Suspected Holcus Leaf Spot. Image Credit: Charles Davis

Remember that the residual on fungicides do not guarantee complete control from tassel to R5 (dent); depending on the application date, the product selected, and sometimes the number of modes of action contained in the product, control may vary from 14 to 28 days. On average, depending on heat unit accumulation, corn can take over 35-40 days to progress from tassel to dent, so holding off on fungicide sprays in situations where the current disease risk is low can help ensure the fungicide is applied at the right time and can help prevent late season disease or warrant a respray on high yielding corn.

If you have any questions on irrigation, fungicide spray timing, or anything else corn related, please contact your local agronomy agent.