Fungicide Use in Corn and Soybeans

Some corn may have been sprayed as early as V5 but now we are approaching tasseling and some fields have already begun to tassel. With the corn prices being what they are more growers are inclined to apply a fungicide. Multiple studies have shown that a single application of a DMI (FRAC Group 3) + QoI (FRAC Group 11) fungicide at VT-R1 provide the best chance of return on investment. The Corn Disease Working Group of the Crop Protection Network updates the corn fungicide efficacy table in the following link.

The last few years we’ve seen more Gray leaf spot on corn in Virginia than other foliar disease. Be aware that if you used Xyway in-furrow you may not be protected against Southern corn rust that comes in later in the season some years. Typically, by the time Southern corn rust is observed in Virginia it will not impact yield greatly. However, for late-planted corn fungicide application must be made by R3 (milk stage) to limit yield loss due to Southern corn rust.

In Virginia, our primary foliar disease of soybean are frogeye leaf spot, Cercospora leaf blight and Septoria brown spot. Scouting for these diseases at or prior to beginning flower (R1) will give an estimation on the amount of disease pressure in a given field if diseases are present. Fields with disease pressure early are more likely to benefit from a fungicide application. Fungicides have the best chance of potentially reducing yield loss to foliar pathogens when sprayed beginning at pod initiation (R3) through seed initiation (R5). If dry conditions are prevalent during this spray window a fungicide spray is not advised. Be aware that foliar fungicides do not offer much protection against seed decay organisms or soilborne pathogens of soybeans. Below is the link for the Crop Protection Network’s soybean fungicide efficacy table.

It is not advisable to spray QoI (FRAC Group 11) fungicides alone as fungicide resistance to this group has been observed in the fungal pathogen that causes frogeye leaf spot.

If you have questions or concerns please don’t hesitate to contact me.

David B. Langston, Jr.
Professor and Extension Plant Pathologist
Tidewater Agricultural Research and Extension Center
6321 Holland Rd., Suffolk, VA 23437
Office (757) 807-6536
Cell (757) 870-8498
FAX (757) 657-9333