Fungicides and Southern Rust in Corn

June 11, 2021

With extensive rains occurring across much of South Carolina in the last week our corn crop is getting back on track and more than half of it is in the process of tasseling.  This is the growth stage where most growers will be deciding whether to spray a fungicide for foliar diseases.  Here are a few tips and questions to ask yourself to help you make this decision.

First you need to check the information sheets on your hybrid.  Most hybrids have good resistance to our primary diseases such as Anthracnose leaf blight, Gray leaf spot, Northern Corn Leaf Blight, and Southern Corn Leaf Blight.

Disease incidence and severity tend to be highest in fields with:

  1. High plant populations
  2. Irrigated fields
  3. Fields that have been planted to corn for 2 or more consecutive years; especially those in minimum tillage systems.
  4. Fields that have received excessive rainfall or had heavy morning dews during and immediately after tasseling

The best results from a fungicide normally are obtained if the fungicide is applied between tasseling and R-3.  After R-3 the return on spraying normally goes down, especially for diseases such as Southern Rust.

Fungicides with multiple modes of action, i.e. multiple active ingredients, tend to having longer efficacy than the 2 weeks normally associated with just one mode of action.

A quick guide to midseason corn growth stages from tasseling to maturity:

VT:  The entire tassel has emerged, regardless of whether it is shedding pollen.

R1:  Silk-one or more silks become visible beyond the husk leaves

R2: Blister – Kernels have a blister like appearance, with clear liquid on the interior and an ivory exterior.  Occurs approximately 10-14 days after silking.

R3: Milk

R4: Dough

R5: Dent

R6: Physiological Maturity

Southern Rust seems to be the foliar disease that most growers worry about on corn.  Southern rust does not overwinter in South Carolina but overwinters in southern Louisiana and Texas.  Inoculum must blow up from those areas.  It usually shows up in Georgia before we see it in South Carolina.  However, there is no definitive time interval between when it is first seen in Georgia and when we first see it in South Carolina.  Likewise, there is no rule defining how long it takes Southern rust spores to travel a given distance.  It is all dependent upon wind speeds and directions.

Southern rust has been detected by the University of Georgia Extension Pathology group led by Dr. Kemerait. They reported Southern rust in Coffee County not far from the Jeff Davis County line.  This is a little more than 100 miles from Hampton County, South Carolina.  So, we are guessing it is probably going to take a week or more to show up in South Carolina, but that is just a guess.  With recent weather patterns the winds are certainly blowing some rust spores our way.

If someone says that you have Southern rust in your field be sure to confirm that it is not common rust.  Pustules, the fruiting structures for Common Rust occurs on the upper leaf surface and sporadically on the bottom leaf surface.  It is typically golden brown to cinnamon in color.  Southern rust occurs primarily on the upper leaf surface and only rarely on the bottom leaf surface.  Pustules and spores are normally bright orange or red.  Pustules of Southern rust can be more densely clustered than common rust pustules.  Both diseases can occur on the same plant or leaves.  Spraying for common rust is usually not warranted.

Fungicides applied at VT to R3 will not control stalk rots.

Below is a link to the “Fungicide Efficacy for Control of Corn Diseases – January 2021” put out by the Corn Disease Working Group which consists of Corn Pathologists from across the United States. Once you have identified the disease you are most concerned with you should be able to find one or more fungicides that will provide VG (very good) to E (excellent) control of that disease and possibly others.

If you want more information on foliar diseases of corn especially Southern rust read: