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July 17, 2014 Southern Corn Rust Update

Confirmation of Southern Corn Rust in Craven County

Confirmation of Southern Corn Rust within Craven County was determined by our office on July 16th. Spores were evident from the bottom of the plant and upward to about the ear. This simply confirms that previous recommendation for a preventative fungicide treatment was warranted. It also changes the current recommendation.

With confirmation of sporolation of the fungus, any fungicide treatment should be changed from a preventative type fungicide to a curative type (triazoles). High rates of triazole materials (products such as Tilt, Domark and Folicur) should provide protection for 10-14 days. Additional treatments may or may not be warranted in about two weeks depending upon the corn growth stage and weather. Once corn has reached the dent stage, minimum damage results from Southern Corn Rust. Until such time, severe yield loss may result if untreated.

Determining Corn’s Vulnerability to Southern Corn Rust

It is not possible to make an exact statement that can provide whether or not a field is at risk based on date of corn planting or to provide some future date that corn will no longer be at high risk. There are simply too many variables of production, varietal factors and weather circumstances. Rather, evaluation of fields for corn maturity will determine whether or not a field is at risk. Data is available to help stage corn and provide general guidelines for reasonable growth stage expectations. One source worth review is an article from Purdue University, Grain Fill Stages of Corn by  R. L. Neilsen. Keep in mind when reviewing these guides that estimations of growth are based upon “typical Growing Degree Days”. Thus, exact calendar days may differ. Complicating this more is that within Craven County, some producers completed corn planting by April 5th while others planted as late as May 15th. Thus, the only true means of determining whether a field is at risk or not is to evaluate corn ears for growth stage. Fields with most ears of corn with all harvestable kernels at dent stage are considered safe from extreme yield loss from Southern Corn Rust. All previous stages of corn are at risk of yield loss from Southern Corn Rust.

In attempt to provide at least some general guidance, it is likely that all fields of corn planted by the first week of April are not likely to require fungicide protection since many of these fields were at dent stage when Southern Corn Rust was discovered. Fields planted around mid-April should have already received a preventative fungicide application. If not, a triazole fungicide application should be applied as soon as possible. These mid-April plantings most likely will not require a second fungicide treatment but the only way to be certain is to monitor growth stage 10-14 days after the initial fungicide application. Fields not at or approaching dent stage at the time of evaluation will require a second fungicide treatment (combination products of strobilurin and triazoles are acceptable at this time). Corn planted in May may require up to three fungicide treatments depending upon weather and corn maturity. Corn planted in May should be scouted about every 10 days to evaluate whether another fungicide treatment will be beneficial.

 Conditions Favorable for Southern Corn Rust Development

Scattered rainfall and temperatures below about 90 degrees Fahrenheit favor rapid development of Southern Corn Rust. Regrettably, this is the type of weather we frequently experience between now and early August. As the temperature increases and rainfall decreases, the probability that additional fungicides will be needed will also decrease.