Alabama Corn: Southern Rust Alert for Southeast Counties

Southern rust was confirmed in two counties in South Georgia last week and more will follow.  This is a little early for that disease to develop and suggests a real threat to corn growers in Southeast Alabama for now, but possibly growers throughout Alabama depending how the situation unfolds in the coming weeks.

Weather conditions are quite favorable for spread of southern rust, but irrigated corn provides ideal conditions for its development. All growers in Southeast Alabama with irrigated corn at or very near tassel are at risk.  Growers across south Alabama should have any action plan in place if or when southern rust arrives in their area.

The main focus for management is to protect against southern rust before it is present in a field, or at least before it is well established. No fungicide works well once southern rust is established in a field. Growers should consider making a fungicide application if corn is at the appropriate growth stage (don’t spray corn that is not at least starting to tassel) and if the crop has good yield potential.

Growers need to consider yield potential PLUS market prices and previous input costs when deciding to spray. A fungicide application for southern rust will cost somewhere between $15 and $30 per acre including the airplane but the crop will be  protected. Failure to spray in a timely manner could result in a 100 bu/A loss in yield based on data from previous years in dealing with this damaging fungal pathogen.

It would be wise to use a fungicide with a  mixed mode of action that combines different chemistries. There are many products on the market, but we have seen success in the Southeast region of U.S. with Trivapro, Headline Amp, Veltyma, Aproach Prima, Stratego YLD, Delano, Fortix, Lucento, among others.

One application applied at the optimum time could do the trick, but a second spray, possibly a cheaper option, may be necessary 2-3 weeks later where high disease pressure exists.

Growers with non-irrigated corn and low yields potential (less than 100 bu/A) might think a fungicide application is not economically unfeasible. Everyone has to make their own decisions on spraying in the end. You know your potential crop yield and budget better than I do, of course.

If you have a good scout who has not found southern rust in your field, you might also consider delaying a spray. Spraying corn in southwest, central, and north Alabama could be delayed until we hear reports of southern rust moving into those regions of the state.

Symptoms of southern rust are similar to those of common rust, but pustules of southern rust are smaller and mainly occur on the upper leaf surface. Pustules are typically circular or oval, and tightly scattered over the leaf surface. Spores are orange when they erupt from the pustule. In time, pustules become chocolate brown to black, often forming dark circles around the original pustule.

Common rust is rarely a concern in hybrid corn. The disease typically occurs early in the season, first appearing on the lower leaves, and does not cause yield losses. Early symptoms of common rust are chlorotic flecks on the leaf surface. These flecks develop into powdery, brick-red pustules as the fungal spores erupt through the leaf.

Pustules are oval or elongated, about 1/8 inch long, and scattered lightly or clustered together on the leaf. Leaf tissue around the pustules may become yellow or brown. As pustules mature, the red spores turn black. Husks, leaf sheaths, and stalks also may be infected.