Alabama Corn: Southern Rust Detected, Southern Counties on High Alert

    Southern corn rust postules. Photo by Dr. Austin Hagan, Alabama Cooperative Extension

    Southern corn rust was confirmed in Baldwin County with the help of Regional Extension Agent Guilherme Morata last week. This is the first report of the disease in Alabama in 2022. The disease had been reported in multiple counties in South Georgia in the past two weeks, so the find in Alabama was expected.

    This is relatively early for southern corn rust to develop in this region and suggests a real threat to corn growers in South Alabama, 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 South Alabama with irrigated corn at or very near tassel growth stage are at risk. Growers across south Alabama should have a management plan in place.

    Management should focus on protecting against southern rust before it is present, or at least well-established in a field. No fungicide works well once southern rust is established. 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 previous years in dealing with this damaging fungal pathogen.

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    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.

    Growers with non-irrigated corn and low yield potential (less than 100 bu/A) might consider a fungicide application as not economically unfeasible. If you have a good scout who has not found southern rust in your field, you might consider delaying a spray. Spraying corn in central and north Alabama could hold off until we hear reports of the disease moving into those regions.

    Symptoms of southern rust can be confused with symptoms 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 loss. 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.

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