Southern rust a threat to corn crop

Given plenty of rainfall and a susceptible variety, one of several leaf spot and blight diseases can cut corn yields.

Of the diseases seen on corn in Alabama, Southern rust consistently poses the biggest threat. This threat may have just gotten worse.

Last summer, a new Southern rust race that can attack once rust-resistant corn varieties with the Rpp 9 gene has appeared. Across the Southeast, late planted corn that followed wheat — or early corn — not only suffered heavy rust damage but fungicides failed to provide effective rust control.

In past years, early planting has been an effective defense against Southern rust and other diseases. The only damaging rust outbreaks I have seen at Fairhope and Brewton occurred in corn planted in early-April. In that area, early to mid-March planted corn always escaped with little if any rust damage.

The situation may be a little different this year. Heavy rains that delayed corn planting over much of Alabama, along with the increased susceptibility of all corn varieties, may open the window for a serious rust outbreak in July. Of course, frequent showers will be needed in June and July on dryland and irrigated corn to trigger a rust outbreak on April-planted corn.

Fungicides are an option for controlling diseases and protecting corn yields.

Fungicide treatments to corn should only be considered when corn prices are good, yield potential is high (more than 120-plus bushels per acre), and weather conditions at tasseling and

silking (i.e. frequent rain showers) favor rapid disease development.

In the Midwest, fungicides are routinely applied to corn to improve “plant health.” Here, no yield gains have been seen, particularly on dryland corn with a yield potential below 100 bushels per acre, from applying fungicides in the absence of diseases.

On 140 bushel-per-acre disease-free corn, a “plant health” yield bump of 7.4 bushels per acre was obtained with Headline 2.09EC in a 2008 Fairhope trial. In this same study, smaller yield gains were seen with several other fungicide treatments.

When damaging Southern rust outbreaks have occurred, yield gains of 15 to 25 bushels per acre above those of the non-treated control have been obtained on high yield corn with timely fungicide treatments.

In a 2007 Fairhope study, highest yield increases were obtained under moderate rust pressure with either two applications of Quadris 2.08SC or Stratego 2.08EC.

Fungicides now registered for disease control, particularly Southern rust on corn, include several formulations of propiconazole (Tilt 3.6E, Bumper, and PropiMax), Headline 2.09EC, Quadris 2.08SC, Quilt 1.67EC, Stratego 2.08EC, and Folicur 3.6F.

On early corn, fungicide applications typically are scheduled at the blister (R2) or milk stage (R3) and if needed about two weeks later. Fungicide application timing for rust control on late corn has yet to be determined.

Corn rust sentinel plots have been established at several outlying AAES units in south and central Alabama. Plots will be monitored for Southern rust and other corn diseases. When outbreaks of Southern rust or other diseases are noted, alerts will be forwarded to the regional Extension agronomists.

TAGS: Corn
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