Monday, August 2, 2021

Southern rust on the move in southern Alabama

• By Ed Sikora •

southern rust
Figure 1: Southern rust seen in Alabama — photo courtesy Alabama Cooperative Extension

Southern rust was found in Crenshaw, Henry, and Monroe counties last week, increasing the number of counties reporting the disease to six for Alabama. Disease severity was low in each case, but I assume the disease is more widespread and has just gone undetected in other areas of Alabama.

Symptoms of southern rust (figure 1) 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.

Southern rust is the most economically important disease of corn in the Southeast, and the presence of the disease in a field can significantly reduce yields. With rains occurring almost daily and constant high temperatures and humidity, the disease can become a problem in fields where corn is still in a vulnerable growth stage (basically anything before the dough stage).

Fortunately, many of the fields I visited in south Alabama this past week seem to be at a reproductive stage beyond the point where the disease will affect yield significantly.

Growers in central and north Alabama with late-planted corn need to be aware of this disease as we move into late July. Growers should consider making a fungicide application if corn is at the appropriate growth stage and if the crop has good yield potential, especially since the price of corn is good.

Mixed mode of action fungicide products would be the way to go right, giving you the best bang for your money. I will add that no fungicide works well once southern rust is well established in a field.

Northern corn leaf blight and southern corn leaf blight as well as the occasional ear infected with corn smut are other diseases I have seen this year.

Contact Dr. Edward Sikora, Auburn Extension plant pathologist, for more information.

Alabama Cooperative Extension contributed this article.

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