Southern rust was found in Union, Clay and Minnehaha Counties at low incidence and severity, announced Emmanuel Byamukama, SDSU Extension plant pathologist.

"Southern rust can develop under warm and wet weather to levels where it can result in reduced corn yields. This disease does not overwinter in South Dakota, however, it still occurs when spores are blown from southern states," Byamukama explained.

Identifying Southern rust

Southern rust can be differentiated from common rust by the color and the arrangement of the pustules on the leaf. "Common rust pustules are dark red in color, while southern rust pustules are orange brown," Byamukama said.

He explained that Southern rust pustules tend to be clustered on the corn leaf whereas for common rust pustules may be scattered over the leaf.

"Eyespot is another fungal disease that can look like southern rust," he said. "While both diseases may have a yellow halo around the lesion, southern rust forms a blister (pustule) that extends slightly above the leaf surface."

These pustules can be scrapped-off leaf surface. Eyespot lesions do not have a blister and the lesions are usually found on the top half of the leaf.

Most corn hybrids are susceptible to Southern rust.

Byamukama encourages farmers to scout their fields and where significant southern rust is developing, plan for a fungicide application for corn not yet at dent growth stage.

Other diseases developing in corn

Goss's wilt: A few corn fields scouted in Union, Turner, and Clay Counties had moderate to severe Goss's wilt.

Goss's wilt is caused by Clavibacter michiganensis subsp nebraskensis. "This disease can be differentiated from corn northern leaf blight by the presence of freckles and water soaking at the edges of the lesion," Byamukama said.

He explained that the bacteria survive on the corn residue and the infection is favored by warm and humid weather.

Goss's wilt can be managed through planting resistant hybrids, crop rotation, and tillage to bury the infected residue. "Risk increases with irrigated corn on corn," Byamukama said.

Gray leaf spot: Most corn fields scouted had low levels of gray leaf spot (GLS).One corn field in Clay County had a moderate level of GLS. "Gray leaf spot is caused by a fungal pathogen, Cercospora zeae-maydis. This pathogen survives on corn residue and infection increases with humid and warm weather," he said.

Gray leaf spot forms rectangular lesions between veins "These lesions can coalesce to form larger lesions," Byamukama said.

He explained that gray leaf spot can be managed through planting resistant hybrids, crop rotation, tillage to burry residue and applying a fungicide when GLS is developing on the leaves below the ear leaf.

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