Foliar disease control begins with hybrids

JOHNSTON, Iowa — Management plans to control common corn foliar diseases like gray leaf spot and northern corn leaf blight are in place each year and a relatively new disease showed up more frequently in fields this past season.

Tar spot in corn was first reported in northwest Indiana and north-central Illinois in 2015 and has since been found in multiple states.

Symptoms

Tar spot is the physical manifestation of circular-shaped, tar colored fungal fruiting bodies developing on corn leaves. Initial symptoms are small brown lesions that darken with age. The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present.

Tar spot lesions cannot be rubbed away completely or dissolved in water. Under favorable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes and eventually the husks of the developing ears.

Severe infection can cause leaf necrosis. Affected ears can have reduced weight and loose kernels and kernels at the ear tip may germinate prematurely.

Management

“Tar spot, even though it’s new to the Midwest, is going to be around for a while,” said Mary Gumz, Pioneer agronomy manager.

Looking ahead toward 2022, Gumz said to manage the corn foliar diseases that can overwinter in fields such as tar spot, gray leaf spot and northern corn leaf blight, growers should make management plans for next season now, starting with hybrid selections.

“No. 1 is pick the right hybrid. We have a lot data and plots showing how different hybrids have handled heavy tar spot pressure. You want to pick a hybrid that show tolerance to tar spot, as well as having overall good overall good foliar disease health and stay green,” Gumz said.

“You probably need to plan to use a fungicide if you’re in a tar spot area, especially if the weather stays good for infection for a long time or we have very heavy pressure, you may need to plan on two applications, particularly if you’re irrigating.”

Tar spot is more likely to develop during cool temperatures of 60 to 70 degrees, high relative humidity over 75%, frequent cloudy days and seven-plus hours of dew at night.

Overwinter

Diseases can overwinter in infected crop residue, allowing inoculum to build up from year to year in fields. Disease spores are spread by rain splash and air currents to leaves of crop plants, where primary infections are produced.

Secondary spread occurs from plant to plant and field to field as spores are carried long distances by the wind. Infections generally begin on lower leaves and then progress up the plant.

Because these diseases overwinter in corn debris, production practices, such as tillage and crop rotation, can reduce the amount of surface corn residue and decrease the amount of primary inoculum.

In severe cases of disease infestation, multiple years of crop rotation away from corn may be necessary.