NASHUA, Iowa — Given the cool wet weather Iowa experienced in May and June, farmers should scout for corn and soybean diseases, said Ed Zaworski, Iowa State University field crops plant pathologist.
In his work at the ISU Plant and Insect Diagnostic Clinic, Zaworski said he has seen a lot of pythium samples. Fusarium and phytophthora are likely to show up as well.
Speaking at the recent ISU Northeast Research and Demonstration Farm field day at Nashua, Zaworski said this is a prime year to scout for Northern corn leaf blight. Gray leaf spot is another disease likely to show up.
Corn and soybeans at Nashua are doing pretty well so far this year, Zaworski said. Corn leaves pulled before his talk showed a little eye spot, anthracnose and rust, but nothing serious.
“You have to go out and scout,” said Angie Rieck-Hinz, ISU Extension field agronomist. “Farmers call and say ‘The planes are flying, what should I do?’ I ask what they’ve seen. They say they haven’t seen anything. ‘Then don’t spray,’ I tell them. You have to get out and scout, and you have to get out of the end rows.”
Tar spot is a corn disease that has moved into eastern Iowa, and farmers should be looking for it, said Zaworski. The fungal disease has been identified as far west as Floyd and Black Hawk counties.
In 2018 tar spot was detected in 12 counties in eastern Iowa. The disease was observed late in the growing season and did not cause a lot of damage. In Wisconsin, Illinois, Michigan and Indiana, however, the disease was observed earlier during grain fill and caused significant yield loss.
Damon Smith, a University of Wisconsin plant pathologist, has developed a tar spot prediction model that is being validated this growing season throughout the Midwest. In Iowa, ISU plant pathologist Allison Robertson is working on the model. Anyone who observes tar spot should contact their ISU Extension crop specialist.
Tar spot is yet to be observed in 2019 in Iowa, Wisconsin, Illinois, Michigan and Indiana, although the weather has been conducive. Zaworski and Rieck-Hinz said to carefully scout fields with a history of tar spot that are at growth stage V6 or older. A fungicide application is only needed if the disease is observed.
Tar spot in corn was first confirmed in the U.S. in 2015 in Illinois and Indiana. The disease was first reported in eastern Iowa in 2016. Since then, the disease has been reported each growing season Researchers believe tar spot spores arrived in the United States in a storm that originated in Mexico or Latin America.
Symptoms consist of small, raised, black, irregular-shaped spots scattered across the leaf surface. Common and southern rust are tar spot look-alikes, Zaworski said.
At the end of the growing season, both rust fungi switch from producing orange-red to black spores. Rust spores can be scraped away with a fingernail. Tar spots cannot be scraped off the leaf tissue.
“If you have a history of tar spot, scout now,” said Rieck-Hinz. “If you are close to a county or in a county with tar spot, you should be looking. If it’s there, you will want to spray a fungicide. Don’t spray a fungicide if it’s not there.”