Universities and industry experts are focusing efforts to deal with tar spot, a new disease in the upper Midwest corn belt. First identified in Illinois and Indiana in 2015, tar spot has now spread into Iowa, Ohio, Wisconsin and Minnesota. In 2018, some farmers in Illinois and Wisconsin experienced 30- to 40-bushel yield losses as well as problems with stalk lodging.
Jeff Housman, a Mycogen Seeds agronomist in Iowa, says tar spot is a disease originally found in Central America, and it is believed that winds from a Gulf Coast tropical storm carried it to the United States.
“Work is underway to understand this disease and develop best management practices,” Housman says. “Some fungicides have received 2ee labels, but recommendations are needed on what threshold triggers a fungicide application, or if multiple applications are needed and the timing for them. The ag industry also is aggressively evaluating hybrids. Some Corteva Agriscience field trials have identified seed genetics that offer some tolerance.”
Like any pathogen, tar spot is weather dependent. Conditions favoring it are wet, cool and when moisture is on the leaves for an extended period, including:
- Above-average rainfall
- High humidity (75% or more)
- Temperatures in the 60 F to 70 F range
- Cloudy conditions
While the disease can appear anytime during the growing season, the greatest damage to yield seems to originate during the early reproductive stages, which was the case in 2018. Disease in 2019 came later in the reproductive stage so not much yield loss or damage was observed. But the important point is that under the right conditions, tar spot can be aggressive. The fungus overwinters on crop residue, but it is not yet known how long it persists. The disease can be in the plant 12 to 14 days before any symptoms are evident. By the time black structures appear, the disease is in the reproductive stage.
Geographically, tar spot is expected to be a problem primarily in the upper Midwest. The University of Wisconsin is working to model predictions of geographic importance and severity.
Tar spot appears as a small black, circular shape on the corn leaf, 1.5 to 2 mm in diameter. While it could be confused with rust, Housman says, the practical test is to rub your thumb on the disease spot and see if the black rubs off. Rust will rub off, but tar spot does not. A conclusive diagnosis can be gained by sending a sample to your university.
Housman says the best advice for farmers is to scout for tar spot when weather conditions favor it. Once discovered, there are a few control methods to consider:
- Crop rotation
- Tillage to bury residue
- Fungicide applications
- Selecting seed genetics with tolerance