As of Sept. 26, South Dakota State University Extension has confirmed the presence of tar spot in 25 counties across the eastern half of South Dakota, according to Madalyn Shires, assistant professor and Extension plant pathology specialist.
The severity of infection has varied anywhere from less than 1% to approximately 20% on individual plants. On average, most fields are under 10% severity, but some fields have hot spots with up to 20% in small areas. Counties north of Highway 34 are typically seeing infection severity of less than 5%. Positive counties south of Highway 34 are seeing anywhere from 1% to 20%, especially in Yankton, Clay and Union counties, which have developed tar spot in both 2022 and 2023.
When scouting for tar spot, growers should be looking for black, circular to diamond-shaped lesions, which may vary in size, the smallest typically being about the size of a pen tip. The lesion should be seen on both sides of the leaf, which will help to distinguish tar spot from insect frass and darker rust spores.
This late in the year, fungicides are not recommended, but residue management should be heavily considered if the severity level is more than 10% in any area of the field.
