Tips for dealing with tar spot disease, hail-damaged crops

In 2022, a fungal plant disease called tar spot was confirmed on corn in southeastern South Dakota.

As of 2024, it was confirmed in 46 counties in eastern South Dakota. Tar spot or Phyllachora maydis is a corn disease that was previously spotted in Indiana, Iowa, Illinois, Minnesota, Missouri, Nebraska, Michigan, Kansas, and Ontario, Canada. In severe incidence, it has been known to cause yield losses of up to 90 bushels per acre.

This fungal disease usually develops mid- to late grain fill and moves very quickly/aggressively compared to other disease like gray leaf spot or northern corn leaf blight. It looks much like southern rust or common rust, though spots are less round and darker in color. The fungus likes mild temperatures and high humidity and has the ability to survive on corn stubble and biomass, making it very difficult to remove from cornfields. Fungal spores are also windblown, which enables broad spread of the disease. Lesions have a fish-eyed appearance with brown tissue around the black lesions on leaves or husks.

According to the Crop Protection Network, tar spot cost an estimated $3 billion in losses to U.S. farmers from 2018 to 2021.

However, South Dakota has yet to see a severe enough instance of the disease to cause treatment or great concern. Should it become more of an issue (if environmental conditions are conducive), it can be helpful to select hybrids that are more resistant to the disease; fungicides are another potential option to manage the disease. The most current research suggests using two different modes of action around the VT (early tassel) growth stage for best protection; however, current unknown incidence in South Dakota means this is not a current, local recommendation.

The last three years, tar spot has been found no earlier than R2/R3 growth stage, so fungicide application would not yield a return on investment nor provide protection. If you spot suspected tar spot, let our Extension plant pathology team know by contacting the SDSU Plant Diagnostic Clinic at 605-688-5545 or contacting Madalyn Shires at 605-651-5631 or [email protected].

Since were talking fungicides, it seems practical to mention how best to handle hail damage situations this summer. Various summer storms have hit this year across South Dakota, and crop growth stage and level of damage are all factors in how to determine the best reactionary steps. Before making any drastic decisions, consider the following:

• Check with your crop insurance agent before making any decisions regarding hail-damaged fields.
• Allow seven to 10 days for plants recover to assess injury as long as some pods or ears are still intact.
• Think twice before applying any fungicides to protect hail-damaged crops. Fungicides are designed to protect against fungal plant diseases. Wounds in growing plants are prone to bacterial infection, which fungicides do not protect against.
• Evaluate local weather conditions and disease tracking maps to determine the environment is conducive for disease development or if crops are even at risk for fungal diseases (i.e. tar spot).
• Check your previously used pesticide labels. Before determining alternative uses for hail-damaged fields, harvest and forage utilization intervals must be checked on all applied pesticides. In addition, check rotation intervals before planting another crop; for a quick look-up on herbicide rotation restrictions, view the SDSU Extension article, Herbicide Rotation Restrictions (search “herbicide rotation restrictions” at extension.sdstate.edu).

Depending on the level of defoliation and plant moisture, row crops left standing may still provide a grain crop following hail damage and, if not, a viable forage option. Iowa Extension has two excellent publications on decision making after hail for corn and soybeans that may be helpful.

If the grain crop is determined to be a loss, there are several options to consider, such as grazing, green chopping, or letting the crop stand. There are many cool season annual cover crops that could be planted in late July and provide soil cover and potentially additional fall/winter forage if conditions allow.

Upcoming events

Register for the following events at extension.sdstate.edu/events.

• July 17 International Farm Transition Network-Fairness in Farm Succession; 10 a.m. CDT, webinar, $30
• July 18 Ag Economic Dialogues: 10 a.m. webinar, no cost
• July 22-24 Women on the Range Grazing Workshop; at Faith Legion Hall, $250 for first timers, $200 or return attendees, $100 for additional attendee from same operation
• July 22 IPM (Integrated Pest Management) Field School; 9 a.m.-4 p.m. near Volga, no cost to attend, lunch included
• July 22 Cover crop and organic vegetable production field day; 10 a.m.-1 p.m. at Haroldson Farms near Bruce, no cost to attend, lunch included
• July 22 High tunnel and vegetable production field day; at Cherry Rock Farms near Brandon, no cost to attend, lunch included
• July 29 Specialty Crop Field Day; 5-8:30 p.m. at Brookings, no cost to attend
• July 31 Forage Field Day; 9 a.m.-3 p.m. at Castana, Iowa, $20/person or $50/farm (up to five people)