Sow and Grow

Farmers must be wary of tar spot, prussic acid

By Sara Bauder

South Dakota Extension forage field specialist

Posted 10/9/24

Tar spot in South Dakota

As of Oct. 2, the SDSU plant pathology team has positively identified tar spot of corn in 44 counties across eastern South Dakota. Infection has ranged from 1% to 20% …

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Sow and Grow

Farmers must be wary of tar spot, prussic acid

Posted

Tar spot in South Dakota

As of Oct. 2, the SDSU plant pathology team has positively identified tar spot of corn in 44 counties across eastern South Dakota. Infection has ranged from 1% to 20% severity on individual plants. Although the average field appears to be under 10% severity, there are fields with up to 20% severity in isolated patches.

So what is tar spot? Tar spot is caused by the fungus Phyllachora maydis and is identified by small, slightly raised circular black spots on corn leaves, stalks, and husks. The black spots are considered the fungal fruiting structures of the disease, called stromata.

These spots can produce thousands of spores. Severity rate of this plant disease depends on environmental conditions, corn hybrid, and amount of pathogen present in the field. Tar spot likes cool (60-70 °F) and humid weather. Long periods of leaf wetness are also ideal for disease establishment. Research in areas where tar spot has been prevalent for several years shows that it can overwinter on infested corn residue, which helps carry the disease from one cropping year to another.

There are currently fungicides available for use on tar spot; however, many disease incident rates in South Dakota are too low for economic threshold (and it’s too late in the season) this year.

Scouting is fairly simple, look for black, circular to diamond shaped lesions that vary in size. Lesions are visible on both sides of the leaf.

If you see tar spot in your field, let Madalyn Shires (madalyn.shires@sdstate.edu or 605-651-5631), our plant pathology state specialist know, as her team is tracking incidence within the state. In addition, consider hybrid selection for next year especially if in Union, Clay, Yankton, Turner, or Bon Homme areas. There are no corn hybrids with complete resistance to tar spot at this time, but some may show partial levels of resistance, which can help reduce the severity of the disease.

Other options to assist in managing this disease include crop rotation and tillage; however, neither completely avoid risk.

Prussic acid concerns

Prussic acid poisoning is an important issue to watch for this time of year, especially during dry periods when light or hard frost is a risk. Ruminants are quite susceptible to prussic acid poisoning, specifically in sorghum, sudangrass, sorghum x sudangrass, and Johnsongrass. Prussic acid is released from the plant very soon after frost/freezing or crushing/trampling/chopping of the plant. Livestock death may occur within 3-4 hours at high levels of toxicity; with fall frosts, this must be taken into consideration before grazing.

Because prussic acid release can happen almost immediately after a frost and may be high in any plant regrowth following one, waiting at least ten days to graze after a frost allows gasses to leave plant tissues and greatly reduces poisoning risks when grazing.

Repetitive light frosts create tough decision-making situations. The safest solution is to check the field for frost damage, and if any signs of frost are present, remove livestock immediately.

Prussic acid is less of a concern when haying or chopping, as ensiling and curing greatly decreases (but does not necessarily eliminate) the prussic acid content of feeds.

There are a few basic steps that can be taken to help reduce the risk of livestock loss surrounding grazing or feeding forages with the potential for prussic acid production:

  • Do not graze sudangrass, sudangrass hybrids, or sorghum until the plants are at least 18 to 24 inches tall.
  • Be especially cautious or simply avoid grazing short regrowth that occurs after grazing, harvesting, or a light frost.
  • Never turn out hungry cattle on new feed; feed livestock well, and then transition.
  • Do not graze susceptible forage crops following a series of light frosts. Wait 10 days (or longer) before grazing under these conditions.
  • Defer grazing after a killing frost until the plant has dried, usually about 10 days.
  • Harvesting as hay or silage usually results in lower concentrations of prussic acid compared to fresh samples. Test any suspect hay or silage samples before feeding.

For more information on prussic acid and safe levels, go here.