Don't rule out late crop disease pressure

Corn Corner: Be aware of southern rust and tar spot as the season continues through grain fill. 
Jul 08, 2020
We sprayed a couple of cornfields with fungicide because of gray leaf spot. Is it still worth scouting remaining fields in case southern rust comes in late? My cousin in northern Indiana is more worried about tar spot late. Should he continue scouting, too?
The Indiana certified crop advisers panel answering this question includes Betsy Bower, agronomist, Ceres Solutions, Terre Haute; Jesse Grogan, regional manager, AgReliant Genetics LLC, Lafayette; and Stan Miles, agronomist, A&L Great Lakes Labs, Fort Wayne.
Bower: You should continue scouting fields in southern counties for southern rust and in northern counties for tar spot, especially if you know there is disease in the area. Both southern rust and tar spot, but especially tar spot, can develop and shut down the plant quickly.
Typically, the R1 to R2 stage in corn is when we target fungicide use for most diseases. Depending on the product used, there is anywhere from two to three weeks of activity. That typically allows for control through the majority of grain fill, but not all of grain fill.
Both southern rust and tar spot can move quickly. Southern rust moves faster in some years than others. We can find new lesions late into grain fill at the R4 to R5 stage.
Should you consider a fungicide then? At R4, dough stage, there is approximately 33 days till maturity. At R5, beginning dent, there is still about 20 days till maturity. A disease that can quickly reduce synthetic capacity in a week’s time like tar spot should be considered for control. Call resources such as your ag retailer, the local representative of the product you’re considering and your seed company representative for advice.
In some hybrids, the extra investment could pay. In other cases, it may not pay. Anecdotal evidence from previous years indicates a late fungicide can pay on genetics that are particularly susceptible to the disease that is present.
Grogan: Scouting fields is worth it when southern rust comes in late, especially in southern Indiana. Most fungicide applications are effective for 14 to 21 days. A second fungicide application for severe rust pressure during the early milk stage, 18 to 24 days after silking, can be beneficial.
Late tar spot control for your cousin in northern Indiana is a more difficult decision. We need to understand more about tar spot control. Tar spot is usually more serious when weather is wet and cool. It diminishes when it is hot and dry. A second fungicide application is not likely a benefit for tar spot.
Miles: Scouting cornfields through grain fill can generate a valuable record of crop performance that could improve this year’s crop management choices and can also add to the producer’s memory bank of experiences for years to come. It is important to monitor the progression of southern rust moving northward on tropical storms, as it usually begins in very small, localized areas of the field and can spread rapidly, causing high levels of yield loss when conditions are favorable for disease progression.
Pay close attention to ear development stages and disease severity to make the best application decisions based on established economic thresholds. As the crop enters the later grain fill stages, watch for stalk quality issues, unexpected insect feeding, nutrient deficiencies and other items that may impact harvest planning.