Blog Archives
Southern Rust Myths
The following article, originally published in CropWatch (https://cropwatch.unl.edu/mythbusters-southern-rust-edition/) , was written by Dr. Tamra Jackson-Ziems and Kyle Broderick. High disease levels of southern rust were present in 2025, impacting yields. This article addresses the misinformation we are hearing about southern rust before decisions are made for next year’s growing season.
“Myth #1: Overwintering Rust: The southern rust fungus cannot overwinter in Nebraska. (It cannot survive in corn residue or soil).
The southern rust fungus (Puccinia polysora) needs to infect living, green corn in order to survive, and there is no known alternate host. Thus, the fungus can’t overwinter anywhere the climate doesn’t support corn growth through the winter months.
In fact, our rust fungi are likely blown north into the United States from subtropical locations, such as parts of Mexico, where corn is grown year-round. The southern rust fungi typically reach Nebraska in late July; however, this year they arrived earlier than usual, with the first confirmed sighting on July 9 — the earliest on record for the state.
Myth #2: Infected Grass: Southern rust doesn’t infect brome or other grasses nearby.
Rust fungi tend to have very narrow host ranges, infecting only one or a few plant species. Because several species of rust thrive under the same environmental conditions, it’s not unusual to see multiple plant species showing rust symptoms at the same time — even though they’re caused by different pathogens.
Myth #3: Super Strains: There is no new “super strain” of southern rust fungus.
… the severity we observed was due to prolonged periods of extremely favorable weather conditions — southern rust thrived under high relative humidity and average temperatures around 80°F. Southern rust…has been confirmed in 19 of the last 20 years…. If you remember 2006, you might recall another historic outbreak centered in south-central Nebraska. That season also brought delayed corn planting from spring rains, followed by ideal weather for rust development during the first two weeks of August. Many fields suffered stalk weakening and lodging, which caused memorable harvest challenges.
Myth #4: Fungicide Failures: Fungicides did not fail to control southern rust this year.
Although yield data are still coming in, most reports indicate that fungicides performed wellagainst southern rust this year. During years with substantial disease pressure, differences in fungicide performance become more apparent, underscoring the importance of selecting effective products and applying them at the right time. Results from multiple states, compiled by the Crop Protection Network, reinforce these findings. Remember: Even the best product can’t perform well without good coverage and proper timing, especially in a season like 2025 when disease pressure was unusually high.” The full article can be viewed at: https://cropwatch.unl.edu/mythbusters-southern-rust-edition/.
Disease and Fungicides
Disease and Fungicides: Tar spot has been found in Polk and York counties at moderate incidence on leaves below the ears in the Shelby/Gresham/Benedict areas thus far. So far, all the samples and photos received have been in fields that were already sprayed. I’ve received a number of questions the past few weeks on why disease is present on fields that were sprayed with a fungicide. In the samples and questions received, I ask where the disease (mostly southern rust or tar spot) is located on the plant. If located below the ear, most likely, the product didn’t penetrate the canopy below the ear leaf. Some producers have asked for increased gallonage (3 gal/acre+) in order to better penetrate the plant canopy. In nearly every case of the questions/samples received, the diseases are below the ear leaf and not above.
If the disease is above the ear leaf in fields sprayed with a fungicide, it’s possible the spores had infected the leaves but hadn’t produced visible signs of lesions until after the fungicide was applied. Products containing triazoles (Group 3) can have some curative (killing) activity to what is already present on the leaf, but they will only work if the fungus was present a few hours to a few days before a leaf was sprayed with a fungicide. It’s also possible with very tall hybrids and lower ear placement that the coverage didn’t reach the ear leaf in some of those situations. Group 7 and 11 fungicides should provide 21-28 days of control. For fields where fungicide was automatically applied at tassel, that residual has most likely worn off or will soon. I’m not aware of any fungicide resistance issues yet in corn in Nebraska.
In corn fields that haven’t been sprayed, I’ve seen low incidence of southern rust at the ear leaf and below. At some point, it may worsen, but as many fields approach dough to dent, it’s helpful that disease didn’t explode yet. Info. about fungicide app if needed at these later development stages is below. Another observation is I tend to see more disease in fields that have been sprayed that had higher rates of nitrogen applied at one time vs. spoon-feeding over time. It’s just an observation and may not hold true, but it’s something observed thus far.
Dr. Tamra Jackson-Ziems had a fungicide timing study for two years (2008 and 2009) at South Central Ag Lab which showed fungicides could be applied through 100% dent (watch pre-harvest intervals) and had no yield difference compared to tassel in a year without southern rust pressure but heavy gray leaf spot pressure. I think we’d be more comfortable with applying no later than dough/early dent instead of 100% dent. The data is why I recommend delaying fungicide apps till needed. Several asked about the need for second fungicide apps if fields have higher disease pressure at ear leaf and below. That’s going to be a farmer/agronomist and field by field decision. The economics are tough this year with one app much less two.
Soybean Management Field Days: Would like to encourage and remind you of the upcoming Soybean Management Field Day at Scott Richert’s farm near Ulysses on Thursday, Aug. 15 from 6-8 p.m. (Reg. at 5:30 p.m.). Google map pin of the location at https://go.unl.edu/smfd3. It’s on the Butler/Seward Co. line on Rd. 21 (Ashland Rd) between Roads D and E in Butler Co. and Roads 434 and 420 in Seward Co. I’m grateful we were able to change this program to be more relevant to the area and showcase what the farmers are doing. If you’re interested in hearing more about how small grains like rye can help with palmer and white mold control, hear about learning experiences with roller crimping, and learn about various soybean seed treatments, we hope you’ll join us! Please RSVP for meal count but walk-ins are welcome (402-624-8030 or https://enreec.unl.edu/soydays).
JenResources 