Southern corn rust has been showing up in cornfields somewhere in Nebraska almost every year over the past decade. But what was different about this season was how early it was discovered in several southeastern counties in mid-July.
Nebraska Extension plant pathologist Tamra Jackson-Ziems says southern rust was confirmed one to two weeks earlier this season than any time in recent memory. "It is caused by an aggressive fungus. It infects plants and can increase quickly, plus it can have a high yield impact. It can spread quicker than gray leaf spot, for instance," she says. "This fungus doesn't overwinter here, so it has to blow in from the South. Southern winds bring in spores from Southern states."
The hot and humid conditions combined with south winds and frequent rainfall through mid-July in southeastern counties provided the perfect conditions for southern rust. "It isn't the hot daytime temperatures that the fungus likes, but the nighttime lows in the upper 70 degrees to low 80 degrees range," Jackson-Ziems says. "Even when the nighttime lows are cooler, then the fungus likes the daytime highs in that same range."
Because it showed up so early in the season, management this year with fungicides was more difficult and caused some producers to apply fungicides twice for control. "Most fungicides typically have residual impact of 21 to 28 days," Jackson-Ziems says. "Early grain fill through dough stage is very critical, but you could see yield loss well into dent stage, because agronomic data suggests that it could impact dry matter accumulation and test weight at that time," she notes. "Early dent is still an important time to monitor." However, late onset of southern rust at that time would not warrant a fungicide treatment because it wouldn't usually be economically feasible, she adds.
"The decision to treat with fungicides in the season should be based on scouting a field and observing disease pressure," Jackson-Ziems says. "Applying too early without disease pressure may mean that a second application will be necessary. Producers have to make a judgment call on the disease development in the field, but they don't want to wait until the entire field has it. You want to be on the front end when you begin seeing it in the field." This year, later planted fields that are earlier in maturity were at the greatest risk as the disease increased in severity.
Most corn hybrids are susceptible to southern rust. "Because it is becoming a regular occurrence, a few seed companies are now rating hybrids for southern rust," Jackson-Ziems says. Over the past 12 years, southern rust has been observed in nearly every section of the state that raises corn, including some sections of south central and southwest Nebraska and occasionally in northern Nebraska into southern South Dakota, she adds.
It is much more of a concern than common rust, which most hybrids have a natural resistance to. Southern rust likes warmer temperatures, humidity and is observed as orange to tan on the leaves. Common rust likes cooler weather and has spores and pustules that are red to brown in color on the upper parts of the leaves.
Jackson-Ziems encourages producers to scout their fields frequently during the season and to make sure they are confirming any disease pressure they observe. Because southern rust has become a perennial problem, growers might consider planning for the expense of a fungicide treatment into their crop budgets every year, especially in areas where it has shown up regularly.
If you'd like to learn more, contact Jackson-Ziems at [email protected]. You can also follow the southern corn rust tracking website at ext.ipipe.org to see where it has already been discovered during the growing season.