The corn disease scene for 2024 could be a paraphrased version of the spaghetti Western movie “The Good, the Bad and the Ugly.”

The good? Well, no corn disease is good. When it comes to common rust, however, “not so bad” could sub for “the good.”

“Yield losses from common rust are really rare,” says Alison Robertson, an Iowa State University Extension plant pathologist. “If I see it in the field, I don’t recommend spraying a fungicide for common rust.”

Not so for the bad (Southern rust) and the ugly (tar spot). Left untreated, both fungal diseases can reduce corn yields by 50% or more during a disease outbreak, Robertson says.

Here’s a breakdown on each disease and how to plan for 2025:

Common rust

Common rust doesn’t overwinter in Midwestern states such as Iowa, as its spores arrive on air currents from more southern locations. Symptoms include circular to rectangular-shaped pustules with an orange-brick/red hue that appear on upper and leaf surfaces. Pustules often surface in early vegetative growth stages, around V6 to V7.  They’re less prevalent after tasseling.

Common rust thrives under 61 to 77 degrees F temperatures and 12 hours of intermittent mist. Symptoms surface quickly after infection due to its short five- to seven-day latent period. A latent period is the time from infection to when new spores are produced. 

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COMMON RUST: Disease is never good. Still, there’s good news about common rust in that even if it emerges in a cornfield, it rarely causes yield loss.

Southern rust

Southern rust outbreaks used to be rare.  No more.

“I’ve been in Iowa for 21 growing seasons, and I’ve never seen Southern rust [appear] like this year,” Robertson says.

Like common rust, this lethal rust also exhibits pustules as its main symptom. Its circular orange pustules are smaller (1/4 inch) and appear in clusters on the upper leaf surface, Robertson says. Pustules often emerge later after tasseling and continue into grain fill.

It also doesn’t overwinter in Iowa, as it rides air currents from Southern areas. It thrives under temperatures of 77 to 82 degrees F and leaf wetness lasting at least two hours. Symptoms emerge quickly after infection due to its six- to 12-day latent period.

Its lethality is enhanced by prolific spore production. “One pustule of Southern rust will produce between 600,000 and 700,000 spores a day for 18 to 20 days,” Robertson says.

SOUTHERN SCOURGE: Southern rust loves warm temperatures and moisture to infect corn. It doesn’t overwinter in Iowa and instead arrives by wind currents from Southern regions.

Tar spot

First, the bad news: Tar spot fungi is already here in crop residue.

“We have tons of residue that was infested with tar spot last year and the year before,” Robertson says.

For tar spot to surface, however, it needs cool 68- to 75-degree F temperatures lasting 30 days and dry weather.

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“There is an inverse relationship between tar spot severity and precipitation,” Robertson says. She notes one 2024 ISU trial had corn irrigated for three days weekly several times per day. No tar spot surfaced.

However, one area had nozzles turned off, as they interfered with irrigation machinery. “There, we saw a ton of tar spot,” Robertson says.

Tar spot’s latent period extends up to 40 days. Thus, it may damage yields before symptoms of irregular black spots are observed. These symptoms on the leaves may occur before tasseling. As the growing season nears its end, spots also may occur on the husks. 

What to do

Planting resistant or tolerant hybrids is the first line of defense, Robertson says.

“Over the last couple of years, companies have been great about scoring their hybrids for tar spot,” she says.

Fungicides are another tool farmers can use. Tar spot’s latent period of 40 days complicates fungicide application decisions, as yield damage can occur before symptoms surface. One tool farmers can use to aid fungicide application decisions is the Tarspotter app. This app forecasts tar spot risk and can predict the best fungicide timing.

In 2024, ISU researchers conducted trials on various fungicides and timing. Three themes emerged, Robertson says:

Related:Scout 2024’s corn diseases this year