Gray Leaf Spot or Bacterial Leaf Streak? Getting the Diagnosis Right Could Save a Fungicide Pass

When corn leaves begin developing long, tan lesions in midsummer, many growers assume they’re looking at gray leaf spot and start considering a fungicide application.

This year, Iowa State University Extension Plant Pathologist Alison Robertson urges caution on making the investment. She reports another disease is showing up in farmers’ fields across the state more frequently, especially those hit by recent storms: bacterial leaf streak (BLS).

The distinction is important because fungicides can help manage gray leaf spot, a fungal disease, but they do nothing against bacterial leaf streak.

“Yes, we have a lot of bacterial leaf streak,” Robertson says. “There’s definitely some hybrids out there that are particularly susceptible to this disease. And then with the storms we’ve had, that also helps drive that disease.”

First identified in Nebraska in 2016, bacterial leaf streak has since spread across much of the Corn Belt and Great Plains, including Iowa, Illinois, Indiana, Kansas, Minnesota, Missouri, Oklahoma, South Dakota, Texas and Colorado. The bacteria enter leaves through wounds caused by hail, wind or driving rain, making storm-damaged fields especially vulnerable to the disease.

How to Tell the Difference: BLS vs. Gray Leaf Spot

The challenge for many corn growers is that BLS looks remarkably similar to gray leaf spot.

Both produce long, tan lesions, but Robertson says a closer look reveals key differences. Bacterial leaf streak lesions have wavy margins and often follow — or even cross — leaf veins. Gray leaf spot lesions are more rectangular with straighter edges and remain confined between the veins.

One of the easiest ways to tell the difference between the two diseases is simply to hold an infected leaf up to the sun or a bright light.

“If you hold a leaf with bacterial leaf streak up to the light, it almost fluoresces with these bright yellow halos around those streaks,” Robertson says. “That’s the tell. You just hold it up and you’re like, ‘Wow.’”

That simple backlighting technique can help growers avoid an unnecessary fungicide application.

What Kansas Growers Are Seeing in Fields

In north-central Kansas, Pioneer Field Agronomist Garrett Kennedy says he’s seeing bacterial leaf streak primarily in older corn hybrids, especially in western Kansas where continuous corn and heavy residue are common.

“You can see on the lower leaves a lot of lesions that have a wavy edge with a yellow halo and are crossing the vein,” Kennedy says.

He also points to wind-driven soil and damaged leaf tissue as important factors that create entry points for the bacteria. While some older hybrids appear more susceptible, he says newer genetics seem to be holding up better.

No In-Season Treatment Available

Unlike gray leaf spot, bacterial leaf streak has no effective in-season treatment.

“There is no treatment,” Robertson says. “So basically, we’re left with using resistant hybrids. If you do have a problem on your farm, don’t grow that hybrid next time.”

That’s why accurate diagnosis is so important. The Crop Protection Network recommends submitting questionable samples to a diagnostic laboratory rather than assuming the disease is gray leaf spot and spending money on a fungicide that won’t provide control.

Researchers Are Getting a Handle on Yield Impact

Researchers are working to better understand bacterial leaf streak and its effect on yield.

“It’s very hard to get a handle on how much yield loss it causes,” Robertson says. “It’s probably affecting yield a little bit just by the damage it does to the leaves. Those leaves are not photosynthesizing very effectively. But I don’t think it’s as bad as another bacterial disease like Goss’s wilt.”

According to the Crop Protection Network, significant yield losses are generally uncommon unless the upper canopy becomes heavily infected early during grain fill. Across annual disease-loss summaries in 28 states and Ontario, bacterial leaf streak consistently ranks well below diseases such as tar spot, southern rust and gray leaf spot for overall production losses.

Current Weather Conditions Limiting Disease Spread

Robertson says Iowa’s recent stretch of hot weather and relatively low dew points means leaves are drying quickly after sunrise, reducing conditions favorable for both bacterial and fungal diseases.

“Those leaves are not going to be wet out into mid-morning,” she says. “There may be a little bit of dew very early in the morning, but those leaves are going to dry quickly. That’s not going to favor a lot of diseases.”

Scout Now, Plan for Next Season

While that won’t erase infections that developed after earlier storms, it does give growers a good opportunity to scout fields, note where bacterial leaf streak is occurring and identify hybrids that may warrant replacement next season.

Speaking with AgriTalk host Chip Flory, Robertson said Iowa’s corn crop remains in generally good condition, but bacterial leaf streak is becoming a disease growers should expect to encounter more often.

“You can have a high grade on crop health now, and in six weeks’ time we can be going, ‘Oh, sherbet, what the heck happened?’” she says.

Her advice: Scout storm-damaged fields, look for the wavy lesions and bright yellow halos that appear when leaves are backlit, and confirm suspicious samples before making a fungicide decision. Correct identification of disease can save farmers both time and money this season.