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Tips for identifying bacterial leaf streak

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NEW DISEASE: This photo of bacterial leaf streak is backlit, with the sun shining through the leaf. Note the irregular, narrow fashion of the lesions, along with the brighter yellow discoloration, which are identifying symptoms of BLS.
Extension Crop Connection: There are subtle clues that differentiate bacterial leaf streak from other diseases.

Farmers and other agriculture professionals began to see the streaks on corn leaves around 2014. The lesions were long and narrow on the lower leaves, so they initially assumed that common gray leaf spot disease had gotten an early start.

Concerned farmers watched as the disease worsened and spread, and some made the difficult and costly decision to spray a fungicide, sometimes twice, to the worst affected fields.

But the fungicides didn’t stop the disease. Some producers assumed there had been a problem with the fungicide application, or that the gray leaf spot fungus was suddenly resistant, making fungicides ineffective. A few producers desperately submitted leaf samples for testing to the University of Nebraska Plant and Pest Diagnostic Clinic in Lincoln to get more answers.

But, instead of a fungus, the clinic diagnostician found bacteria oozing from the corn leaves. In 2016, UNL plant pathologists and collaborators from Colorado State University, Iowa State University and USDA's Animal and Plant Health Inspection Service reported the bacterium Xanthomonas vasicola pv. vasculorum had been confirmed in several Nebraska fields causing bacterial leaf streak — the first time the disease had been found in the U.S.

Courtesy of Tamra Jackson-ZiemsMap of bacterial leaf streak which has been confirmed in 75 (marked in red) out of the 93 counties in Nebraska.

MAP IT OUT: Nebraska Extension plant pathologist Tamra Jackson-Ziems notes that bacterial leaf streak has been confirmed in 75 (marked in red) out of the 93 counties in Nebraska.

Since then, the disease has been confirmed in samples from cornfields in 75 of the 93 Nebraska counties. The disease also has been found in Colorado, Kansas, South Dakota, Iowa, Illinois, Minnesota, Oklahoma, Texas, and is suspected in other states.

New to U.S.

Decades ago, the disease was confirmed on corn in South Africa, but the bacteria are better known for causing a serious disease of sugarcane almost worldwide. Since its confirmation in the U.S., bacterial leaf streak also has been confirmed in Brazil and Argentina.

A flurry of research was initiated on the disease in the U.S., and the results have indicated that local strains are genetically more closely related to those from South America than those found in South Africa.

Research on the impacts of the disease on corn yield have been inconclusive and continue, but corn hybrids vary widely in their reaction to the disease. The most severely affected, especially some susceptible popcorn hybrids, likely experience yield loss to varying degrees, depending upon how much leaf area is affected by disease lesions.

Although, testing in the field to generate disease has been difficult and has slowed seed company efforts to test hybrids.

What we know

We’ve learned that the bacteria survive in infested corn debris, and that the pathogen can infect more than a dozen other plant species. Some of the most notable hosts include some of the most common native perennial prairie grasses such as big bluestem, little bluestem and indiangrass, and weeds like bristly foxtail, green foxtail and yellow nutsedge.

Sorghum species, including grain sorghum, were susceptible in greenhouse testing, as well as some varieties of oats and rice, but disease has not been found in commercial production fields other than corn.

In spite of all that we’ve learned from research on bacterial leaf streak, frequent misidentification of the disease in the field continues to be a challenge. Mistaking the disease for a fungal disease can mean ineffective use of fungicides that can’t control bacterial pathogens and increases in input costs.

BLS vs. GLS

There are some considerations and subtle clues that may help to identify bacterial leaf streak and differentiate it from other common diseases that we encounter in cornfields, especially gray leaf spot:

When? Bacterial leaf streak can develop any time of year, including during corn seedling stages when temperatures are cool. In contrast, gray leaf spot needs the warmer (70 to 80 degrees F), more humid conditions that we usually don’t experience in Nebraska until July or closer to tasseling. If night conditions have been consistently cool, it’s probably not gray leaf spot.

This table identifies a few subtle differences between bacterial leaf streak and gray leaf spot in corn.

COMPARING DISEASES: This table identifies a few subtle differences between bacterial leaf streak and gray leaf spot in corn.

What? The most common bacterial leaf streak symptoms are narrow, tan, brown-to-yellow streaks between the veins. Look closely at several lesions. If the edges are generally wavy and irregular in shape, it’s likely bacterial leaf streak. Holding leaves up to backlight them with the sun will usually show bright yellow discoloration, too. In contrast, mature gray leaf spot lesions have smooth edges and are rectangular in shape with much less yellow around them.

Where? Both bacterial leaf streak and gray leaf spot usually start in the lowest leaves and continue to move to higher leaves if weather conditions are favorable. If you need assistance, contact us in Nebraska Extension for more information, and submit samples to the UNL Plant and Pest Diagnostic Clinic.

Jackson-Ziems is an Extension plant pathologist with the University of Nebraska-Lincoln.

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