Tips for managing bacterial diseases in corn

When hail, high winds, or heavy rains damage crops, even minor bruising of corn leaves can give bacterial diseases an opportunity to get started. Once a corn crop is infected, the disease organism can overwinter in crop residue, giving way to a repetitive cycle of disease.

With bacterial diseases in corn, in-season management options to control the pathogen are limited. But accurate diagnosis can kick-start longer-term management changes and prevent misapplying fungicides, which are not effective in controlling corn bacterial diseases.

“In Nebraska, the most common bacterial diseases of corn are bacterial leaf streak and Goss’s bacterial wilt and blight,” says University of Nebraska Extension plant pathologist Tamra Jackson-Ziems. “In the rest of the country, bacterial leaf streak is not as common as it is in the western Corn Belt. It was first reported in Nebraska in 2016, which was the first time the disease had been reported in the United States. Since then, it has been confirmed in at least eight more states.”

With bacterial leaf streak, yield loss is hard to confirm, Jackson-Ziems says, partly because corn hybrids differ in their reactions to the disease. Yield losses resulting from Goss’s bacterial wilt and blight have been easier to pin down because the disease has a longer track record of infecting crops.

In general, Jackson-Ziems says, corn yield losses from bacterial diseases can run 5% 

to 15%, and as high as 50% when disease is severe in susceptible hybrids. “In the last two years, we’ve had a lot of storm damage, and in years like that we’ll see more outbreaks of Goss’s wilt,” she says. The yield loss depends on the disease’s severity. “In areas where we’ve had early storms and systemic Goss’s wilt development, the loss can be 100% in isolated areas of the field,” she notes. “But that’s uncommon, except in some areas of Colorado, and western Kansas and Nebraska.”

Rather than causing a systemic plant infection, Goss’s bacterial wilt and blight typically results only in a leaf blight, with lesions on leaves.

Correct identification

Besides reducing yields, bacterial diseases can lead to misapplying fungicide when diseases are misdiagnosed. 

“Sometimes, when producers first find disease lesions on corn leaves, they panic and think they need to spray right away with fungicides,” Jackson-Ziems says. “But it’s critical to first get an accurate disease diagnosis.”

Bacterial leaf streak, in particular, can be difficult to differentiate visually from other corn diseases, she says. “Its initial symptoms on some hybrids can easily be confused with those of gray leaf spot, which is caused by fungus,” she points out. “Proper identification is important because as a bacterial disease, bacterial leaf streak is not effectively controlled by the fungicides used to manage gray leaf spot.”

One way to differentiate between bacterial leaf streak and gray leaf spot is noting their development timing. Bacterial leaf streak may develop earlier in the season, during cooler weather. This contrasts with gray leaf spot, which develops during midseason, when conditions are warmer.

Another way to determine whether lesions result from bacterial leaf streak or gray leaf spot is to study the mar-gins between the leaf veins.

“Gray leaf spot, the fungal disease, produces rectangular lesions that are smooth along the edges,” Jackson-Ziems says. “Lesions caused by bacterial leaf streak generally have messy, wavy edges. The bacterial leaf streak lesions also tend to be a much brighter yellow — especially when backlit by the sun — compared to those caused by gray leaf spot, which may only have a narrow ring of yellow around them in some hybrids.”

Goss’s bacterial wilt and blight lesions have freckling or little dark spots around the edges, Jackson-Ziems says. “Along with that, the leaves will often have a glossy exudate on the surface,” she adds. “It sparkles and can occur on either the top side or bottom side of the leaves.”

Goss’s wilt can resemble the fungal disease northern corn leaf blight. However, the Goss’s wilt lesions tend to appear darker and water soaked, with dark freckles. The northern corn leaf blight lesions can be small to very large and elliptical, usually with smooth, round ends.

Sending tissue samples to a plant diagnostic lab is the most effective way to identify the disease, and the pathogen causing the symptoms.

Disease pressure

Because bacterial pathogens often enter corn plants at wounded tissue points, their subsequent diseases tend to begin after damaging weather events such as hail or high winds, resulting in sandblasting.

“But we’ve also seen bacterial leaf streak, for instance, get started in early summer after cool, rainy weather,” Jackson-Ziems says. “Such weather can also cause fungal diseases.” Water splashing during a rain event or irrigation can spread pathogens from infected plants.

“We’ve seen both bacterial leaf streak and systemic Goss’s bacterial wilt disease occur in small corn plants as early as the V6 leaf stage,” she says.

Routine scouting is the best way to get an early handle on disease pressures and their pathogens. Step up scouting after inclement weather, which could wound corn leaves.

“It takes a couple of weeks before you’ll see lesions developing,” Jackson-Ziems says. “But if you have an experienced eye, you can look earlier than that for precursors of lesions. You may be able to see evidence of water soaking, where damaged tissues along wounds are beginning to darken. These areas look much like plant leaves in the field after a light frost.”

Prevention

Managing or preventing bacterial diseases in corn presents no cut-and-dried options. The best ways to control them, Jackson-Ziems says, are to select resistant hybrids and practice crop rotation. 

“Work with your seed agronomist to see disease ratings and learn how various hybrids have reacted to bacterial diseases,” she says. “Resistant hybrids can have much less disease and yield loss than others.”

Because bacterial disease organisms can overwinter in residue or infected corn plants, diversifying rotations can give fields a break from corn. “Rotating to another crop for one or more years gives time for the infected crop debris to break down,” Jackson-Ziems says.

Rotating to crops and cover crops not susceptible to bacterial diseases (see side-bar) reduces the likelihood of future disease development. Controlling volunteer corn plants also helps prevent infection.

Limiting the relocation of harvested residue — such as in bales — from affected fields to unaffected areas also can help slow the disease’s spread.

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