A number of diseases have been reported in Nebraska in the last few years. In late August, a different disease, bacterial leaf streak, was confirmed in Nebraska cornfields. It's the first time the disease has been confirmed in the U.S., although it had been previously confirmed in corn in South Africa. The pathogen that causes the disease, Xanthomonas vasicola pv. vasculorum, also causes gumming disease in sugarcane in a number of countries.

SYMPTOMS TO WATCH FOR: While bacterial leaf streak can look similar to gray leaf spot, the edges of the lesions can give some subtle clues. Gray leaf spot causes smooth linear margins on rectangular lesions in most cases, while bacterial leaf streak can cause lesions that are wavier or more irregular.

But it may not necessarily be a new disease to Nebraska; it's just the first time it's been positively identified. Unusual symptoms were first reported in corn samples received by the University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic in 2014. Since the announcement, the disease has been confirmed in several other states, including Colorado, Iowa, Kansas and Illinois.

Multi-agency collaboration
UNL's Plant and Pest Diagnostic Clinic collaborated with Colorado State University, Iowa State University, the Nebraska Department of Agriculture and USDA’s Animal and Plant Health Inspection Service for the final diagnosis. Identification was delayed because of the lack of historic research on the pathogen and limited data available on this pathogen and its close relatives.

"This is a disease we've been watching for a couple years now. Some people report they've seen it for longer than that. It's possible we've all overlooked it because it can look like gray leaf spot," says Tamra Jackson-Ziems, University of Nebraska Extension plant pathologist. "Our primary message is to make sure people know there's something else out there that may look like gray leaf spot, but it's not. Because it's bacterial, a foliar fungicide is not going to effectively manage it."

With little research on the disease at this point, there isn't much information available on the yield impact of the disease and what the best practices for control are. Currently, however, the best approach is to follow similar management practices used to control other bacterial diseases, like Goss's wilt.

"Continuous corn seems to be more favorable, and a crop rotation could help. But I have some doubts about some rotations, as we don't understand the host range that this pathogen has. Sorghum, for instance, has been listed as a host, but we haven't yet confirmed it in a sorghum field," says Jackson-Ziems. "Tillage might help, but I understand that's not practical or favorable for many people. Because the pathogen overwinters in residue, if we can promote degradation, it might be beneficial. But none of those practices will completely eliminate the possibility of the disease occurring the next year or following year."

One of the challenges in identifying the disease is its resemblance to fungal diseases like gray leaf spot, which also has appeared in Nebraska cornfields over the last couple years.

Lesions vary
"There are pretty subtle differences. The lesions themselves have some clues. One of the best things to do is look at the edges of the lesions," explains Jackson-Ziems. "While gray leaf spot has smooth linear margins on rectangular lesions on most hybrids, the lesions caused by the bacteria might be wavier and/or irregular. You may have to look at quite a few leaves and lesions to make sure that's what you're seeing. And it is more difficult when both diseases are on the same leaf."

Over the last two years, bacterial leaf streak has developed much earlier in the season than gray leaf spot — one of the indicators that made it stand out as a separate disease. The disease seems to be favored by warm, moist conditions — like many diseases — but can still be active at cooler temperatures than diseases like gray leaf spot. However, as a recently confirmed disease, there is no temperature curve yet. If you suspect you have bacterial leaf streak in your cornfields, the best course of action is to have leaves tested to confirm the disease to make sure you aren't trying to control it with a fungicide.

To learn more, contact Jackson-Ziems at [email protected].