Some corn fields are starting to dry down faster than others in the area. However, there might be quite a few fields that are still green, especially if they were planted late. Since most of the corn is at or nearing dent, if not farther along, most of the fungicide applications for the year have come to a complete stop. There were some interesting diseases out there this growing season, but there was one you might have seen in the field but weren’t quite certain what it was.
Last week, the USDA released an official report stating that Bacterial Leaf Streak has been confirmed in Nebraska. Various entities, including Nebraska Extension, have been monitoring the disease this growing season and have been trying to learn and understand as much as possible about this bacterial disease of corn. Throughout the season, symptoms have been seen in both the lower and upper leaf canopy. This disease has often been mistaken as gray leaf spot on corn. While symptoms can often be mistaken, especially at the early stages of infection when lesions are small, there are some ways that we can differentiate gray leaf spot from bacterial leaf streak.
Gray leaf spot is caused by a fungal pathogen. The fungus overwinters in corn residue remaining on the soil surface. Once temperature and humidity are optimal, spores are produced which either can be blown or splashed onto the lower leaves in the corn canopy. Typical lesions of this disease are gray and rectangular in shape. The lesions are typically limited by the leaf veins which gives them that characteristic rectangular shape. After infection, more spores will develop and this disease will gradually move up into the leaf canopy. If these lesions coalesce, or join together, they can be confused for diseases like Goss’s bacterial wilt and leaf blight. Fungicides have been shown to be effective if applied at certain times. Multiple hybrids also exhibit partial disease resistance which could help in controlling this fungal pathogen.
Bacterial leaf streak is caused by a bacterium, as the name indicates. Currently, there is a lot that we do not understand about this disease due to limited research. We do know that from our observations, infection can develop earlier in the growing season compared to gray leaf spot. As mentioned previously, lesions can develop in the upper or lower canopy. At first glance, these lesions look a lot like gray leaf spot. They are long but not rectangular in shape. Also, these lesions have wavy margins that may not stay within the leaf veins as gray leaf spot normally would. Infected leaves held up, using the sun as a back light, can exhibit a yellow halo directly around the lesion. Often times you can spot lesions directly surrounding the midrib, but that does not mean it is limited to this region. Like gray leaf spot, bacterial leaf streak might become more difficult to identify as lesions mature or join together. Since this is a bacterial disease, fungicide applications will not work as a treatment option. At this time, there is still a lot that we do not know about this disease in terms of hybrid susceptibility or resistance.
Since the USDA released the confirmation of bacterial leaf streak in Nebraska, multiple media sources have become available as public sources of information. The University of Nebraska-Lincoln released a Market Journal interview with Dr. Tamra Jackson-Ziems, our state corn and sorghum pathologist, on August 26th, 2016. CropWatch also released a couple of articles on August 26th for those interested in learning more about bacterial leaf streak.
Husker Harvest Days is rapidly approaching as it will be held in Grand Island on September 13th-15th. As usual, Nebraska Extension will be there willing and ready to help if you have any questions. With irrigation season rapidly winding down for the season, it’s time to start thinking about next year. We will have free testing available for irrigators interested in having their center pivot irrigation pressure regulators checked at the IANR building during Husker Harvest Days. We encourage irrigators to bring two regulators per pivot span for the most accurate reading of your system. Pressure regulators can be dropped off at the IANR building in the morning and picked up, with your report, later during the day for those who are interested.