September 8, 2016
New Corn Disease: Bacterial Leaf Streak
Some of you may have heard recently that a new disease had been discovered in Nebraska corn fields. On Friday, August 26, the Nebraska Department of Agriculture announced that this new disease called bacterial leaf streak of corn had been identified in Nebraska. This is the first time this disease had been confirmed in the United States, but it has been observed in other countries and also attacks other crops, most notably sugar cane.
This disease has been observed in 49 counties in Nebraska as well as in Colorado, Kansas, Iowa, Illinois, Oklahoma and Texas. It has been identified in a number of counties in northeast Nebraska although the worst infestations appear to be in south central and southwest Nebraska. Being a new disease, much is not known about it or its potential impact on yields, but we do know several things about it.
The initial infection usually starts on the lower to mid-canopy leaves and then moves up the plant. Infection can start early in the season, having been observed on V7 corn in June this year. It forms lesions that run between the veins on a corn leaf and can be brown, tan or yellow. The lesions are very similar in appearance to another common fungal corn disease, gray leaf spot.
Its similarity to gray leaf spot helped identify this disease. Farmers with fields infected with bacterial leaf streak thought they had gray leaf spot and treated their fields with a fungicide, the normal treatment for gray leaf spot. However, the disease continued to spread and they were concerned that they had gray leaf spot that was resistant to fungicides. On further examination, it was discovered that rather than having a fungal disease that would normally be controlled with fungicides, they had a bacterial disease which fungicides would not control. Most crop protection products are ineffective in controlling bacterial diseases.
There are several ways to help distinguish this disease from gray leaf spot. First, it can be observed much earlier in the growing season, although it won’t always show up that early. The better ways to distinguish this disease is to hold a leaf with lesions up to the sun or a bright light. Bacterial leaf spot lesions are strikingly yellow when backlit. The other way is to take a close look at the lesion itself.
Gray leaf spot lesions are typically very rectangular in shape with smooth edges and flat ends. Bacterial leaf streak lesions, although rectangular and about the same size as gray leaf spot lesions, tend to have wavy or jagged margins along the sides and the ends of the lesions are not as square as gray leaf spot.
Other bacterial diseases such as Goss’s wilt are most common when there has been injury to the leaf, a lot of leaf tattering following high winds or after a hail storm. This injury provides a place for the pathogen to enter the leaves. Bacterial leaf streak can enter a leaf through the natural openings in the leaf, the stoma, and does not require injury to the leaf to infect the plant.
We’re late enough in the growing season that this disease, or other diseases, are not going to cause significant yield loss so treatments are not warranted. However, it’s important to identify this disease, if present, because we know it can overwinter on crop residue. So fields with continuous corn or reduced or no-till are more likely to become infected again in future years.
It is also important to distinguish it from gray leaf spot so ineffective and costly fungicide applications are not made that will not control the disease. Our observations indicate there are differences between hybrids in their susceptibility to this disease so hybrid selection will be important when planting corn in fields with a history of bacterial leaf streak.
If you think you might have a field infected with bacterial leaf streak, contact your local Nebraska Extension office. The Nebraska Corn Board is supporting a program to cover the cost of analysis to determine if this disease is present at no cost to the producer or agronomist submitting the sample.
For more information on bacterial leaf streak of corn, visit the CropWatch website at http://cropwatch.unl.edu/ or contact your local Nebraska Extension office.