Corn Disease Update
Bacterial Leaf Streak
Bacterial leaf streak (BLS), caused by Xanthomonas vasicola pv. vasculorum, was reported for the first time in the United States in Nebraska in 2016. Since then, the disease has been confirmed in 60 Nebraska counties and eight additional states: Colorado, Kansas, Illinois, Iowa, Minnesota, Oklahoma, South Dakota, and Texas. Previously, the pathogen had only been confirmed on corn in South Africa and on sugarcane in numerous countries around the world. Numerous other grass and palm hosts were identified in other countries, as well, including sorghum species. Results from additional host range testing conducted in Nebraska confirmed several additional crop, weed, and native perennial grass species as hosts.
Symptoms on corn can be difficult to differentiate from other diseases, especially the gray leaf spot fungal disease. Typical symptoms of the disease on corn and other hosts are narrow interveinal streaks that can appear bright yellow when backlit. The pathogen overwinters in infested crop debris thus, disease develops in the same areas repeatedly when susceptible hybrids are grown and favorable weather conditions persist. Severity of the disease varies considerably on corn hybrids, particularly on some popcorn hybrids that can be quite susceptible. High relative humidity and leaf wetness favor disease development. Results from additional research trials will be shared, including yield trials and mitigation experiments evaluating the effects of corn-soybean rotation sequences and tillage regimes.
Goss’s Bacterial Wilt and Blight
Goss’s bacterial wilt and blight was more common in 2017 than in recent years. Lesions often have small, dark “freckles” and a water-soaked appearance on the edges and may appear similar to other fungal diseases, such as Northern Corn Leaf Blight and Diplodia Leaf Streak. The bacteria causing Goss’s bacterial wilt and blight overwinter in the infested crop residue from the previous season(s). Development of the disease again this year was most often in fields where corn sustained injury caused by severe weather, especially hail and wind storms. These bacteria commonly utilize plant wounds to infect and cause disease.
Development of the disease again in 2017 was a reminder that the pathogen is still widespread throughout much of Nebraska and capable of causing disease even if we haven’t observed disease in one or several years. With this in mind, producers with fields that have a history of the disease should still carefully select and plant resistant corn hybrids in those fields that have a high level of resistance to Goss’s bacterial wilt and blight to avoid severe disease that could impact yield. Crop rotation and tillage, where practical, may also help reduce overwintering inoculum and disease severity.
Diplodia Leaf Streak
In 2017, Diplodia (Stenocarpella) leaf streak, caused by the fungus Stenocarpella macrospora (syn. Diplodia macrospora and S. zeae) was confirmed in samples from Madison and Platte counties in eastern Nebraska. This fungal disease had been previously identified in several other states, but this was the first time it was been confirmed on corn samples from Nebraska. The disease was reportedly at low incidence and severity in these locations.
Symptoms are similar to those of some other common diseases that will make it difficult to recognize. For example, the large tan lesions caused by Diplodia leaf streak may look similar to the large lesions of northern corn leaf blight or Goss’s bacterial wilt and blight. Lesions may begin as small elliptical tan to brown spots expanding into very long streaks that usually have tapered, pointed ends. Fungal reproductive structures, called pycnidia, may develop within the lesions and look like black dots and release two-celled spores on the surface.
The fungus survives in infested plant residue, in the soil, or on seed. Infection and disease development are favored by warm, wet conditions. Disease primarily occurs on the leaves and is related to the fungus causing Diplodia (Stenocarpella) ear rot. Because the fungus primarily survives in infested residue, the most effective management strategies in other states have been crop rotation and tillage as resistant hybrids are not available. The disease is genhas been largely considered a minor disease in other states where it occurs now.
In 2017, some producers were surprised to see more Holcus (bacterial) spot, than in previous years. Symptom distribution in the field and on the plants themselves can help to differentiate this disease from others or from pesticide drift. Development of more disease, bacterial or fungal, in the lower leaves is common, but eyespot lesions are often much smaller, only about 1/8 inch. Holcus spot lesions are white to tan and usually ½-¾ inch. Herbicide drift is usually on one side of the field and appears suddenly, not spreading.
Stalk Rot Diseases and Lodging
Harvesting the 2017 corn crop was very difficult for many Nebraska producers dealing with downed corn. During the early fall, stalk rot diseases in some parts of the state prematurely killed some corn plants and possibly led to some yield loss. However, as harvest began, those areas affected by stalk rot were not common and overall standability was very good. However, heavy rainfall events and prolonged wet conditions delayed harvest in some areas for as much as four weeks. The prolonged wet conditions favored many of the existing stalk rot pathogens and other common (mostly beneficial) fungi that occur naturally in our agricultural fields. These fungi perform beneficial functions of breaking down crop residue to recycle nutrients in our soil. Some of these fungi are the same ones creating large amounts of black spores on the surfaces of senesced corn plants in the fall that release black clouds that may look like smoke from combines as they harvest.
Unfortunately, the delayed harvest allowed more time for degradation of stalks by all of these microorganisms that severely compromised stalk integrity. The prolonged wet conditions that significantly delayed harvest were soon followed by very high wind events statewide that led to the lodging of many acres of corn with weakened, vulnerable stalks. Some of these conditions were unavoidable; however, selecting hybrids with good resistance to stalk rot diseases and good standability may help reduce losses. Our research at the South Central Ag Lab near Clay Center has consistently shown improved standability following timely foliar fungicide applications around VT/full tassel emergence states. However, by late season if harvest is delayed significantly by adverse conditions, the benefits of fungicide application and those of disease-resistant hybrids may be negligible.
Find more information and photos of these diseases in the Corn Disease section of CropWatch.