By Adriana Murillo-Williams, Alyssa Collins and Paul D. Esker (PennState)

Whether you are scouting corn fields to determine if a fungicide application is warranted or evaluating the effectiveness of a previous fungicide treatment, accurate disease identification is critical. But are you sure you know what you are looking at when scouting?

Foliar diseases can reduce both grain yield and silage quality by decreasing the leaf area available for photosynthesis, increasing the risk of stalk rots and lodging, and causing premature plant death. Along with understanding whether environmental conditions are favorable for disease development, regular scouting and accurate disease identification are essential components of an Integrated Pest Management (IPM) program.

Disease scouting provides the information needed to answer key management questions:

• Are diseases and/or their vectors present in the field?
• Is the crop at a susceptible growth stage?
• What are the incidence and severity levels of the disease?
• Are disease levels approaching economic thresholds?
• How susceptible is the corn hybrid being grown?

Foliar disease pressure often increases during grain fill, particularly when humid conditions and moderate temperatures persist. Fungicide activity also declines over time, and disease symptoms may become more apparent several weeks after an application. Diseases such as gray leaf spot and northern corn leaf blight are favored by wet, humid conditions and can increase rapidly in susceptible hybrids, especially in no-till or continuous corn production systems. In addition to these common diseases, scouts should be aware of emerging diseases such as southern rust and bacterial leaf streak, which can be easily confused with other foliar diseases and may require different management approaches.

Below are key identification features of several important corn diseases.

Tar spot

Tar spot is caused by the fungus Phyllachora maydis. Symptoms and signs include small, raised, black spots called stromata that develop on both upper and lower leaf surfaces, as well as on husks. Stromata are shiny and cannot be rubbed off with a finger. They may be surrounded by tan or brown necrotic halos known as fish-eye lesions (Figure 1). Disease development is favored by cool temperatures and prolonged periods of high relative humidity during late summer.

Figure 1. Signs of tar spot of corn. Photo credit: Adriana Murillo-Williams. Penn State

Northern corn leaf blight (NCLB)

Northern corn leaf blight is caused by the fungus Exserohilum turcicum (Setosphaeria turcica). Symptoms appear as long, gray-green to tan lesions that are widest in the middle and taper at both ends, giving them a characteristic cigar-shaped or fusiform appearance (Figure 2). Lesions run parallel to leaf veins and may reach up to six inches in length. Under favorable conditions, lesions may develop dark margins and become covered with olive-gray fungal growth.

Figure 2. Symptoms and signs of northern corn leaf blight. Photo credit: Adriana Murillo-Williams. Penn State.

Gray leaf spot (GLS)

Gray leaf spot is caused by the fungus Cercospora zeae-maydis. Early symptoms appear as small tan lesions with yellow halos. As the disease progresses, lesions expand parallel to the leaf veins and become rectangular and gray to tan in color (Figure 3). Lesions are typically one to two inches long. Symptom appearance can vary depending on the hybrid and its resistance level.

Figure 3. Gray leaf spot symptoms. Photo credit: Adriana Murillo-Williams, Penn State.

Northern corn leaf spot

Northern corn leaf spot is caused by Bipolaris zeicola (syn. Cochliobolus carbonum). Symptoms can vary depending on the pathogen race and the resistance of the corn hybrid. Race 3 commonly produces small tan to brown lesions arranged along leaf veins in a pattern often described as a "string of pearls” (Figure 4).

Figure 4. Northern corn leaf spot. Photo credit: Adriana Murillo-Williams. Penn State

Common rust

Common rust is caused by the fungus Puccinia sorghi. Early infections appear as small chlorotic flecks on the leaves. These flecks develop into elongated, blister-like pustules called sori, which contain masses of brown to brick-red spores and are often surrounded by a yellow halo (Figure 5). Common rust pustules occur on both the upper and lower leaf surfaces and are usually scattered across the leaf. The disease is very rarely problematic because almost all hybrids carry good resistance to it.

Figure 5. Common rust pustules. Photo credit: Adriana Murillo-Williams, Penn State.

Southern rust

Southern rust is caused by the fungus Puccinia polysora. Symptoms include small, circular pustules filled with bright orange spores (Figure 6). Unlike common rust, southern rust pustules are only found on the upper leaf surface and often occur in dense clusters. The disease can be easily confused with common rust, which is frequently observed late in the growing season. While common rust is rarely economically important because most modern hybrids carry good levels of resistance, southern rust can cause significant yield losses depending on the crop growth stage at infection and weather conditions following disease establishment. For this reason, growers and crop advisers should remain vigilant when scouting fields and carefully distinguish southern rust from common rust.

Southern rust does not overwinter in Pennsylvania. Each year, spores are carried northward from tropical and subtropical regions by wind currents. Growers can track the movement of southern rust across the United States using the Corn IPM PIPE monitoring system.

Figure 6. Southern rust pustules. Photo credit: Edward Sikora, Auburn University. Bugwood.org.

Bacterial leaf streak (BLS)

Bacterial leaf streak is caused by the bacterium Xanthomonas vasicola pv. vasculorum. Although it has not been confirmed in Pennsylvania, the disease has been reported in multiple corn-producing states across the United States and remains an important disease to watch for during scouting activities. Symptoms often resemble gray leaf spot, making an accurate diagnosis especially important. Lesions are narrow, elongated, and tan to brown in color, developing between the leaf veins and creating a streaked appearance. Unlike gray leaf spot, which is caused by a fungus, bacterial leaf streak is caused by a bacterium. As a result, fungicide applications are not effective for disease management. Because bacterial leaf streak is relatively new in many production areas, researchers are still working to better understand its distribution, spread, economic impact, and management options. If bacterial leaf streak is suspected, contact your local Extension educator or diagnostic laboratory for assistance with identification and confirmation. Additional photographs and management resources are available through the Crop Protection Network.

Figure 7. Symptoms of bacterial leaf streak of corn (Photo credits: Vinicius Garnica, Bugwood.org)

Corn stunt

Corn stunt is a disease complex transmitted by the corn leafhopper (Dalbulus maidis). It can be caused by one or more of the following pathogens:

• Corn stunt spiroplasma (Spiroplasma kunkelii)
• Maize bushy stunt phytoplasma (Candidatus Phytoplasma asteris)
• Maize rayado fino virus (MRFV)

Symptoms include yellow spots that develop into stripes on the leaves, shortened internodes that result in plant stunting, and the production of tillers. As the disease progresses, leaves may develop a reddish discoloration, and ears can become abnormal or poorly developed (Figure 7). Symptom severity depends on the timing of infection and the pathogen involved,

Corn stunt has not yet been reported in Pennsylvania. However, the disease was confirmed in New York in 2024, and the corn leafhopper population continues to expand its geographic range. To monitor the spread of the vector, consult the Corn Leafhopper Map, and for disease reports, refer to the Corn Stunt Pathogens Map.

Figure 8. Corn stunt (Spiroplasma kunkelii). A) Photo credit: Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org; B-C) Photo credit: Department of Plant Pathology , North Carolina State University, Bugwood.org