15 MIN READ

Managing the Early Onset of Foliar Diseases in Corn

June 5, 2023

  • Early scouting and correctly identifying an infection are key for determining the status, progression, and best management practices for foliar diseases.

All plant diseases require three components for an infection to occur: the pathogen must be present, the environment must be suitable for the pathogen, and a susceptible host plant must be available. Many sources can lead to a pathogen becoming present in a field. Depending on the disease, some pathogens may survive the winter on previously infected crop residue (e.g., northern corn leaf blight, tar spot) and some may be moved into northern growing areas on winds from southern locations (e.g., southern corn rust). For a suitable environment, many foliar diseases need warm, humid, and wet conditions to propagate. Fungal diseases (e.g., grey leaf spot) can infect and penetrate plant tissue without a wound. However, bacterial diseases (e.g., Goss’s wilt) can only infect and penetrate plant tissue via a wound to the plant. Finally, in terms of susceptibility, many Bayer seed products have been bred to have genetic resistances to many common plant diseases. Your seed provider can provide information on a corn hybrid’s susceptibility levels to many of the common foliar diseases.

When to Scout

Depending on the disease, field history, and the susceptibility of the corn product, scouting should start at the whorl or tasseling stage (Table 1). If a susceptible corn product is planted in an area with a history of foliar disease, that area should be scouted prior to tasseling to determine the presence and severity of the disease.


Table 1. Suggested sampling window, favorable environmental conditions, and management options for common foliar diseases of corn.

Suggested sampling window, favorable environmental conditions, and management options for common foliar diseases of corn

Impact on Yield and Standability

Corn foliar diseases are a concern when they develop early and progress up the plant before grain fill is complete. Disease development in corn around the tasseling stage can result in yield loss, particularly if favorable environmental conditions support the continued infection and the top 8 to 9 leaves above the ear become infected, as the photosynthetic capacity of these leaves provides at least 75% of the carbohydrates needed to complete ear fill.10 Studies conducted at Iowa State University demonstrated that when grey leaf spot and common rust were controlled using a fungicide, the incidence of stalk lodging was reduced.11

Common Corn Foliar Diseases

Anthracnose leaf blight. This disease is very common in fields that are in a continuous corn cropping system. Anthracnose leaf blight spores are spread primarily by splashing water from the crop residue onto the corn leaf. The disease develops soon after planting and continues to develop until canopy closure. Plants develop oval- or spindle-shaped lesions that are brown with very dark-brown or purple margins (Figure 1). Lesions can coalesce to create larger areas of dead tissue, and leaves may die. Spores formed on the dead tissue will look spiny under magnification, much like a sea urchin.

anthracnose leaf blight produces oblong, water-soaked lesions up to 6 inches long, with tan centers and brown borders
Figure 1. Anthracnose leaf blight.

Northern corn leaf blight (NCLB). Typical symptoms of NCLB are large (1 to 6 inches long), cigar-shaped lesions (Figure 2). Lesions are initially grey green with a water-soaked appearance and turn tan to brown as infected tissues die. A distinct margin between the infected and healthy tissue is often apparent. Distinct dark areas of fungal sporulation develop within necrotic lesions when weather is humid. Mature NCLB symptoms can look similar to the leaf blight phase of Goss’s wilt or to drought/heat stress.

Northern corn leaf blight
Figure 2. Northern corn leaf blight.

Grey leaf spot. This disease causes grey to tan, rectangular lesions on leaf, sheath, or husk tissue (Figure 3). The spots are opaque and long (up to 2 inches). Lower leaves are affected first, usually after silking. Lesions may have a grey, downy appearance on the underside of leaves where the fungus sporulates. Grey leaf spot has become more prevalent with increased use of reduced tillage and continuous corn.

Gray leaf spot
Figure 3. Grey leaf spot.

Physoderma brown spot. The symptoms of this disease include very small (approximately ¼ inch in diameter), round- to oval-shaped lesions that are yellowish brown in color, occur in high numbers, and appear in broad bands across the leaves (Figure 4). In addition, dark purple to black spots occur on the midrib. The midrib lesions help distinguish this disease from other diseases such as eyespot and southern rust. The infection requires a combination of light, free water, and warm temperatures during the whorl stages, which results in the commonly seen alternating bands of infected and non-infected tissues on the plant. Symptoms may also appear on the stalk, leaf sheath, and husk.

Figure 4. Physoderma brown spot.

Eyespot. This disease causes small (about 1/16 of an inch), circular, translucent lesions surrounded by yellow to purple margins that give them a halo effect (Figure 5). Lesions occur on leaves (most commonly as plants approach maturity), sheaths, and husks. The disease is favored by cool, moist weather.

eyespot
Figure 5. Corn eyespot.

Common rust. Infections begin as light-green to yellow spots on leaves and develop into circular or elongated, small (2-10 mm long), reddish-brown, raised pustules.6 Pustules rupture the leaf epidermis and contain small, cinnamon-brown, powdery spores that can become darker brown to black later in the season (Figure 6). Pustules are often found in bands or patches, indicating that infection occurred while the leaf was in the whorl. Pustules can form on the upper and lower leaf surfaces. Severely infected plants may have issues with water balance and show symptoms of moisture stress during hot, dry weather, even when soil moisture is adequate.

Common corn rust.
Figure 6. Common rust pustules on the underside of a corn leaf.

Southern rust. Early symptoms of southern corn rust are small, circular- to oval-shaped lesions, which are often accompanied by a light-green to yellow halo. Unlike common rust, the lesions are almost exclusively located on the upper leaf surface. Within the lesions are light-orange to cinnamon-red pustules, which are unique to this disease and key to identification (Figure 7). Southern rust pustules tend to be smaller, have a more circular shape, and are more densely packed than common rust pustules. Also, unlike common rust, the lesions can develop on tissues other than the leaves, including the stalk, husk, and leaf sheath.

Southern rust corn image
Figure 7. Southern rust on corn.

Tar spot. A preliminary identification of tar spot can be done in the field, but a laboratory diagnosis is required to definitively distinguish this disease from other pathogens. Leaves with tar spot have small, raised, black, circular spots, which are called stromata (Figure 8). Stromata can be present on the healthy or dead tissue of leaf sheaths, stalks, and husks, and can be surrounded by a narrow, tan halo. The stromata are raised, bumpy, and vary in shape from small, pinhead structures to more elongated structures. Tar spot stromata can be easily confused with structures associated with other fungal diseases, such as the black pustules produced by the corn rust pathogen as it ages. Tar spot can also be easily confused with the black saprophytic organisms that grow on dead leaf tissue. However, saprophytes can be rubbed off, whereas stromata cannot.

Tar Spot
Figure 8. Tar spot of corn.

Goss’s wilt. The leaf blight symptoms of Goss’s wilt usually appear as long, grey-green to black, water-soaked streaks extending along leaf veins. Small, dark, water-soaked flecks referred to as “freckles” often occur inside larger lesions and at the edges of lesions where symptoms are advancing. Leaf freckles are luminous when lighted from behind, such as when the sun is used as backlighting. Bacterial cells may ooze from infected leaves and dry on leaf surfaces, forming a shellac-like sheen. As lesions mature, large areas of tan to brown, dead leaf tissues become visible (Figure 9).

Goss's Wilt freckling observed against the light
Figure 9. Goss's wilt of corn.

Bacterial leaf streak. Disease symptoms have been observed in corn as early as the V7 growth stage, in which lesions first appear on the lower leaves (Figure 10). Incidences of bacterial leaf streak are more common under continuous corn production, overhead irrigation, or rainfall during hot weather. Symptoms in the upper canopy are more common when the disease occurs after tasseling. It can be confused with grey leaf spot and common rust, so a laboratory test is often needed to confirm if an infection really is bacterial leaf streak.

Bacterial Leaf Streak
Figure 10. Bacterial leaf streak on corn.

Fungicide Considerations12

While fungicides can help reduce the incidence of fungal diseases, they will have no effect on the bacterial diseases that infect corn. Triazole and strobilurin fungicides are labeled for corn to help manage foliar fungal diseases. Triazole fungicides interfere with fungal membrane structure and function, and must be applied preventatively or in the early stages of infection. Following application, the active ingredients in triazole fungicides move locally into the leaves on which they were applied, but they are not necessarily transported to other leaves. Strobilurin fungicides inhibit fungal respiration and should be applied preventively or as early as possible in the disease cycle. They are absorbed into the leaf and have some upward movement in the xylem. Most triazoles and strobilurins have some residual activity based on the rate of application, coverage, and environmental conditions. Consult individual product labels for harvest interval and other restrictions for use.

In most cases, fungicides should be applied at or after tasseling. For example, a fungicide has been shown to be most effective for northern corn leaf blight and grey leaf spot when applied between two weeks prior to tasseling and two weeks after tasseling.13 Follow all individual product label instructions for proper application timing, application volume, application equipment, and environmental and harvest interval precautions.

Making an Application Decision

There are many factors to consider when determining if a fungicide application is warranted. Prior to making an application, evaluate each field for the susceptibility of the corn products to the diseases, the current yield potential of each field, disease severity, and corn stage of development. Then check the weather forecast to evaluate if upcoming conditions will continue to promote disease development. Finally, consider the cost of treatment and corn price to determine if an application is likely to provide an economical return in each field.


Sources:

1Stoetzer, E. and Robertson, A. 2019. Signs of Anthracnose leaf blight on your seedlings? No need to fret! Iowa State University Extension. https://crops.extension.iastate.edu/blog/alison-robertson-ethan-stoetzer/signs-anthracnose-leaf-blight-your-seedlings-no-need-fret

2Vittetoe, R. and Robertson, A. 2016. Seeing northern corn leaf blight? Iowa State University Extension. https://crops.extension.iastate.edu/blog/alison-robertson-rebecca-vittetoe/seeing-northern-corn-leaf-blight

3Stoetzer, E. and Robertson, A. 2018. Weather conditions ripe for Physoderma brown spot and node rot and gray leaf. Iowa State University Extension. https://crops.extension.iastate.edu/cropnews/2018/06/weather-conditions-ripe-physoderma-brown-spot-and-node-rot-and-gray-leaf-spot

4Smith, D. 2015. Corn diseases of 2015 and should I spray fungicide? University of Wisconsin Extension. https://ipcm.wisc.edu/blog/2015/07/corn-diseases-of-2015-and-should-i-spray-fungicide/

5Thiessen, L., Rivera, Y.R., and Kinczyk, J. 2018. Corn rusts: Common and southern rust. North Carolina State University Extension. https://content.ces.ncsu.edu/corn-rusts-common-and-southern-rust

6Telenko, D. and Creswell, T. 2019. Tar spot. Purdue University Extension. BP-90-W. https://www.extension.purdue.edu/extmedia/BP/BP-90-W.pdf

7Bradley, C.A., Mehl, K., and Pfeufer, E. 2017. Stewart’s wilt of corn. University of Kentucky Extension. PPFS-AG-C-04. https://plantpathology.ca.uky.edu/files/ppfs-ag-c-04.pdf

8Wise, K., Ruhl. G., and Creswell, T. Goss’s bacterial wilt and leaf blight. Purdue University Extension. BP-81-W. https://www.extension.purdue.edu/extmedia/bp/BP-81-W.pdf

9Stoetzer, E. and Zaworski, E. 2018. Identifying, scouting and management of bacterial leaf streak. Iowa State University Extension. https://crops.extension.iastate.edu/cropnews/2018/07/identifying-scouting-and-management-bacterial-leaf-streak

10Rees, J.M. and Jackson, T.A. 2008. Gray leaf spot of corn. University of Nebraska Extension. G1902. http://extensionpublications.unl.edu/assets/pdf/g1902.pdf

11Robertson, A., Abendroth, L., and Elmore, R. 2007. Yield responsiveness of corn to foliar fungicide application in Iowa. Iowa State University Extension. Integrated Crop Management News 498(26): 281-285. https://crops.extension.iastate.edu/encyclopedia/yield-responsiveness-corn-foliar-fungicide-application-iowa

12Smith, D. 2020. Field crops fungicide information. University of Wisconsin-Madison. https://badgercropdoc.com/field-crops-fungicide-information/ and https://ipcm.wisc.edu/blog/2020/07/fungicide-for-field-crops-information-updated/

13Smith, D. 2018. Late season corn foliar disease update and hail damaged corn. University of Wisconsin-Madison. https://ipcm.wisc.edu/blog/2018/08/late-season-corn-foliar-disease-update-and-hail-damaged-corn/

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